A Systematic Study of Wave Phasing on Righting Arm Curves for
Fishing Vessels - Paper #D15 - 07-11-05
John Womack, Member, Mid-Atlantic Shipwrights
Bruce Johnson, Fellow, U. S. Naval Academy
ABSTRACT
This paper summarizes the results from the SNAME funded T&R grant titled Preliminary Development of the Next
Generation of Stability Criteria for Small Fishing Boats. The principal goal of this project was to take a broad look
at effects of head and following waves on the current still water based stability evaluation methods to focus the needs
for future research in the development of new performance based stability criteria. The wave effects were calculated
using an off the shelf naval architect software package of the type typically used in small fishing vessel stability
evaluations to explore the ability of these software packages to perform cost effective meaningful stability research.
The use of the off the shelf naval architect software package also allowed the authors to explore new performance
based stability criteria formats that utilized software and basic concepts already available to the naval architect.
With the recent availability of affordable software packages that
have been developed for the efficient analysis of the static
stability of small working vessels operating in various regular
wave conditions, these goals are now readily obtainable.
NOMENCLATURE (ITTC symbols when
applicable including computer compatible
symbols for graphics)
From the results that were obtained from these goals, it is also
now time to consider proposing affordable new fishing vessel
stability evaluation methods and criteria. To do so will require
considerable additional research and development efforts, but
this can be divided into short and long term goals to speed up
this process and introduce the needed safety benefits as soon as
is practical.
AM - Amidships
AP - After Perpendicular
cG – Wave Group velocity or celerity
cW – Wave Phase velocity or celerity
FP - Forward Perpendicular
GHS – General Hydrostatics Program
GM – Metacentric Height
HW, HW – Wave height
KG - Center of Gravity Above Baseline
LPP - Length Between Perpendiculars
λ, Lw, LW - Wave Length
LWL – Waterline Length of vessel
TW – Basic wave period
US - United States
USCG - United States Coast Guard
Vs – Speed of vessel (ship)
WC – Wave Crest (graphs)
WT - Watertight, Wave Trough (graphs)
σ - Wave Encounter Angle - Relative to the Bow
λM – Linear Scale of ship model
The short term goals of such an effort should be to explore ways
to improve on the current static stillwater righting arm curve
approach. The basic goals are two fold. One; provide some
level of capsizing risks as opposed to the current strictly
pass/fail approach. And two; take into account a vessel’s unique
characteristics, operating conditions, and stability “Achilles
Heels” such as downflooding through watertight openings and
large enclosed deck house spaces.
The long term goals are to develop prediction methods for a
vessel’s response to given sea conditions to allow the;
- Determination of the maximum expected roll/pitch/etc.
This is useful for both vessel survivability and crew
working safety.
- Determination of the risk of capsize from broaching,
breaking waves, and other similar capsize events.
- Determination of the likelihood of boarding seas on decks,
pilothouse, etc.
- Determination of the likelihood of watertight openings
being submerged and the risks from any resulting
potential internal flooding
INTRODUCTION AND PROJECT GOALS
This SNAME T&R supported project was developed to achieve
the following goals.
1. Investigate the relative effects of head/following seas on the
intact righting arm curve evaluated at various ratios of wave
length to ship length, wave steepness and position on the
wave. Discuss the significance of the results on current still
water based stability evaluation criteria methods.
2. Investigate the relative effect on the area under the righting
arm curve up to watertight openings which are not evaluated
in current stability criteria.
The process required to achieve the short and long term goals
will be a multifaceted project using the following four principal
investigation techniques.
- Analytical investigations using existing naval architecture
software packages. This would principally be for short
1
term improvements to the current static still-water
righting arm curve stability evaluation methods.
- Theoretical research using advanced computational fluid
dynamics (CFD) methods. (Beck and Reed 2001)
- Model tests to confirm theoretical research and to develop
data when theoretical methods will not work or are
impractical.
- Realistic experimentation by outfitting research vessels
with motion and sea state monitoring systems such as
Ship Motion Control’s SMCFish and Ocean Wave’s
WaMos II. This is also to confirm theoretical research
and to develop data when theoretical methods will not
work or are impractical.
The systematic model tests on the series 60 hull form followed
Paulling’s “Transverse Stability of Tuna Clippers” (Paulling
1960) in which he confirmed that for vessels with high
freeboard and watertight buoyancy forward and low freeboard
and watertight buoyancy aft (as is the case for many types of
working vessels) “the basic assumption of the usual cross-curve
computation, i.e. no heel induced trim, is violated and the results
so calculated will not accurately represent the actual behavior of
the vessel.” This comment is true for many types of fishing
vessels “which exhibit a strong tendency to trim by the stern at
angles of heel greater than that at which the deck edge is
submerged.” Thus Paulling concluded that “the initial
metacentric height is even less valid as a criterion of stability
than in the case of most other vessel types.”
BACKGROUND AND STATE OF THE ART
Paulling (1960) also stated the following with reference to using
static ship-wave orientation: “Preliminary tests showed that the
measured righting moments are independent of model speed for
a range of zero speed up to a model speed equal to the wave
speed. From the agreement between computed and measured
values, it is therefore concluded that the assumption of static
equilibrium and hydrostatic pressure distribution leads to results
which are of acceptable accuracy in representing the following
sea situation.
Model Tests and Analysis of Vessel Stability in Following
and Quartering Waves
The effect of the seaway on vessel stability has been researched
for many years, albeit with limited computational tools until
recently. Paulling (1961) and Du Cane (1962) review the history
at their time of writing of attempts to analyze the effects of
ahead and following seas on the overall stability of various
classes of vessels. In the pre-personal computer age, Kempf
(1938), Graff (1941), Grim (1952) and Wendel (1954)
performed experimental investigations in Germany on the
subject.
Since the most dangerous situation occurs when a wave crest is
amidships (in that investigation) Paulling also concluded that “it
may be possible to serve the needs of the fishing method by
keeping freeboard to a minimum aft and, at the same time,
materially improve the seagoing stability of the vessel by raising
the freeboard amidships.”
The first systematic look at the “Transverse Stability of a Ship
in a Longitudinal Seaway” was done by Paulling (1961). He
used a DTMB Series 60 parent with three beams and three
freeboards of ± 25% to the parent hull which was towed at a
fixed angle of heel in various overtaking constant steepness
waves. At this wave steepness, little change in inclined GZ with
wavelength was noted for the parent hull form, so the standard
wavelength equal waterline ship length in λ/20 waves was used
for the rest of the tests. (It is possible that λ/20 waves
represented the practical steepness limit for nearly sinusoidal
wave generation using wavemakers installed before the mid 70s.
That was the case at the Naval Academy before the powerful,
low harmonic distortion hydraulic wavemakers were installed in
1975 in the new Hydromechanics Lab.)
Du Cane and Goodrich (1962) examined the broaching problem,
including loss of stability on waves using a free to surge subcarriage arrangement. A major focus is on directional stability
and the ratio of ship speed to wave speed has a large influence
on whether or not the wave can accelerate a slower moving
vessel up to or slightly more that the wave speed. To quote from
the paper “The instability of surge velocity at what has been
called the critical wave speed (Vs/ cW > 0.7) can result in a very
large change in speed due to a slight increase in wavelength.
The most interesting fact illustrated by these experiments was
the large range of wavelengths over which the model was
carried along at the wave crest speed.” One of the discussers (H.
Lackenby) of the paper pointed out that the critical wave speed
is probably a function of the wave height (held constant during
these experiments) and slope. The authors also did not discuss
the influence of vessel inertia on the critical wave slope for the
surge acceleration to take place. Later investigations on
capsizing in following and quartering seas determined that
breaking waves can accelerate the vessel quickly to the crest
velocity, especially in the light loaded condition with a critical
vessel speed as low as 0.5 of the wave speed (Grochowalski,
1989). These investigations also showed that accelerating to a
surf-riding situation is important to both pure loss of stability on
a wave and broaching when a breaking wave impacts the vessel
in a quartering condition.
