(This document will be updated frequently with new question and answer postings)
December 12, 2016
1. Q: How do we combine PDFs without Adobe Pro?
A: There are many solutions found online. A simple one is pdfmerge.com. It does not
require any software download. Simply upload the PDFs in the order in which you want
them merged, then download the merged PDF they have created for you.
December 5, 2016
1. Q: Several of our teams are having difficulty when calculating metacentric radii. When
calculating the moments of inertia using Inventor, I am fairly certain we have everything
set up properly. In some cases, the BMT seems inappropriately large. When calculating
the BML, some students are initially coming up with a negative number for IL when using
the parallel axis theorem.
A: Iyy in Inventor is the moment of inertia from the reference point, while the correct
moment of inertia is Iy (about the center of flotation). This is explained in the Inventor
stability video available on the YouTube channel. Do not use the parallel axis theorem;
it is not necessary since you are using CAD software.
November 28, 2016
1. Q: Would the 4 finalist groups have to worry about possible heavy winds or other forms
of heavy weather during the competition day? Or would the date of the competition
change due to heavy weather conditions?
A: You should definitely take into account environmental factors when designing your
craft. Unless the conditions are dangerous to the participants/boats, race day is
scheduled for May 6th. According to the Guidelines, the rain date is scheduled for the
following day, May 7th.
If the conditions are too windy to fly the blimp on the 6th, then this part of the scenario
will be dropped in favor of keeping race day on the 6th.
2. Q: Would the four finalists be able to alter their boat’s design after being announced as
being a finalist, or would the design be locked in and unchangeable after submission?
A: Once the boat designs are submitted and finalists announced, there will be no
changes allowed to the designs.
1
3. Q: Are we allowed to create a cylindrical tube like object around the propeller to create
a more streamline power?
A: No. See page 20 of the competition guidelines:
“[The motor] Shall be able to be quickly attached/detached via the screw clamps without
interference or require disassembly of other components or require disassembly of the
motor itself (i.e. the motor is one piece).
Since the addition of some sort of apparatus around the motor would require attachment
to the motor itself, this would considered “required disassembly of other components” to
attach/detach the motor.
4. Q: Is there a minimal distance between each pontoon?
A: While there is no required distance between any hull portions of a multi-hull design,
you should take into account impacts to the ships functions (such as maneuverability and
stability) when making your design decisions.
November 21, 2016
1. Q: For the length between perpendiculars, could you please explain the point at the stern
of the boat that the measurement reaches? I am confused if the LBP should go from the
front waterline to the rear waterline or to the rudder stock, and I cannot seem to find a
clear answer online. Between what two points is the length between perpendiculars
measured?
A: The boat design competition has modified the traditional definitions that you
probably found online. Per the guidelines (see pp. 48, 66-68), the LBP should be the
total length of the [watertight] hull form.
2. Q: Can we use our own border and title block drawing files?
A: Yes, there is no requirement to use the ones available for download.
November 14, 2016
1. Q: If there is a relatively small difference in freeboard from bow to stern and minimum
freeboard requirements are met, am I correct to assume that the boat is more or less
complete? If there is a drastic difference, do the kids need to go back and rearrange
loads, etc. and redo everything?
A: If the fully outfitted boat meets the minimum freeboard requirement at both the bow
and the stern, then yes, the design of the boat can be considered complete. The difference
in trim between the bow and the stern is the student’s discretion as naval architects, but
typically the trim at the bow is slightly greater than the trim at the stern.
2
The amount of rework depends on the correction that is necessary.
 Any change to the definition of the hull form requires another iteration (or two) to
calculate the new design waterline and resulting hydrostatics.
 Moving a component requires an update to the calculations the move impacts; i.e.
moving the battery six inches aft requires an update to the battery’s local LCG on the
weights spreadsheet which then leads to all the LCG based calculations for the boat,
ultimately leading to different trim values at the bow and at the stern. See below:
Local LCG impacts LMOM which impacts LMOMTotal which impacts LCG which impacts
trim which impacts f which impacts TF & a which impacts TA.
2. Q: Does the sandbag(s) have to be factored into the weight spreadsheet or can it/they be
added as an afterthought to trim the boat?
A: Please see pages 18 and 30 of the guidelines for full details, but the short answer is
yes, the sandbag(s) must be factored into all the calculations.
November 7, 2016
1. Q: How exactly do we attach the beach ball payloads to the attachment points for the
loading diagram?
A: A single beach ball will be inflated with air and not use the attachment points. This
single beach ball must be included in the loading diagram as a spare, non-deployed
barrage balloon.
2. Q: Should the beach balls be floating above the attachment points? Should we try to lay
the beach balls in the attachment points?
A: The attachment points are for securing a single barrage balloon which is available
for download as the “blimp” drawing file. Unfortunately, the competition team has not
yet purchased this item and don’t know the exact attachment of the string to the blimp.
The team is considering ~10-13 ft long blimps, which should look something like the
image below (with 2 attachment points fwd and 2 aft). For now, space the attachment
points equally and insure the string does not interfere with any moving parts. Further
clarification will be provided once the purchase has been finalized.
3
3. Q: Do we use the large or small beach balls? Or perhaps a combination of the two?
A: Use the large diameter beach ball drawing file. The small beach ball drawing file in
the downloadable payload folder is from a previous competition year and should not be
used.
To clarify, the boat will carry one aloft barrage balloon (which uses all four of the
attachment points) and carry one stowed barrage balloon, represented by the large
diameter (8 inches) beach ball.
October 31, 2016
1. Q: If we are to cover our hull(s) completely except for a 1 in x 1 in square cut out of the
top, are we not following the guidelines?
