One
of the problems any yacht design company faces when it embarks upon a
new design is being able to assess the vessels performance
characteristics. If the design office dose not have an extensive
database of past proven designs to emulate, it is very important to the
success of the project that a thorough research and development program
be initiated.
There are many tools available to the designer to draw upon such as
velocity prediction programs for sail boats (VPP), speed/power programs
for power boats, computational fluid dynamic programs (CFD) for both
power and sail, and the tank testing of scale models. These tools, used
alone or in combination, can yield extremely valuable information and
greatly contribute to the success of a design.
Of course these R&D efforts can be taken to the extremes where
they
will often cost more than, or almost as much, as the boat is actually
worth. This level of effort only warrants the expense when the stakes
are high, as seen with the Americas Cup, Whitbread Round the World
projects, and Trans-Atlantic Power Boat record attempts. On a somewhat
smaller scale, Van Gorkom Yacht Design faced this challenge with the
design of their Mount Gay 30 and came up with a workable, cost
effective solution which has yielded a wealth of knowledge and insight
into the performance characteristics of their design.
This high-tech, low-budget approach served to not only validate this
design and significantly reduce the overall cost of the boat, but give
the client the confidence to fund the tooling and building of the first
boat having been given a reasonable guarantee of the on-the-water
performance. VGYD's research and development program comprised of three
phases: the model development by way of a complete parametric analysis,
a one-model tank test series, and definitive post-analysis and
optimization to formulate the final set of lines for the production
female tooling.
Model
Development One
of the underlying prerequisites for this design was to create a strong
windward performer in wind speeds of between six to sixteen knots of
true wind. This was deemed necessary to make the boat attractive to
both East Coast and Great Lakes owners who typically race
windward/leeward courses in light to moderate breezes. This is not to
say that off-the-wind performance was compromised, as key features have
been incorporated into the design to produce a strong all-round
performer for both offshore and around-the-buoys events.
With an intuitive sense for what makes a yacht perform well, plus a
complete parametric analysis based on a range of 30 footers with
similar performance criteria to the Mount Gay 30, a set of parameters
was defined in order to develop the lines for the tank test model, all
while optimizing to the Mount Gay 30 Rule. This analysis included
comparing physical parameters such as prismatic, water plane, block,
and midship area coefficients, along with the displacement/length, sail
area/displacement, sail area/wetted surface, beam/draft, and
length/beam ratios. IMS rating certificates were also scrutinized for a
series of the markets top 30 footers. This analysis readily illustrated
the trends in performance based on the above physical ratios and
velocity prediction outputs.
The choice of certain parameters was obvious, such as waterline length
and displacement. Waterline length was maximized to reduce drag, and
displacement was minimized to the Rule limits (2300 kgs). The most
crucial choice was the beam. For off-the-wind sailing the narrower form
is best for reducing wetted surface and wave drag, but for upwind, the
hull must be wide enough to produce adequate stability to resist
heeling force from the sails.
Considering
that the boat can take on water ballast (and the further outboard you
can get the weight the more righting moment, and consequently the more
horsepower you will have) we decided to go with the maximum beam
allowed by the Rule (3.35m). The hull form was given a fine bow entry
for a strong windward performance and a gentle rocker and after
sections conducive to off-the-wind sailing.
The lines were then created using Nautilus/Prosurf for Windows, an
advanced lines fairing program featuring full NURB (Non-Uniform
Rational B-spline) definition for curves and surfaces, facilitating the
creation of a truly fair hull shape. This computer model was then given
to Lite System for the milling of the model hull. Computer milling
produced a very close-toleranced surface at a very affordable price.
The model was built to 2/3 scale, approximately 7.0 meters in length,
and 1000kgs displacement. Building to such a large scale had several
major benefits. Firstly, the dynamometer, which is the apparatus that
connects the model to the towing carriage and measures loads resulting
from heave, pitch, and yaw, required an optimum model weight of
1000kgs. This meant the best possible results could be achieved without
using counter balances and risk inducing extraneous errors into the
data collection. Also, the greater the scale of the model the cleaner
the resistance data produced.