Paulling performed a number of model experiments in following
seas and experimentally measured the righting moment at
various heel angles. The model was fixed at various heel angles
and allowed to heave but was not free to surge. His results
confirmed that “relatively narrow, high freeboard vessels suffer
substantially smaller stability changes in a seaway.” However,
beamy, low freeboard vessels suffer dramatic reductions in their
righting arm curves when “perched on a wave”.
The concluding statement from Paulling (1961) is "The effect of
the seaway should be taken into consideration in judging the
stability of vessels, particularly those of large beam-to-draft and
low freeboard-to-beam ratios, if a more concise representation
of the vessel's ultimate performance at sea is to be obtained."
2
Using the development of hydrostatic software for personal
computers, Storch (1978a and b) analyzed crab vessel losses
including loss of stability on waves properly corrected for heel
and trim. He analyzed 13 specific vessels for waves of λ/LWL =
1, height equal to 1.1 √LWL and crest at amidships. Although this
condition was considered by many a worst case scenario at the
time, it is not the most dangerous or common scenario for small
working vessels encountering gales of significant wave height
considerably greater than 1.1√LWL, corresponding periods much
longer than √LWL*2π/g and wave celerities, cW =λ/T much
greater than √LWL*g/2π, the criteria used by Storch.
perched on a wave were calculated but the wave conditions were
not given.
The multiple capsizes during the Fastnet Yacht Race in 1979
brought increased interest in capsize studies and the
development of capability to generate large breaking waves in a
towing tank for beam sea testing. (Hirayama and Takezawa
1982, Salsich, et al, 1983, Zseleczki, 1988, 1989).
Various modes of capsize were discussed by many authors at the
Stability 82 conference in Japan including a good summary by
Takaishi (1982). Many of the following sea model tests at that
time were still towed by a carriage in which surfing is not
allowed by the towing rig (Hamamoto and Nomoto 1982). This
enables direct measurement of corrections to the righting arm
curve at various fixed angles of heel while the model travels at
the wave phase velocity (as was done previously by Paulling).
The loss of the 57 m stern trawler Gaul with all 36 crew in very
heavy seas north of Norway in February, 1974 spurred a number
of investigations which continue to the present. The first of
these was published by Morrall in 1980. The investigation
included an evaluation of the variation in righting arms in waves
equal to the vessel length and wave height equal to LS / 20
(2.85m) with the crest at the FP and AP, 0.25LS from the AP,
crest at amidships and crest at 0.25 LS from the FP. The effect
of the wave position on the righting arm curve was typical for a
“shelter deck” vessel as described later in this paper. The
unfortunate use of the LS / 20 rule of thumb was quite
inappropriate for the estimated significant wave height of 6.7 m
and estimated significant wave period of over 11 seconds. The
reported heavy seas could have easily included wave groups of
around 11 m high approximately 1 percent of the time. (Kriebel,
1993, Dawson, 1996, 2002). One of the discussers (Gilfillan)
gave an example of comparing the results of LS / 20 waves with
LS / 7 breaking waves and mentioned that 1.2 LS / 7 waves were
even more dramatic in their effect on righting arms.
However, several self-propelled tests which allowed the free to
surge condition were reported. Blume and Hattendorff (1982)
reported their initial results of an extensive investigation on the
intact stability of free running fast cargo liners in both regular
(λ/LWL = 1) and irregular waves. They investigated the effect of
Froude Number, initial heel angle, and position on the wave for
the λ/LWL = 1 case. Rennilson (1982) reported on his
investigation the likelihood of broaching to in following seas
with 1 < λ/LWL < 2. His suggested guidelines for avoiding a
broaching situation include the following comments “The
predominant wave length must be of the order of the ship length
or greater, with the wave amplitude the order of the ship draft or
greater…Since the critical wave lengths are dependent on the
ship length, shorter ships will encounter more severe conditions
and, hence, are more likely to broach.”
The intact stability of towing and fishing vessels was the subject
of a major set of model experiments at Hydronautics in the mid
70s (Amy, Johnson and Miller 1976). These tests involved both
free running and towed tests. “In all the free-running tests in
following seas, capsizings and extreme rolling were due to
either a complete loss of stability with the model poised on the
wave crest or to rolling at a period equal to twice the wave
encounter period. In no case did the model capsize in stern
waves while running free at a low speed–length ratio. It was
necessary to have waves which were long enough ( λ/LWL = 1.5)
and a high enough model speed (speed/length ratio of approx. 1
corresponding to Fn = 0.3) for the model to surf for some time
with the crest amidships. No models capsized by broaching.”
Corrections to the vessel righting arm curves for the tugboat
Table 1. Regular Wave Conditions for Scale Ratio of 14
Regular
Wave
Wave
Wave
Wave
Height
Period
Length
λ/LWL
m
Sec
m
#
#1
2.52
3.37
17.7
.95
#2
3.78
4.12
26.4
1.42
#3
5.32
4.86
36.9
1.99
#4
6.16
5.24
42.8
2.30
#5
7.00
5.61
49.2
2.65
#6
9.10
6.36
63.2
3.40
An extensive series of intact stability model tests was performed
at the SSPA model tank during the 1980s (Grochowalski, 1989,
1990, 1993, 1994). The fishing vessel used for these tests was
based on a 1/14 model of a typical small Canadian, hard-chine
stern trawler of 19.8 m length. The model was self-propelled
and was tested at three different model speeds in following and
quartering seas using four loading conditions, two of which met
the IMO criteria and two which did not. Six breaking (spilling)
regular wave conditions at H/λ = 1/7 and two irregular wave
conditions with numerous breaking waves were used for the
testing. The regular wave conditions are summarized in Table 1
(Table 4 from Johnson and Grochowalski 2002). .
Wave
Steep.
0.142
0.143
0.144
0.144
0.142
0.144
3
Wave
Celerity
m/sec
5.26
6.43
7.59
8.16
8.76
9.93
Ratio of Vessel Speed to Wave Celerity
Wave
Vessel
Speed
Celerity
5.1
8.0
10.2
Knots
Knots
Knots
Knots
10.2
0.50
0.78
1.00
12.5
0.41
0.64
0.81
14.8
0.34
0.54
0.69
15.9
0.32
0.50
0.64
17.0
0.30
0.47
0.60
19.3
0.26
0.41
0.53
No capsizes occurred in wave #1 with a λ/LWL = 0.95, even for
model conditions which were below IMO minimums. The
model conditions not meeting the IMO criteria generally
capsized in 1.42 <= λ/LWL <= 3.4. A significant result occurred
while testing the light displacement (port departure) model IA
(which satisfied the IMO criteria) in #6 waves of 9m full scale
wave height which can occur 1% of the time in 6m significant
wave height. While traveling at 8 knots full scale in quartering
waves (corresponding to 41% of the linear wave celerity), the
model did not capsize in a wave group, but at 10.2 knots full
scale (corresponding the 53% of the linear wave celerity), it
experienced some breaking wave-riding and capsized on the
passage of the second wave. The model condition IIB with an
extended range of stability did not capsize in any of the wave
conditions. A comprehensive table of the capsize conditions and
reasons for capsize is included in Grochowalski 1994. In the
response to the discussion of that paper, Grochowalski pointed
out that when the energy of extreme wave impact contributes
significantly to accelerating the ship, it may stay in the breaking
portion for some time, which is not really surf-riding and
requires further studies. Note that this condition cannot be
analyzed correctly by assuming trochoidal waves acting to
reduce the hydrostatic GZ curves for “perched on a wave”
captured by a large wave it cannot escape by reducing propeller
RPM.”
Figure 2. Fig. 16 from Kan (1990)
Blume (1990) tested in irregular waves two extreme models
designed to experience small and large variations in righting
levers in waves. The loss of righting lever on a wave crest with
λ/LWL = 1 and H/λ = 1/15 was calculated for each model. Blume
concluded that “stability parameters derived from righting levers
on a crest itselves are not suitable for a fair judgment of the
safety against capsizing. The hydrostatically calculated values
do not reflect the physical phenomena. Stability parameters
derived from these curves therefore also can be seen only as
comparative values which are not necessarily better than those
derived from the calm water righting lever curves.” It should be
noted that λ/LWL based on the peak period of the irregular wave
spectrum was about 2 rather than 1 used to calculate the righting
lever on a crest.
