In the tank

One of the problems any yacht designer faces when he embarks on a new design is how to assess a prospective vessel’s performance characteristics before it’s actually built. If the designer doesn’t have an extensive database of past designs to emulate, it’s often important to the success of the project to do a thorough research and development program.

There are many tools available to the designer to draw upon, such as velocity prediction programs for sailboats (VPP), speed/power programs for powerboats, computational fluid dynamic programs (CFD) for both power and sailing vessels, 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, if these research and development efforts are taken to extremes, they will often cost almost as much as or more than the boat. This level of effort only warrants the expense when the stakes are high, as seen with the America’s Cup, Whitbread Round the World projects, and Trans-Atlantic Powerboat record attempts.

On a somewhat smaller scale, Van Gorkom Yacht Design (VGYD) faced this challenge with the design of its new Mount Gay 30 racing boat. We came up with a workable, cost-effective solution that yielded a wealth of knowledge and insight into the performance characteristics of the design. This same research can be performed for voyaging boats as well as racers.

The high-tech, low-budget approach taken with the Mount Gay 30 served not only to validate the design and significantly reduce the overall cost of the boat, but it also gave the client the confidence to fund the tooling and building of the first boat since there was a reasonable guarantee of good on-the-water performance. Our research and development program can be broken into three phases: 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 true wind speeds of between six to 16 knots. 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 to develop the lines for the tank test model, all while optimizing to the Mount Gay 30 Rule.

This analysis included comparing physical parameters (e.g., prismatic coefficient, ratios of displacement/length, sail area/displacement, sail area/wetted surface, beam/draft, and length/beam, among others). The IMS rating certificates of a few comparable 30-footers were scrutinized. This illustrated the trends in performance based on the physical ratios and the 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 (2,300 kgs). The most crucial design choice was the beam measurement. For off-the-wind sailing, a narrower form is best for reducing wetted surface and wave drag, but for upwind sailing 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, we decided to go with the maximum beam allowed by the rule (3.35 m). The hull form was given a fine bow entry for a strong windward performance and a gentle rocker and after sections for better off-the-wind sailing abilities.

The lines were then created using Nautilus/Prosurf for Windows, an advanced lines-fairing program featuring full Non-Uniform Rational B-spline (NURB) definition for curves and surfaces, facilitating the creation of a truly fair hull shape. This computer model was then given to Lite Systems for the milling of the model hull. Computer milling produced a very close-tolerance surface at a very affordable price.

The model was built to scale, approximately seven meters in length, and 1,000 kgs displacement. Building to such a large scale had several major benefits. First, the dynamometer, which is the apparatus that connects the model to the towing carriage and measures loads resulting from heave, pitch, and yaw, had an optimum model weight requirement of 1,000 kgs. This meant the best possible results could be achieved without using counterbalance weights. These weights risked inducing extraneous errors into the test data. Second, the greater the scale of the model, the cleaner the resistance data produced.


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 Council’s Institute for Marine Dynamics (IMD) in St. John’s, Newfoundland. Fluid Thinking Proprietary Ltd. and the Australia One team used this facility in preparation for their 1995 America’s Cup challenge. IMD is regarded as one of the preeminent 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 IMD personnel specifically for America’s 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 on board the towing carriage. This information was then used to create a matrix of data that 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 our Velocity Prediction Program for the Mount Gay 30. It also allowed us 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 base design that had been tank-tested. 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 design 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 the winning Team New Zealand in the last America’s Cup. YRI performed the regression analysis on the final set of tank data to create a Mount Gay 30 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 our VG-Mount Gay 30.

The total cost for the research and development 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 his investment: the building of the model, tank-testing, regression analysis of the tank-test data, VPP software, and miscellaneous costs such as travel and communications. These factors will have a direct bearing on the overall economics of the project by allowing the client to bypass the building of a prototype hull. The knowledge gained from this analysis has allowed VGYD to fine-tune its design and has given the company a high degree of confidence in predicting the performance of their VG-Mount Gay 30 before it is even built.

Geoffrey Van Gorkom is a naval architect in Newport, R.I. He can be reached at P.O. Box 982, Newport, R.I. 02840; phone/fax: (401) 849-6090.

By Ocean Navigator