SAILBOAT DESIGN
Markku Hentinen 2016
NEEDS OF A SAILOR
• What type of use?
• Cruising: archipelago, inshore/lakes, offshore, oceans;
daysailing, weekend overnighting, holiday voyages,
ocean sailing
• Racing: dinghies, centerboard boats (may capsize,
recovered by crew actions), keelboats (round the buoys
sailing), offshore racing yachts, multihulls
• Often combinations of the above
Racing boat concepts
• Performance nr 1. priority
• Functionality during the race (rig, deck arrangement, sea
bunks...)
• One type classes, handicap rules
• Manufacturing technology: uniform quality, high strengthweight ratio
• Price, second hand value
Cruising yacht concepts
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Interior space prioritized, also safety and comfort on deck
Quality, finishing, equipment and outfit
Appearance important; impressive/classic/sporty looks
Functionality when sailing short-handed (alone or
small/inexperienced crew)
• Price, financing, image
Challenges of the boatyard
Target group and product range
• See previous points
• consistent/versatile product range that suits for the
market situation – but does not compete with own
models
Production arrangement
• Series production benefits (moulds, jigs, purchasing) vs.
tailoring
• subcontracting: moulds, FRP-parts, carpentry,
installations?
Sales arrangement
• Directly from the boatyard or via dealers?
• Boat shows, test sailing
• In racing boats, the activity of the racing class
Choice of the designer
• References and reputation of the designer have high
importance
• Earlier only few well-known designers, now more and
more: S&S, Frers, Holland, Peterson, C&C were ”the names” at
1970’s; Dubois, Finot, Norlin, Pettersson, Södergren at 1980’s;
nowadays at least Farr, Castro, Reichel/Pugh, Judel/Vrolijk,
Jeppessen, Berret/Farroux/Finot, Joubert, Irens, Jakopin,
Kouadajoumoun
• In Finland at least Groop, Still, Mannerberg, Stråhlman...
• Design groups are nowadays formed around large
projects (Americas Cup, Volvo Ocean race, megayachts)
having several specialised designers involved
• Responsibility of structures, installations, details and
production friendliness still remains at the boatyard
Designer’s tasks
• Ref. Needs of the sailor
• Balancing and know-how of different parameters
(f. ex. sail area/stability; upwind/downwind
performance; comfort/seakeeping aspects
• Industrial design more and more important:
functionality, dimensions, looks, ease of
manufacturing
• No possibility to build prototypes of larger boats,
the first sample must be soldable
Sailing ability and characteristics
• The basic challenge: the ability to advance to all
directions using the wind only
• Ability to sail windward includes: hydrostatics
(stability), hydrodynamics (lift and drag of the
foils and hull), aerodynamics (lift and drag of the
sails)
• Light wind/hard wind
• Wind triangle: true vs. apparent wind
• Force balance: side forces, driving
forces/resistance, heeling/righting moment,
yawing moments
Performance prediction
• Rough performance by parameter ratios: SA/AW, SA/ 2/3,
L/ 1/3, HM/RM
• Velocity Prediction Programs: based on calculating the
force balance
• Polar diagrams for showing the results
V
= 12 m/s
T
V
= 7 kn
S
Safety in extreme situations
• Keelboats: range of
positive stability
• Centerboard dinghies:
floatability and recovery
• Multihulls: floatability
and emergency exit
Research and testing
• In universities:
Delft (prof. Gerritsma), MIT, Southampton, Davidson
lab., Chalmers, TKK: Tank testing (ref. Delft systematic
series), Wind tunnel testing, CFD
• In projects:
Americas Cup, Volvo Ocean race, Megayachts etc.
• Published in:
HISWA-symposiums, RINA-publications (HPYD),
SNAME, AIAA
Volume and shape of the hull
• Total weight and center of gravity are determining, but…
• Interior requirements are often decisive
• Length-breadth ratio: resistance, stability (also for
multihulls)
• Length-displacement ratio: high speed resistance,
surfing ability
• Wetted surface, viscous resistance; how smooth shall
the surface be?
Form coefficients
• Wave-making (effective waterline length), wake,
pressure drag
• Optimum values for prismatic coefficient can be found as
a function of Fn, but does not take surfing ability into
account
Motions in waves
• Affecting parameters: Total weight, LCG vs. LCF, radius
of inertia, block&prismatic coefficients
• F.ex. Effect of radius of inertia (k) to VMG (Sunshine):
Handicap rules, scantlings affecting to
concept design
• Possible effects of handicap rule;
– Old IOR (section shape), nowadays IMS-rule (vertical
stem etc.)
– Also affect to cruising yachts (marketing)
• Scantlings
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ISO 12215
earlier NBS, ABS/ORC
Material supplier’s guidelines
Loadings, material values and analysis method in
balance
– Max. Speed (racing boats) may be difficult to
determine (rudders)?