By the time of the 1990 STAB conference, many investigations
of the effect of waves on transverse stability and surf-riding
were reported. Kan (1990) extended Rennilson’s guidelines to
avoid surf-riding study from 0.5 < λ/LWL < 3 and determined
theoretical critical wave heights as a function of Froude number
and thus estimated critical speed for a 124T Stern Trawler as in
his Figures 15 and 16. (From Kan 1990)
At the 1994 STAB conference, Kan, Saruta and Taguchi, (1994)
extended Blume’s comparative model tests on capsizing of
container ships in following and quartering seas with somewhat
smaller models. The only correction to the GZ curve for riding
on a crest was again for λ/LWL = 1 and H/λ = 1/15. The regular
waves used for the testing ran from 0.5 < λ/LWL < 2.25 with H/λ
= 1/20, 1/15, 1/12 and 1/10. The longer wave tests λ/LWL > 1.5
were run only at H/λ=1/20 and naturally fewer capsizes
occurred. The irregular wave tests were run at a peak period
which gave λp/LWL = 2. Capsizing vanished in the irregular
waves at FN <= 0.26.
Figure 1. Fig. 15 from Kan (1990) (Note that the labels “critical
wave height” are actually “critical wave steepness”.)
The following quotes from Kan (1994) bear repeating, “It has
been said that the most dangerous wave length is λ/LWL = 1.
However, it is obvious from these tables that many capsizings
were observed in longer waves such as λ/LWL = 1.25 to 1.75”
(Note: beyond which the steepness decreased to 1/20 in these
tests.) “Therefore, we should recognize that the most dangerous
wave length is not restricted to λ/LWL = 1…..”
Umeda (1990) used Kan’s surf-riding results as data points in a
theoretical probabilistic study in irregular following seas. He
suggested “that it is possible to escape from surf-riding by
lowering propeller RPM when the regular wave steepness H/λ is
1/20 or less, but is not possible to escape when H/λ is 1/10. The
criteria are more complex in irregular waves and once a vessel is
4
In their Risk Analysis applied to Capsize of Fishing Vessels,
Dahle and Myrhaug 1995 assumed pure loss of stability only
occurs at λ/LWL = 1 and when Vs equals the wave celerity. This
constraint reduced the probability of capsizing from pure loss of
stability to near zero for the cases considered.
- An area under the RA curve of at least 1.72 m-deg (5.6 ftdeg) from 30 degrees of heel up to the lesser of a heel
angle of 40 degrees or the angle of downflooding.
- A minimum range of positive righting arms of at least 60
degrees.
On the other hand, Umeda, et al in 1999 carried out free running
model experiments in extremely steep regular waves and
concluded that capsizing is caused by broaching, loss of stability
on a wave crest, or bow diving. Also that a ship capsizes in
following and quartering seas more easily than in beam seas.
(Previously shown by Grochowalski, 1989). Based on some
previous suggestions by Takaishi (1982) that capsizing due to
regular excitation is more dangerous than that due to random
excitation, Umeda proposes to carry out capsizing model
experiments in extremely steep regular waves. “If the model
does not capsize, we can presume that the possibility of
capsizing from this ship in any irregular waves is negligibly
small. Because of wave breaking, the measured wave steepness
can be smaller than 1/7 but should be larger than 1/10. The
wave length to ship length ratio, λ/LWL , is set at about 1.5,
because ship motions become significant in this wave
condition.”
These are purely static criteria based on the original stability
criteria developed by Dr. Rahola in his 1939 PhD thesis “The
Judging of the Stability of Ships and The Determination of the
Minimum Amount of Stability Especially Considering the
Vessels Navigating Finnish Waters” (Rahola 1939, Cleary 1993,
Johnson and Womack 2001). While ground breaking in its time,
two fundamental problems remain using this approach.
The first is that the current design of US fishing vessels has
deviated significantly from the characteristics of the one fishing
vessel used in Dr. Rahola’s analysis.
(Womack 2002)
Freeboards have become lower, beams have widen, watertight
openings have been lowered and moved outboard, and capsizing
forces have increased from larger nets and more powerful
engines since the 1930’s. When at sea, a fishing vessel is
operating in a highly dynamic environment; and the smaller the
vessel the higher the dynamic impacts are for a given sea
condition. (Johnson and Grochowalski 2002)
At the STAB 2000 Conference, Rahim and Khondoker (2000)
report on their investigation of the effects of the presence of
waves on the stability of 6 inland passenger vessels by showing
the variation in GZ correction for 16 equally spaced positions
along the hull. “To conform to the “Strathclyde Method”, the
length of the wave is taken equal to that of the vessel.”
However, it is uncertain what wave steepness was used in the
investigation.
Attempts have been made to reflect the realistic dynamics in
criteria such as the IMO or USCG’s Severe Wind and Roll
Criteria. The problem is these criteria are still based on the
static still-water RA curve and are applicable only for beam seas
and winds. From model tests carried out at the SSPA Maritime
Consulting facilities in 1984 to 1985, it was shown that for
fishing vessels following and stern quartering seas may be
significantly more dangerous that beam seas. All loading
conditions, both meeting and failing IMO standards, survived
irregular beam seas, but loading conditions that met existing
IMO criteria capsized in certain following irregular sea
conditions. (Grochowalski, 1989, 1990, 1993, 1998)
Thus, in the stability literature, there appears to be no
universal agreement on what wave conditions constitute the
most dangerous situations.
In addition to the dynamic failings, the “one size fits all vessels
and all seas” nature of the criteria also severely limits their
effectiveness in analyzing a fishing vessel’s stability. The
current criteria;
- Do not differentiate between the amount of the hull and
the deckhouse which is included in the buoyant volume
used in the calculation.
- Do not adequately account for the locations of watertight
openings such as doors or fish hold hatches. These are
currently assumed closed for the calculations but in the
real world are often used at sea and could be
inadvertently opened at the wrong time.
- Do not take into account the internal configuration of the
spaces, particularly any large enclosed spaces on the
main deck or above that if partially flooded could
significantly reduced stability included introducing a
large loll angle.
- Do not take into account the actual sea conditions the
vessel will operate in such as sea state or wave encounter
angle.
Comments on Existing Intact Stability Criteria
The intact stability criteria currently used in the United States
for fishing vessels (Cleary 1993, IMO 1995, CFR 46.28
Subpart E), harkens back to the days of the paper drawings, the
slide rule, the planimeter, and for those high end shops, an
integrator. All of the criteria are based on the static still-water
righting arm curve. A typical intact stability criteria would be;
- A minimum GM value of at least 0.35m (1.15 ft).
- The maximum righting arm occurs at a heel angle of at
least 25 degrees.
- A righting arm of at least 0.20m (0.66 ft) at a heel angle of
not less than 30 degrees.
- An area under the RA curve of at least 3.15 m-deg (10.3
ft-deg) up to a heel angle of 30 degrees.
- An area under the RA curve of at least 5.16 m-deg (16.9
ft-deg) up to the lesser of a heel angle of 40 degrees or
the angle of downflooding.
5
- Are not applicable for vessels under 24 meters (79 feet) in
length.
- Are minimum recommended values only; no guidance is
provided on when these minimums may not be adequate
for certain fisheries.
- The criteria are purely pass/fail and do not provide any
level of risk advice.
- The criteria fail to provide adequate stability guidance for
near shore “day trip” fisheries such as the Mid-Atlantic
Clam Fishery and derby style fisheries such as the Gulf
of Mexico Red Snapper of Bearing Sea Opilio (Snow)
Crab Fisheries.
Figure 4. Shelter Deckhouse
For this preliminary study only the style of the deckhouse is
being varied as this the most prominent difference in a typical
fishing vessel’s watertight envelope. Additional research will
be needed to investigate the effect of other non-dimensional
parameters such as the length to beam ratio.
Clearly at the least the current criteria are in the need of a
significant reassessment (Francescutto, 2002, Cramer and
Telkamp, 2002). The following quote from Daniel Parrott in his
book, “Tall Ships Down”, sums up that author’s opinions why
more has not been done in the United States to rectify this.