A: Yes, this would be in violation of the Guidelines. Specifically, this would violate the
whole inaccessible area clause, not the watertight clause. On race day, our outfitting
team wouldn’t have access to the bottom of the boat. If you put payload there, we
couldn’t load it. If your battery was there, we couldn’t install/connect it. If your boat
started to take on water, we couldn’t bail it out.
2. Q: Do we get to decide the depth of the holes in the payload?
A: No, the depth of the holes are specified on Figure 2 of the Guidelines as thru
holes. Either way, the diameter of the hole is what determines how the golf balls will stay
in place (assuming the depth of the hole is sufficient for the golf balls to not bottom out).
3. Q: Do we need a working ramp to “deploy” the payload?
A: No, the payload will not undergo deployment. While we would love to simulate the
scenario as true as possible, the scope of the construction does not allow for the
precarious nature of swamping a bow door.
4
October 24, 2016
1. Q: It is my understanding that the waterplane and station area spreadsheets are
completed to approximate the volume of water displaced and then adjust the waterline
accordingly. The delta should be as close to the weight of the boat (but greater than) as
possible. Is my understanding correct?
A: Correct, this is the application of Archimede’s Principle where the weight of the
water displaced (object’s displaced volume * density of water) must be greater than or
equal to the weight of the object.
For determining the design waterline, the weight of the water displaced should be equal
to the weight of the boat. If the weight of the water displaced is greater than the weight of
the boat then the design water line is too high and should be lowered. Conversely, if the
weight of the water displaced is less than the weight of the boat then the design water line
is too low and should be raised.
2. Q: Should the waterline first be adjusted using waterplanes then fine-tuned with the
stations? I guess I am looking for a more clear understanding of the purpose of doing
both the waterplanes and station planes.
A: You’re on the right track, however there is no fine tuning by calculating the station
areas. Both the waterplane and station areas must be modified after adjusting the
waterline to accurately capture the volume. We require to calculate both for the
following reasons:
1. Calculating using two different areas provides a check for the accuracy of each
calculation. If the two displacements are not close to equal, there is likely a
mistake that needs corrected.
2. The calculation for VCB depends on displacement based on waterplanes and
vertical lever arms
3. The calculation for LCB depends on displacement based on stations and
longitudinal lever arms
After moving the design waterline, adjust the spacing of the waterlines between baseline
and the new design waterline then adjust the locations of the stations. Populate the new
offsets and calculate the displaced volume (by waterplane and station area) at the new
design waterline. Note: The results of moving the design waterline depend heavily on
the shape of the hull form. The picture below shows a simplified process.
5
October 3, 2016
1. Q: Any advice for estimating the design waterline?
A: Calculating the design waterline is an iterative process that begins with an estimate.
If the initial design waterline calculations result in the displacement being greater than
or less than the weight of the fully outfitted boat, it simply means the initial guess was not
the final design waterline. Adjust the design waterline and offset values then perform the
calculations again; each iteration should bring you closer to the final design waterline.
6
The competition team recommends starting with a design waterline either (1/2) or (2/3)
the total depth of the hull. Remember, the higher the accuracy of the design, the higher
the quality and higher the score!
2. Q: Am I correct to assume that the CG has to be calculated for every block [of wood]
and every golf ball on board?
A: Yes, but the method to capture each element is up to the student team. The decision
should be based on level of accuracy vs. level of effort where the highest accuracy would
be to individually calculate each golf ball and block as a separate line in the weight &
CG calculation sheet; this approach requires a significant level of effort to initially
populate and to update as the design changes. The competition team suggests grouping /
assembling a loaded vehicle [block of wood + golf balls] and calculating the weight and
composite CG. Use the loaded vehicle assembly in the weight & CGs sheet as needed for
the design. There are easier approaches that sacrifice accuracy which can be provided if
necessary. Be sure to document the approach and methodology!
3. Q: Any specifics on the block of wood used on the motor mount?
A: The block of wood is a standard 2x4 that is slightly larger than the spacing between
the screw clamps. The weight and CG are negligible and can be omitted from the
calculations.
4. Q: Both the rudder stock and motor shaft are able to be adjusted to specific depths, right?
A: Yes, both the rudder stock and trolling motor heights are adjustable. The rudder and
propeller will be placed in an open water channel by the outfitting team based on the
Loading Diagram. If the rudder stock height or propeller height is crucial, clarify the
positions on the Loading Diagram with dimensions for installed heights.
5. Q: Rudder area calculations are resulting in areas that far exceed the provided plywood
sheet. Is this due to an error in the estimated design waterline?
A: The error with the rudder area calculation is not a bad design waterline length. The
Det Norske Veritas calculation may be completed with units in feet or in inches (you’ll
find the results are the same by a factor/product of 144 in2/ft2); however, the A’ equation
must be done in both units. Then a reasonable rudder size can be chosen. Smaller boats
may need to do A’ with units in inches whereas larger boats complete A’ with units in
feet.
Also noteworthy is that the Det Norske Veritas is only valid for the rudder shape in the
guidelines (whose area is A’). The dimensions for H, B and b can change (see below),
but any exotic shapes will not be accurately predicted by the equation.
7
September 1, 2016
1. Q: In the weight spreadsheet for the TASNAMEA example boat, the total weight of the
boat is displayed as 366.43 lbs; however, when I sum the weight column I get 333.35 lbs.
A: When summing the weight column using Excel, you get 333.35 lbs because Excel
does not recognize the four cinder blocks (8.27 lbs each) since their digits include
asterisks (to reference the disclaimer). So that additional weight is added to get 366.43
lbs.
8