Tank
Testing Since
only one model was being tested with no keel and rudder variations, it
was imperative that the resistance curves generated be as realistic and
as accurate as possible so they could be used as an effective tool for
the calibration of the VPP and ultimately in defining the final hull
form.
The tank test experiments on the VG-Mount Gay 30 model were conducted
at the National Research Councils Institute for Marine Dynamics in St.
John's, Newfoundland. Fluid Thinking Pty Ltd and the Australia One team
used this facility in preparation for their 1995 Americas Cup
challenge. IMD is regarded as one of the pre-eminent marine research
facilities in the world. It boasts a 220 meter towing tank, a
sophisticated towing carriage, and a hydraulically driven wavemaker
capable of generating a wave height of up to one meter. The dynamometer
was designed and built by Institute personnel specifically for Americas
Cup Research.
Approximately 150 runs were made in the tank at various speeds, heel
angles, yaw angles and rudder angles. A record of each run, describing
the magnitude of the hydrodynamic forces that the canoe body and foils
generated was recorded by the computer onboard the towing carriage.
This information was then used to create a matrix of data which modeled
the hydrodynamic characteristics of the hull form. The preliminary VPP
modeling that was conducted during this testing proved to be extremely
encouraging.
Post-Analysis
& Optimization
The data collected from these experiments has allowed VGYD to calibrate
their VPP, and to simulate, in the computer, the same performance
characteristics that the tank test model exhibited at IMD. A series of
canoe body variations was then created and compared against the tested
base boat. The goal was to create a computer model that made it
possible to quantify those variations in terms of performance by racing
the hull variations against the base boat in the velocity prediction
program. This allowed us to fine tune the hull form resulting in what
we considered to be the best performing hull form.
The
velocity prediction program used in the post-analysis and the
performance optimization was Winn Design VPP developed by Clay Oliver
of Yacht Research International, Inc. YRI's software was used
exclusively by Team New Zealand in the last two Americas Cup. YRI
performed the regression analysis on the final set of tank data to
create a M30 module, proprietary to VGYD, used in their VPP.
Six systematic series were created, each with three to six boats. Each
series explored different parameters such as: the beam/draft ratio,
prismatic coefficient, slight changes in the after body, flare
variations, and combinations of each. A fleet, consisting of the best
performing hull forms from each series, was then raced in the VPP over
a one mile windward/leeward course to come up with the fastest boat.
Making this final analysis, allowed VGYD to enhance and optimize the
performance of their VG-Mount Gay 30.
Cost
Analysis
The total cost for the R&D of the VG-Mount Gay 30 was
approximately
one third of the total cost to put a boat on the water. That's not bad
considering what the client got for their investment: the building of
the model, the tank testing, regression analysis of the tank test data,
VPP software, and miscellaneous costs such as travel and
communications. These factors had a direct bearing on the overall
economics of the project by allowing the client to bypass the building
of a prototype hull.
To summarize, it is possible to mount a thorough research and
development effort for a fraction of the expense that is usually
associated with this degree of effort. This is accomplished by
utilizing some of the design tools and building processes that are now
commercially available and are quite cost effective such as VPP and
lines fairing software and computer milling. Development time is
significantly reduced which also enhances the economics of the program.
The knowledge gained from this analysis has allowed VGYD to fine tune
their design and given them a high degree of confidence in predicting
the performance of their VG-Mount Gay 30 before it was even built. In
fact, the correlation between VPP produced target boat speeds and
actual on-the-water performance was very close. The design office is
now working on several extrapolations of their M30 and feel extremely
confident in their ability to gauge the performance of the boats based
on their VPP development.
Since
only one model was being tested with no keel and rudder variations, it
was imperative that the resistance curves generated be as realistic and
as accurate as possible so they could be used as an effective tool for
the calibration of the VPP and ultimately in defining the final hull
form. 
The
velocity prediction program used in the post-analysis and the
performance optimization was Winn Design VPP developed by Clay Oliver
of Yacht Research International, Inc. YRI's software was used
exclusively by Team New Zealand in the last two Americas Cup. YRI
performed the regression analysis on the final set of tank data to
create a M30 module, proprietary to VGYD, used in their VPP.