“When there is a precedent for a particular activity,
human nature is such that people are inclined to
perpetuate that activity rather than analyze it afresh:
“If it ain’t broke, don’t fix it.” But sometimes things
are broken we just don’t know it yet.”
The model configurations used for the study is based on the
fishing vessel Arctic Rose (See also Appendix D). The Arctic
Rose was a 31 meter (100 foot) catcher/processor trawler
working in the Bearing Sea when she was lost in 2001(Borlase,
2002, Johnson and Borlase 2003). This vessel offers the
following advantages for use in this study.
- The vessel was originally built as a short forecastle vessel and
later converted to a large shelter deck style. The parent
vessel’s actual configurations covers both of the model
configurations used in this study.
- The vessel’s single hard chine hull shape is typical for most
US fishing vessels.
- The vessel’s size is mid-range for most US offshore fishing
vessels.
- The reported on-scene weather and sea conditions before the
time of the casualty were typical for the Bearing Sea with
winds and seas estimated at 20 knots and 2-2.5 meters (6-8
feet). These are not anywhere near extreme conditions for
this area. Unfortunately, the closest NOAA data buoy in the
Bearing was out of commission at the time. so the Marine
Board requested a hindcast from NWSFO, which estimated a
maximum significant wave height at 7.3 meters (24 feet) and
a wave period between 8 and 12 seconds corresponding to a
peak period wave length to vessel length (LW/LPP) of
between 3.6 and 8.
- The vessel at the time of the casualty is believed to have met
all applicable USCG intact stability criteria.
- The vessel’s loss was likely due to a watertight door being
open which compromised the vessels’ stability from
flooding. The location of the WT door is not reflected in the
current USCG intact stability criteria.
- Due to the vessel’s very low main deck freeboard, the
watertight door would become submerged at a heel angle
of around 20 degrees in still water conditions. (The
vessel’s most recent stability letter required a minimum
freeboard at the lowest point of 150mm (6”)).
- The watertight door opened to a large full width main deck
processing space. A small amount of water in this space,
approximately 150mm (6”) deep, would create a loll
angle of 20-25 degrees.
Recent losses such as the Scalloper F/V Northern Edge (2005),
the Trawler/Processor F/V Arctic Rose (2001), and the
Clammers Beth-Dee-Bob and Adriatic (1999) indicate that
operations using the current stability criteria may have some
problems. In all of these cases the vessels were lost in moderate
sea conditions that they had experienced many times in the past.
PROJECT DESCRIPTION
Study Vessel Characteristics
Two hull and deckhouse configurations were used to reflect the
most common fishing vessel configurations.
The two
configurations are:
- The short forecastle is typical of house forward trawlers,
crabbers, scallopers, and ocean clammers that have a
large open after working deck. The configuration covers
a majority of US fishing vessels. See Figure 3 for a
typical example.
- The large shelter deck configuration typical of catcher
/processors and longliners. See Figure 4 for typical
example.
Figure 3. Short Forecastle
6
each of the two model versions by incrementally raising the KG
until the USCG stability criteria outlined in the Introduction
Section were met. This results in a KG value of 3.25 meters
(10.4 feet) for the shelter deckhouse model and 2.80 m (9.0 feet)
for the short forecastle model.
The lines plan for the study’s two models are shown in Figure 3
and the basic parameters are listed in Table 3. The shelter
deckhouse version has a main deck house length approximately
75% of the LPP. For the short forecastle version has a main
deck house length approximately 25% of the LPP.
The use of two different KG values reflects the realistic
differences that occur between the two versions. For this study,
these KG values will give the minimum acceptable still water
righting arm curve and any deviations below current stability
standards from a wave effect will be clearly evident. Further
since this a study in the relative changes in the righting arm
curves and each model configuration is being studied
independently, the use of two different KG values will not effect
this studies conclusions.
Table 2. Study Model General Characteristics
Shelter Deckhouse Short Forecastle
Length Overall:
31.25m 102.5ft 31.25m 102.5ft
Waterline Length:
28.01m 91.90ft 28.01m 91.90ft
Hull Beam:
7.42m 24.35ft
7.42m 24.35ft
Hull Depth:
3.55m 11.65ft
3.55m 11.65ft
Molded Draft:
3.10m 10.17ft
3.10m 10.17ft
KG Abv BL
3.25m 10.40ft
2.80m 9.00ft
Displacement:
360.7mt 354.9lt 360.7mt 354.9lt
Selection of Analysis Software
The two model versions were developed in MaxSurf’s NURBS
based MaxSurf Pro surface modeling software. The hull
consists of four separate surfaces; a bottom, side, main deck,
and transom. Each deckhouse was modeled with three separate
surfaces; a side, 2nd deck, and aft bulkhead. A WT door was
also modeled in the after main deck house bulkhead to visually
represent the existing WT door on the Arctic Rose. For the
short forecastle model the WT door was located the same
distance off center and with the same sill height as on the Arctic
Rose.
Today’s powerful personal computers and integrated naval
architecture software packages now allow for more
sophisticated evaluation of a fishing vessel’s overall stability at
costs feasible for small and large design offices. High end
personal computers can be obtained for $1,000 to $2,000 with
all the memory, processing power, storage devices, and graphics
display power required for today’s software packages. And
integrated naval architecture software packages can be obtained
for around $4,000 for a basic modeling and intact hydrostatics
suite to around $10,000 for advanced modeling capabilities,
damaged hydrostatics, and ship motion predictions.
These integrated naval architecture software packages allow the
naval architect to easily analyze a fishing vessel’s stability with;
- The ability to use sophisticated models that include both
exterior watertight hull and deckhouse surfaces and
internal boundaries for tanks and compartments.
- The ability to locate watertight openings anywhere on the
model.
- The ability to flood hull and deckhouse spaces.
- The ability to account for the true free surface effect of
complex compartments and tank spaces.
- The ability to calculate the static righting arm curve using
user define wave profiles and crest/trough locations.
- The ability to predicate basic vessel motions in response to
user define sea states.
This study used Formation System’s MaxSurf HydroMax Pro
hydrostatics software package (Courser, 2003). This package
was selected for several reasons.
- The software package is an off the shelf package that is priced
at levels that allow it to be affordable for use in small
architecture firms that work on fishing vessels.
- The software easily handles varied wave profiles and
configurations.
- The software’s output is easily copied into a Microsoft Excel
or similar spreadsheet to allow for convenient numerical and
graphical comparisons of the results.
- The software is fairly intuitive to use, which will allow
infrequent users to effectively use methods similar to this
study to evaluate the stability of a fishing vessel..
Figure 5. Study Models Lines Plans
A fixed displacement of 360.7 mt (354.9 LTs) was used for all
calculations, this is the estimated value at the time of the Arctic
Rose’s loss. A fixed center of gravity (KG) location was set for
7
and calculating the resulting righting arm curves with currently
available off the self naval architecture software. The wave
length to height ratios and wave crest/trough locations defined
below outline the scope of the wave profiles used in this study.
For all calculations, a wave encounter angle σ=0/180° (head or
following seas) was used.
Analysis Methodology
For each of the wave length to height and wave crest/trough
location combinations (see next section), MaxSurf’s HydroMax
Pro hydrostatics software was used to calculate the resulting
righting arm curve using the software’s built-in trochoidal wave
profile. In all calculations no free surface effects are included
and the initial trim was set at even keel to remove any potential
skewing of the results. The vessel was free to trim and rise or
sink during all calculations to be in quasi-static equilibrium at
each heel angle.
Wave Length to Height Matrix
The wave matrix (Table 3) was used to define the various wave
height and length combinations. The wave length was varied
from a short wave of one times the vessel’s length between
perpendiculars (1 X LPP) to a long wave of 8 X LPP. The wave
height was varied from a steep wave with a wave height to wave
length ratio of 7.5 (LW/HW=7.5) to a shallow sloped wave with
a LW/HW=60. A maximum wave height of 15 meters (49 ft)
was selected as reasonable extreme for the subject vessel’s 28
meter (98 ft) LPP.