Design of foils
• Maximising lift-drag –ratio:
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profile thickness: better L/D with small thickness, but less total force
effect of the aspect ratio AR:
Flow separation
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On thick profiles separation starts at the trailing edge. Lift is not
falling, but drag increases significantly
On thin profiles lift is falling as the separation starts, and drag
increases
Keels/centerboards
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In ballast keels: volume required. In bulb keel the two tasks (creating
side force & lower the CG) are separated, in canting keels especially
Asymmetrical profiles? Used in multihulls, Open 60, VOR, or trim
tabs in classic boats
Lifting/folding centerboard beneficial in downwind
Aspect ratio compromised with depth limits and/or grounding forces
Standard concept 20 years ago
Standard concept in new yachts
Rudder
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Angle of attack typically larger than in keels
In upwind conditions lift-drag ratio dominating
Maximum lift force significant in (hard) downwind conditions (preventing
broaching)
thicker profile, max thickness more forward
Profile shapes
• Laminar profiles beneficial at small angles of attack
• In practical conditions, laminar or turbulent flow?
Taper ratio and sweep angle
Other aspects in foil design
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Lateral balance, center of effort of the lateral
force
Scantlings for keel attachment: ISO 12215-9
(+old ABS/ORC, NBS)
Rudder stock scantlings: ISO 12215-8 (+old
NBS)
Roller
bearing
Bending moment
Torsion
where
where
and kSEA = 1.0…1.4
depending on design category
Rig and sails
• In downwind (and light wind) creating enough total force
is dominating
fuller sails
• In upwind the lift-drag ratio is dominating:
– Aspect ratio, flat sails
– Sail interference
• Lateral balance, center of side force
• handling: nr. of sails and masts, sail forces, furling sails,
wing sails
• adjustability: reefing, flattening; partial rig (3/4, 7/8), top
rig, mast flexibility, twist
• Parasite drag (windage): shrouds, spreaders; freestanding rigs
Sail
interaction
Light wind vs. hard wind; sail depth
and twist
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Light upwind, low boatspeed:
more force by increased sail depth,
more twist to adapt the vertical wind
gradient
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Light side/downwind: max force
by increased sail depth
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Medium upwind, medium
boatspeed: flatter sails for better
L/D, less twist because boat speed
decreases the vertical apparent wind
gradient;
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Hard upwind: stability limits the side
force
flat sails, increased twist to
decrease heeling force
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High-speed boats: Flat sails for
best L/D, small twist because boat
speed decreases the vertical apparent
wind gradient
Rig scantlings
• Mast compression force is affected by:
– Chain plate distance
– Angles of diagonals and stays
• Mast buckling is affected by:
– Panel lengths (also at deck level)
– Longitudinal and transversal moment of inertia
• Shroud scantlings
– Fatigue may be critical
• See ISO-12215
Auxiliary engine
• (Small) outboard or inboard engine
• Direct shaft or S-drive on inboard engines
• Propeller: Fixed blades, or folding or feathering
blades
• Electric propulsion: regenerating electric power
to batteries under sail
Deck arrangement
• Safety: roughening, clear surfaces; lifelines, pushpits,
pulpits, hand holds, hiking straps
• Sail control lines lead to cockpit: functionality, safety
• ergonomics: sitting positions at different heel angles,
winch placement
• Small friction in running rigging: as few turning blocks as
possible, and small angles in them
• Large enough winches
• Lockers for lines, anchor, fenders etc., place for liferaft
• Deck scantlings: feeling of robustness: stiffness
important; ISO 12215
Sailboat interiors
• Significance increases during last years
• Functional only in harbour or also at sea?
Interior volume
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dimensions: standing/sitting height, berths, doors and hatches,
toilet...
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Stowage amount and functionality in harbour/at sea
Ventilation (air conditioning), water, electric power
Space requirements for tanks, pumps, hoses, etc.
Firmness and form stability of interior components:
doors, attachments, hard points; subject to hull loads
and displacements?
Handicap ratings and their development
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Meaning: to make it possible for different boat types to race against
each other
Construction rating rules (measurement formula based classes): the
boat is designed to reach certain calculated rating, no time
handicaps in the races
Handicap rules: a handicapped time is calculated after finishing,
based on the handicap rating and time sailed.
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Gross tons
Seawanhaka
Froude
Univ. Am. Rule
RORC
IOR
international Rule
scandicap
LYS
IMS, ORCi
IRC
1773
L,B
1890
L,S
1896
L,B, girth lengths
1930
(L,S)/displacement
1931
(L,S)/Dspl+L+S
1971
5,6,8,10,12 m
in Nordic countries n. 1970-2000
Nordic countries, empirical, cf..PHRF in USA
1990
vpp-program
2000,
hidden formulas
”Box rules” – almost one design
• VOR 60
• IAAC, AC70
• Mini Transat, Open 50, Open 60
ORCi hull measurement
Thank you,
The exam is 15.04.2016 at 13-16 in F175b