The trochoidal wave assumption used in this study, while not
capable of modeling spilling breakers commonly found in
extreme steep waves, represents the best affordable compromise
for the analytical analysis of small fishing vessel righting arms
in steep waves. Even more advanced and costly CFD methods
are forced to use similar non-breaking wave approximations.
The values for the typical stability criteria applicable to vessels
of this type were also calculated using the software’s built-in
criteria evaluation features. The criteria used were the same
ones listed above that were used to set a KG value for each
model version.
The area under the righting arm curve to a typical watertight
deckhouse door downflooding heel angle was also calculated in
this study. Currently the area under the righting arm curve is
calculated to 40 degrees or the angle of uncontrollable
downflooding. Uncontrollable downflooding is defined by the
USCG as any opening in the watertight envelope that cannot be
made weathertight. While this definition may be adequate for
most vessels, fishing vessels present a unique situation that
based on past vessel casualties indicate the definition may be
inadequate.
For example, on fishing vessels, watertight doors that access
working decks are in constant use in all weather conditions. In
addition, these watertight doors are often located on decks with
relatively low working freeboards and may be located toward
the vessel’s sides. For most other vessel types, low working
decks simply are either non-existent or not used once underway
in heavy seas and thus at sea watertight doors will likely remain
closed. Because of this key difference, the area to a typical
deckhouse watertight door is an important piece of information
in a fishing vessel’s stability analysis.
Table 3. Wave Height to Length Study Matrix
This range of wave steepness ratios was selected to cover the
operating conditions typically seen by small fishing vessels.
These vessels generally operate in near coastal waters that see
the full range of sea conditions from short steep seas created by
shallow waters, land masses, and converging weather systems to
long high waves from offshore storms.
Wave Crest/Trough Location Matrix
All results from HydroMax Pro were copied into Microsoft
Excel spreadsheets. From these spreadsheets the resulting
righting arm curves and stability criteria values were grouped by
desired common parameters such as wave height and
comparative graphs created.
The position of the crest and trough was varied along the length
of the model’s hull as shown in Appendix A and defined as
follows. The following fixed locations were used; Forward
Perpendicular (FP), 1/4 LPP aft of FP, Amidships (AM), 3/4
LPP aft of FP, After Perpendicular (AP) and the wave’s Front
and Back.
Wave Conditions Analyzed
The project involved imposing a series of standard trochoidal
wave profiles on two sets of vessel hull and deckhouse forms
8
dips in the graphs when going from Figures B7 to B10, B8 to
B11, or B9 to B12.
SHORT FORECASTLE MODEL - Results and
Discussions
Short Forecastle Model - Effect of the Wave Crest and
Trough Location - Figures B1 to B3 and B7 to B12
For the short forecastle model the location of the wave crest and
trough along the hull determined the magnitude of the change to
the model’s stillwater righting arm curve. Figures B1 to B3 are
the righting arm curves for 3.73m (12.25 ft), 7.47m (24.49 ft)
and 14.93m (48.98 ft) high waves at the key crest and trough
locations along the hull length. Figures B7 to B9 and B10 to
B12 are the graphs for the area under righting arm curve to the
three key heel angles in this study as a wave passes along the
hull for 7.47 m (24.49 ft) and 14.93m (48.98 ft) wave heights.
Figure 7.
Short Forecastle Model - Effect of Wave Steepness
Figures B4 to B6, B13 to B15, and B16 to B18
-
The steepness of the wave for the short forecastle model also
had a significant determination on the size of the impact on the
short forecastle model’s stillwater righting arm curve. Figures
B4 to B6 are the righting arm curves for a 7.47m (24.49 ft) high
wave for varying wave steepnesses at the key crest and trough
locations along the hull. The 3.73m (12.25 ft) and 14.93m
(48.98 ft) wave heights had similar results.
Figure 6.
Figures 6 shows the typical large reductions from the stillwater
righting arm curve when the wave crest is located at or near
amidships. For the 7.47m (24.49 ft) and 14.93m (48.98 ft) high
waves the righting arms were negative throughout and near zero
or negative for the 3.73m (12.25 ft) high wave.
The location of the wave trough on the other hand had at most
only a moderate impact on the stillwater righting arm curve
when compared to the location of the wave crest. And in all
cases this impact was an increase in the righting arms,
particularly in the higher heel angles. (See Figures B1 to B6)
And interestingly as the wave length to hull length ratio
increases from LW/LPP=1 to LW/LPP = 4 for a given wave
steepness (LW/HW constant) the range of the wave crest
locations that these righting arm curve reductions occur
increases. At LW/LPP=1 the reduction occurs when only when
the wave crest is at amidships. But from Figure 7, by the time
the wave length has increased to LW/LPP=4 the reductions now
occurs when the wave crest is located to between 1/4 and 3/4 of
the LPP. This is also seen by the increase in the width of the
Figure 8.
The primary observation from Figure 8 is that as the wave
steepness decreases, i.e. the wave length is increasing for a
given wave height, the wave righting arm curves quickly
9
The other observation from Figure 9 is that higher waves’ area
under the righting arm curve values converge quicker to the
stillwater values than the smaller waves. For example from
Figure 9, the 7.47m (24.49 ft) wave had the larger reduction in
the stillwater area to 40 degrees of heel than the 3.73m (12.25 ft)
high wave at a LW/HW=7.5. However, by a LW/HW=12 both
wave heights had the same reduction value and for LW/HW
values greater than 12 the 3.73m (12.25 ft) had the larger
reductions.
converge to the stillwater righting arm curve. The steep wave,
LW/HW ratio= 7.5, (Figure B4) had the most effect on the
model’s righting arm curve with the impacts then rapidly
diminishing as the wave steepness decreased as shown in Figure
B5 and then Figure B6.
This effect can also be shown in Figures B13 to B15 and B16 to
B18. These graphs show the lowest and highest areas under the
wave righting arm curves to the study’s three key heel angles as
compared to the stillwater value. The lowest and highest values
were obtained from area under the righting arm curve graphs as
shown in Figures B7 to B12. Figures B13 to B15 use the wave
length to wave height ratio (LW/HW) as the comparator.
Figures B16 to B18 use the wave length to hull length ratio
(LW/LPP) as the comparator.
Short Forecastle Model - Effect on Area Under the Righting
Arm Curve - Figures B7 to B9 and B10 to B12
Another observation is that as a wave passes the area under the
righting arm curve to the forecastle’s watertight door follows the
same trend as the area under the righting arm curve to 30 and 40
degrees. For example when the area under the righting arm
curve to 30 and 40 degrees dips (Figures B7, B8 & B10, B11)
the area under the righting arm curve to the watertight door also
dips (Figures B9 & B12) in roughly the same proportion. This
result is logical as the areas under the righting arm curve are all
derived from the same righting arm curves.
For all three wave heights the steepest waves have the most
impact with the curves quickly converging to the stillwater
levels. And interestingly the larger waves, 7.47m (24.49 ft) and
14.93m (48.98 ft) high, caused the largest reduction in the areas
under the righting arm curve to the three key heel angles. From
Figure 9 at a LW/HW = 7.5, both the 7.47m (24.49 ft) and
14.93m (48.98 ft) high waves had a worst case reduction of
about 110% to 120% while the 3.73m (12.25 ft) high wave only
had a worst case reduction of about 90%.
Short Forecastle Model – Summary of Results
The following summary of the impact of following or head
waves on the short forecastle style fishing vessel are;
One, the principal reduction in the stillwater righting arm curve
and in the area under the righting arm curve occurs when the
wave crest is at or around the amidships position. (Figures B1
to B3)
Second , the location of the trough had minimal effects on the
stillwater righting arm curve and the area under the righting arm
curve. As has been demonstrated in many investigations the
area values are slightly above the stillwater values. (Figures B1
to B3)
Third, in the steeper wave profiles (LW/HW=7.5) the areas
under the righting arm curve dip well below the minimums
required by the current stability criteria and is actually negative
for the larger wave heights (i.e. the vessel will capsize on its
own given enough time while riding in the crest area). (Figures
B7 to B9 and B10 to B12)
Fourth, as the wave steepness decreases, the wave righting arm
curves and the areas under the righting arm curve quickly
converge to the stillwater values. (Figures B4 to B6, B7 to B9,
and B10 to B12)
Figure 9.
A similar result also exists when comparing wave length the to
hull length (LW/LPP). In this case the wave length equal to hull
length condition (LW/LPP=1) also does not cause the worst case
reduction in the stillwater areas under the righting arm curve.
Both wave lengths two and four times the hull length (LW/LPP
= 2 & 4) showed higher potential reductions of about 110% to
120% while the 3.73m (12.25 ft) high wave only had a worst
case reduction of about 90%. (See Figures B16 to B18)
Five, sharp reductions in stability can occur at waves several
times the length of the vessel in addition the currently used
standard of wave length equal to ship length (LW=LPP). These
reductions at the longer wave lengths and higher steepnesses are
larger than when the wave length is equal to ship length.
(Figures B16 to B18)
10
Six, the larger waves’ areas under the righting arm curve values
converged quicker to the stillwater values than the smaller
waves. (Figures B13 to B15 and B16 to B18)
Seven, the area under the righting arm curve to the forecastle’s
watertight door follows the same trend as the area under the
righting arm curve to 30 and 40 degrees as a wave passes.
When the area under the righting arm curve to 30 and 40
degrees dips the area under the righting arm curve to the
watertight door also dips in roughly the same proportion.
(Figures B7 to B9 and B10 to B12)
Based on the past studies and the above observations, the key
areas of concern when evaluating the impact of a wave on a
short forecastle fishing vessel’s stability is steep waves
(LW/HW=7.5) ranging in heights up to the maximum expected
for the area the vessel will operate in.
SHELTER DECKHOUSE MODEL - Results and
Discussions
Figure 10.
Shelter Deckhouse Model - Effect of the Wave Crest and
Trough Location - Figures C1 to C3 and C7 to C12
These results are similar to the findings by Pauling (1961) in his
study using the series 60 cargo ship hull form. Because of the
series 60’s wall sidedness and relatively high freeboards, this
approximates the general configuration of a shelter deckhouse
style fishing vessel.
As opposed to the short forecastle model, the location of the
wave crest had minimal impact on the shelter deckhouse
model’s righting arm stillwater curves. In the shelter deckhouse
configuration neither the location of the wave crest and trough
had any significant impact on the righting arm curve as shown
in Figure 10. Figures C1 to C3 are the righting arm curves for
3.73m (12.25 ft), 7.47m (24.49 ft) and 14.93m (48.98 ft) high
waves at the key crest and trough locations along the hull.
Figures C7 to C9 and C10 to C12 are the graphs for the area
under righting arm curve to the three key heel angles in this
study as a wave passes along the hull for 7.47 m (24.49 ft) and
14.93m (48.98 ft) wave heights.
Shelter Deckhouse Model - Effect of Wave Steepness
Figures C4 to C6, C16 to C18, and C19 to C21
-
As with the short forecastle model, the steepness of the wave for
the shelter deckhouse model also had significant determination
on the size of the impact on the shelter deckhouse model’s
stillwater righting arm curve as shown in Figure 11. Figures C4
to C6 are the righting arm curves for a 7.47m (24.49 ft) high
wave for varying wave steepnesses at the key crest and trough
locations along the hull. The 3.73m (12.25 ft) and 14.93m
(48.98 ft) wave heights had similar results.
However, unlike the short forecastle model, the location of the
wave trough had no consistent impact on the shelter deckhouse
model’s righting arm curve. The righting arm curve was
slightly reduced for the 14.93m (48.98 ft) high wave and slightly
increased for the 3.73m (12.25 ft) and 7.47m (24.49 ft) high
waves.
The primary observation from Figures C4 to C6, the steep
waves, LW/HW ratios = 7.5, (Figure C4) had the most impact
on the model’s righting arm curve. As the wave length
increases, i.e. the waves become less steep, (increasing LW/HW
ratio) the impacts rapidly diminish as shown by Figures C5 and
then C6. The results for the 3.73m (12.25 ft) and 14.93m (48.98
ft) high waves were similar.
In addition from Figures C1 to C3 the height of the wave
appears to only a small impact on the magnitude of the changes
in the shelter deckhouse model’s righting arm curve as the wave
passes by the model. The 7.47m (24.49 ft) high wave showed
the most variation in maximum righting arm value, ranging from
0.455m (1.46 ft) to 0.587m (1.88 ft), a 29% variation. The
14.93m (48.98 ft) high wave followed with maximum righting
arm value ranging from 0.475m (1.52 ft) to 0.578m (1.85 ft), a
22% variation. The 3.73m (12.25 ft) high wave had the least
variation with a maximum righting arm value ranging from
0.470m (1.51 ft) to 0.550m (1.76 ft), a 17% variation.
This effect can also be shown in Figures C16 to C18 and C19 to
C21. These graphs show the lowest and highest areas under the
wave righting arm curves to the study’s three key heel angles as
compared to the stillwater value. The lowest and highest values
were obtained from graphs as shown in Figures C7 to C15.
Figures C16 to C18 use the wave length to wave height ratio
(LW/HW) as the comparator. Figures C19 to C21 use the wave
length to hull length ratio (LW/LPP) as the comparator.
11
following or head waves would become evident when evaluated
by the current USCG criteria.
This though could give a false indication that the vessels
stability was adequate in all aspects. Though the impacts on the
stillwater righting curve and the subsequent area under the
righting arm curve to 30 and 40 degrees were minimal, there
was a significant change in the area to the heel angle at which
the shelter deck watertight door submerged. This is shown in
Figure 13 for the 3.73m (12.25 ft) waves, particularly in the
steeper waves as previously discussed. The 7.47m (24.49 ft)
and 14.93m (48.98 ft) high waves showed similar results.
(Figures C17 and C18)
This is primarily due to the fact that the watertight door is
submerged at lower heel angles in the smaller waves because
there are minimal changes in the righting arm curves as
previously discussed. In fact for the 3.73m (12.25 ft) high wave
the watertight door is submerged, i.e. the submergence heel
angle is negative, with the vessel upright with the wave crest at
the location of the watertight door as shown in Figure C22.
Figure 11.
Shelter Deckhouse Model - Effect on Area Under the
Righting Arm Curve - Figures C7 to C22
As shown in Figures C7/C8, C10/C11 and C14/C14 (Example
of is Figure 12), the areas to the fixed 30 and 40 degree heel
angles has little variation from the stillwater values. And in all
cases the calculated areas never dipped below the minimum
values required by the current USCG stability criteria. This
result is logical as there is little variation in the righting arm
curves as a wave passes by the model as discussed above.
Figure 13. Appendix C Figure C16
For this shelter deckhouse model the worst reductions the area
under the righting arm curve to the watertight door submergence
occur when the wave crest is at approximately 3/4 of the hull’s
length from the forward perpendicular, which is coincidently or
not the same location as the WT door. This result will be more
apparent in the following discussions.
From Figures C9, C12, and C15, when the wave crest was
located along the hull at the watertight door there is a much
larger dip in the area under the righting arm curve to the
watertight door (“Area to WT Door”) than to either the 30 or 40
degree heel angle (Figures C7/C8, C10/C11, and C13/C14).
This dip is most evident in the steep waves, with it quickly
dissipating as the wave length increases to near nothing as
discussed in the previous section.
Figure 12. Appendix C Figure C8
This is also shown in Figures C16 to C18 and C19 to C21
(Example of is Figure 13) which only show a small, 25% or less,
reduction in the “Area to 30 Deg” and “Area to 40 Deg” curves.
Therefore for this hull and deckhouse configuration, it could be
concluded that little impact on the models stability levels from
12
Third, in the steeper wave profiles (LW/HW=7.5) when the
wave crest is located at the shelter deckhouse’s aft watertight
door, there is a larger reduction in the stillwater area under the
righting arm curve to the angle of the watertight door
submergence. (Figures C16 to C18 and C19 to C21)
This observation means that for this model, which is based on
the F/V Arctic Rose, a smaller 3.73m (12.25 ft) high wave can
present a higher risk for potential flooding through the shelter
deckhouse watertight door than the 7.47m (24.49 ft) or 14.93m
(48.98 ft) high wave. From Figure 14 the maximum reduction
in the area to the watertight door for the 14.93m (48.98 ft) high
wave is 79% and 90% for the 7.47m (24.49 ft) high wave while
the corresponding maximum reduction is 102% for the smaller
3.73m (12.25 ft) high wave.
Four, these sharp reductions in the stillwater area under the
righting arm curve to the angle of the watertight door
submergence can occur at waves several times the length of the
vessel in addition the currently used standard of wave length
equal to ship length (LW=LPP). These reductions at the longer
wave lengths though are smaller than when the wave length is
equal to ship length. (Figures C16 to C18 and C19 to C21)
The last observation from Figure 14 is that higher waves area
under the righting arm curve values converge quicker to the
stillwater values than the smaller waves. For example from
Figure 9, the 7.47m (24.49 ft) wave had the larger reduction in
the stillwater area to 40 degrees of heel than the 3.73m (12.25 ft)
high wave at a LW/HW=7.5. However, by a LW/HW=12 both
wave heights had the same reduction value and for LW/HW
values greater than 12 the 3.73m (12.25 ft) had the larger
reductions.
Five, the larger waves’ areas under the righting arm curve
values converged quicker to the stillwater values than the
smaller waves. (Figures C16 to C18 and C19 to C21)
Based on past studies and the above observations, the key area
of concern when evaluating the impact of a wave on a shelter
deckhouse style fishing vessel’s stability are any watertight
openings that if inadvertently opened at the wrong time would
lead to the loss of sufficient stability from flooding. The
principal concern is while operating in steep waves
(LW/HW=7.5) ranging in heights up to the maximum expected
for the area the vessel will operate in.
GENERAL CONCLUSIONS
This study achieved two major results. The first is that existing
off the shelf hydrostatics software with good graphical displays
can be used to investigate in a systematic approach the general
effects of following/head waves on the overall stability of
fishing vessels. The second is that the same software can then
be used to provide useful stability information from the effect of
waves on typical fishing vessels righting arm curves for
providing stability guidance to the fishing vessel crews and
designers in a cost effective manner. From the discussions
above the following conclusions are put forth.
As in previous investigations discussed in the Background
section for short forecastle style deckhouses, when the wave
crest is located around amidships there is a significant reduction
in the stillwater righting arm curve. This reduction can occur at
wave lengths significantly longer the length of the vessel, and
the longer the wave length, the broader the range centered
around amidships of the wave crest locations when the
reductions occur. The magnitude of the reduction was also
highly dependent on the steepness of the wave. The steep waves
(LW/HW=7.5) had the most effect which then quickly
decreased as the wave steepness dropped and eventually
converged on the stillwater righting arm curve values.
Figure 14.
Shelter Deckhouse Model – Summary of Results
The following summary of the impact of following or head
waves on the shelter deckhouse style fishing vessel are;
One, the location of the wave crest or trough had minimal
effects on the stillwater righting arm curve and the area under
the righting arm curve to 30 or 40 degrees of heel. (Figures C1
to C3)
Two, as the wave steepness decreases, the wave righting arm
curves and the areas under the righting arm curve to 30 or 40
degrees of heel quickly converge to the stillwater values.
(Figures C7 to C9, C10 to C12, and C13 to C15)
For the shelter deckhouse arrangement though, the location of
the wave crest had little significant impact on the righting arm
curves. However the location of the wave crest did have a
significant impact on the area under the righting arm curve to
the heel angle for submergence of a typical watertight door in
13
space it doesn’t come back out due to watertight door sills and
can lead to capsizing. (i.e. it takes only a brief moment to get a
shelter deckhouse vessel into trouble from which it can not
recover) One possible solution would be requiring a minimum
area under the righting arm curve to all watertight openings.
This would be similar in concept to requiring minimum sill
heights for a watertight door.
the aft end of the shelter deckhouse. When the wave crest is
located near the watertight door there was a large reduction in
the area under the righting arm curve to the watertight door
submergence when compared to the stillwater values. As with
the short forecastle style, the magnitude of the reduction was
highly dependent on the steepness of the wave. The steep waves
(LW/HW=7.5) had the largest effect which then quickly
decreased as the wave steepness dropped and eventually
converged on the stillwater righting arm curve values.
The other area of concern for future investigation into
performance based stability criteria is loss of stability in short
forecastle style deckhouse fishing vessels when a steep wave’s
crest is located near the vessel’s amidships. As this and many
other investigations have shown, there is not much that can be
done about the loss of stability when the wave crest is near
amidships. This is a phenomenon that is mostly a function of
freeboard which for many fisheries is dictated by the fishing
gear used. This is not to say that additional investigations
should not be done. This phenomenon is likely a major
contributing factor in many of the recent losses in the US
Northeast and needs to be addressed.
For both the short forecastle style deckhouse and the shelter
deckhouse arrangement the location of the wave trough has
minimal impact on the righting arm curves when compared to
the stillwater righting arm curve. Because of this the effect of
the wave trough can probably be ignored in any performance
based criteria.
“Freeboard is the key to stability” has been said since before
Rahola’s ground breaking thesis. This is clearly supported by
the results from this study as the negative impacts from waves
that occur in the low freeboard short forecastle arrangement are
not present in the shelter deckhouse arrangement. The shelter
deckhouse creates a high effective freeboard as far as stability is
concerned even though the after main deck has a low freeboard.
In essence, if you were to add freeboard the short forecastle
model it basically will become equivalent stability wise to the
shelter deckhouse arrangement.
Some of this can be addressed with operator training. If the
wave crest passes by the fishing vessel quickly, it will not
remain in the danger zone long. It takes time for the reduction
in stability on a wave crest to take effect thus the old seaman’s
adage in following seas your speed should be less than 1/2 of the
wave speed to prevent surfing and capsizing. And this adage
has been confirmed by the many model tests as discussed in the
background section.
This higher effective freeboard in a shelter deckhouse style
fishing boat though can create an “Achilles” heel in its stability
that is not addressed in the current USCG and IMO stability
criteria. As noted previously, the area under the righting arm
curve to the submergence of the watertight door on the aft end
of the shelter deckhouse was significantly reduced by a passing
wave crest. And for the steep small wave the reduction actually
created a negative area under the righting arm curve to the
watertight door, that is the watertight door was submerged by
the passing wave with the model still in the upright position.
However there probably still should be a maximum reduction
allowed from stillwater righting arm curves set by the current
USCG and IMO stability criteria. This would force a reasonable
minimum freeboard to allow a fishing vessel a chance to survive
being inadvertently “perched” on a wave.
Based on these conclusions and using the software currently
available, significant improvements in creating stability criteria
that reflect an individual vessel’s actual characteristics and
operating conditions can be done now with simple additions to
the existing USCG and IMO stillwater based stability criteria.
These additions such as requiring a minimum area to watertight
openings or maximum allowable reductions in the righting arm
curve from waves are doable now and would address the causes
of many fishing vessel losses.
This can occur because the buoyant volume from the shelter
deckhouse will compensate for the loss of buoyant volume from
a low freeboard main deck and allow the vessel to still meet
current USCG or IMO stability criteria. The watertight doors
needed to access the very low aft working main decks typical of
shelter deck fishing vessels, if placed outboard will then be
submerged at relatively low heel angles. And with the interior
spaces of many shelter deckhouses being large and the full
width of the vessel, a small amount of water accidentally
flooding these spaces will result in large free surface effects.
This effect was likely the reason for the loss of the F/V Arctic
Rose in moderate sea conditions. See Appendix D for
additional discussion on this and water on deck considerations
for all types of watertight openings (watertight doors, fish hold
hatches, vents).
ACKNOWLEDGEMENTS
The authors wish to thank the SNAME T&R Steering
Committee for sponsoring this research and Formation Systems
for the kind loan of their MaxSurf Software used in this paper.
The authors also wish to thank those who reviewed this paper;
their comments were very instrumental in assisting the authors
in the development of this paper.
Because of this, this phenomena should be one of the areas of
concern for future investigation into performance based stability
criteria. One relatively small wave can quickly submerge a low
“freeboard” opening. And generally once the water enters the
14
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16
APPENDIX A - Definition of Wave Crest and Trough Locations
Wave Crest @ Fwd Perp
Wave Crest @ 1/4 LPP
Wave Crest @ Amidships
Wave Crest @ 3/4 LPP
Wave Crest @ Aft Perp
Wave Front
Wave Trough @ Fwd Perp
Wave Trough @ 1/4 LPP
Wave Trough @ Amidships
Wave Trough @ 3/4 LPP
Wave Trough @ Aft Perp
Wave Back
17
APPENDIX B - Short Forecastle Model Figures
Figure B4
Figure B1
Figure B5
Figure B2
Figure B6
Figure B3
18
Figure B7
Figure B10
Figure B8
Figure B11
Figure B12
Figure B9
19
Figure B16
Figure B13
Figure B17
Figure B14
Figure B18
Figure B15
20
APPENDIX C - Shelter Deckhouse Figures
Figure C1
Figure C4
Figure C2
Figure C5
Figures C3
Figure C6
21
Figure C7
Figure C10
Figure C8
Figure C11
Figure C9
Figure C12
22
Figure C13
Figure C16
Figure C14
Figure C17
Figure C15
Figure C18
23
Figure C22
Figure C19
Figure C20
Figure C21
24
to all values of GZ. This factor attempted to account for the loss
of waterplane area as the vessel travels up the face of the wave.
APPENDIX D
During the “Ride Wave Crest” time step, the water was assumed
to have cleared off the deck through the freeing ports and aft
stern ramp and a righting arm factor of between 0.7 and 0.9 was
applied to estimate the loss of waterplane area at the bow and
stern of the vessel at the crest of the wave. The wave was
assumed to be up to 2 feet above the normal still waterline.
Water on Deck considerations.
A number of recent investigations concern dynamic simulations
of how to handle water on deck for stability calculations. The
standard model of the water on deck can be calculated as an
asymmetrical heeling arm curve superimposed on the righting
arm curve. This is the method the current Code of Federal
Regulations, CFR 28.560 specifies and is discussed briefly in
Francescutto et al (2001). The corrections to the effect of wave
conditions on the righting arm curve are covered in this
investigation, but no new criteria are suggested.
During the “Wave lowers Stern”, the vessel would shed the
water on deck, including that which flowed out of the
processing space because of the attitude of the vessel on the
wave and a pitch up correction could be applied to the internal
gravity head flooding calculations.
This crude method of estimating the righting arm correction can
now be replaced by the methods outlined in this paper.
The various rules incorrectly call the area under the righting and
heeling arm curves righting and heeling energy.(Figure 28.565
in CFR 28.560.) They are actually righting and heeling energy
per unit displacement since the area under the righting moment
curve is the actual righting energy.
Vessel responses in long quartering or following seas which
break but do not induce surf-riding can still involve several
seconds in the breaking portion of the wave. If the vessel
already has water on deck and possibly downflooding started,
serious consequences can result.
The Arctic Rose Time to Flood analysis as a motivation for
this investigation.
APPENDIX E
The righting arms for the ARCTIC ROSE were calculated from
the cross-curves of stability tables using GHS at the estimated
at-loss displacement and KG. This curve was used as the
baseline for the righting arm calculations at each time step. A
second righting arm curve was then calculated from the crosscurves of stability tables based on the displacement and trim of
the vessel and the weight of the flooding water using look up
tables for all corrections including the trim vs heel correction to
the assumed roll axis. The current investigation could have
correctly computed the quasi-static GZ curves directly, had it
been available.
Current IMO Review of the Intact Stability Code
SLF 47/6/4 June 2004
Quoting from SLF 47/6/4 “This document suggests a structure
for these dynamic criteria and within this proposed structure one
set of criteria will account for a minimum stability limit required
to ensure that these minimum stability standards will provide the
ships with sufficient safety.”
However in section 28 “The righting levers shall be computed in
a wave equal to the wetted length of the ship, wave height
according to the 90% quantile (see above) with the vessel
trimming freely. The crest shall be located at the half of the
wetted length fro the crest condition and at AP for the trough
condition.”
“For all waveheights above the limiting wave height according
to the failure criterion, we assume that the ship will capsize or
be exposed to a large rolling angle that leads to a loss. For all
waveheights below this limiting wave height, we assume that
the ship is safe and the probability is set to zero.”
As described in Johnson and Borlase, 2003, the Arctic Rose’s
motion in following waves was broken into four distinct
motions based on equal time steps equal to one fourth of the
encounter period, Te.
During the “Vessel in Trough” time step, the vessel was
assumed to be traveling in the trough of the wave, and
accumulate water on the starboard side aft deck well if the heel
angle was less than the angle of bulwark submergence. No
righting arm correction was applied because no loss or gain of
waterplane area was assumed for such long waves. The
estimated 10 second wave period from hindcasts gave a λ/LWL =
5.4.
There are two fundamental reasons to reject this proposal as a
step backwards to oversimplified pass/fail criteria.
1. The use of λ/LWL = 1 as a worst case criteria ignores
the many model tests and the results of this
investigation which demonstrate that there are many
other wave conditions which can cause a high a
probability of capsize. .
2. The use of a 90% quantile wave height ignores that
dangerous groups of high waves can occur 1% of the
time in severe seas.
During the “Wave Raises Stern” time step, the wave at the aft
deck was assumed to be two feet above the normal waterline
and a pitch down correction could be applied to the internal
gravity head flooding calculations for sensitivity purposes.
Additionally, in the absence of the results of the current
investigation, a righting arm correction factor of 0.9 was applied
25
for smaller vessels of various shapes not coinciding with the
ship type covered in this proposal, the author’s feel that from the
results of this investigation, especially Figures B1 to B3,
illustrate that the λ/LWL = 1 is not the worst case for alterations
between the wave crest and trough.
To illustrate this point, Section 17 of SLF47/6/4 has been
reworked to show that the table uses critical waveheights and
steepnesses that are much too small for the assumed mean
periods.
The major focus of the proposal is to prevent serious parametric
rolling and for large ships, this may be a useful tool. However
IMO SLF47/6/4
Modified Table
on page 7
Wave
Period
sec
2.5
3.5
4.5
5.5
6.5
7.5
8.5
9.5
10.5
11.5
12.5
13.5
14.5
15.5
16.5
17.5
18.5
19.5
20.5
Wave
Length
m
9.76
19.13
31.62
47.23
65.97
87.83
112.81
140.91
172.14
206.49
243.96
284.56
328.28
375.12
425.08
478.17
534.37
593.71
656.16
H_1/10
H_1/10
Wave
LW/H
Height
m
0.49
19.86
0.73
26.21
1.44
21.96
1.98
23.85
2.72
24.25
3.70
23.74
4.36
25.87
5.43
25.95
6.53
26.36
7.43
27.79
8.44
28.91
9.37
30.37
10.30
31.87
10.95
34.26
12.06
35.25
13.10
36.50
14.30
37.37
15.28
38.86
16.35
40.13
Implied
Implied
Deep
Deep
H_1/3
H_1/100
H_1/100
Water
Water
Wave
Wave
LW/H
Wave
Group
Height
Height
Celerity Velocity
m
m
Knots
Knots
0.386
0.644
15.16
7.59
3.79
0.573
0.957
20.00
10.62
5.31
1.131
1.887
16.76
13.66
6.83
1.555
2.594
18.20
16.69
8.35
2.137
3.564
18.51
19.73
9.86
2.907
4.848
18.12
22.76
11.38
3.425
5.713
19.75
25.80
12.90
4.266
7.115
19.80
28.83
14.42
5.130
8.556
20.12
31.87
15.93
5.837
9.735
21.21
34.90
17.45
6.630
11.059
22.06
37.94
18.97
7.361
12.277
23.18
40.97
20.49
8.091
13.496
24.32
44.01
22.00
8.602
14.348
26.14
47.04
23.52
9.474
15.802
26.90
50.08
25.04
10.291
17.165
27.86
53.11
26.56
11.233
18.737
28.52
56.15
28.07
12.003
20.021
29.65
59.18
29.59
12.844
21.423
30.63
62.22
31.11
26