Tank Tested
All the simulations money can buy can’t beat an in-the-water, 1:13 model test at the highest-speed tank in the world.
Boating feels far away. It’s a chilly February morning in Hoboken, New Jersey, and it’s still a few months before we’re back on the water in the Northeast, a few months before the wind is whistling in our ears as our boats pound through familiar Atlantic chop. We’re not listening to the wind and the waves now, just the constant hum in the Davidson Laboratory at the Stevens Institute of Technology, the sound of mechanical procedures at work. These are the procedures that underpin many of the aspects of boating we know and love; this is the behind-the-scenes design process that is often forgotten once the boat hits the water.
I wouldn’t normally expect to meet a yacht designer and his client on a college campus, but the Davidson Lab offers a service that is a rare find in the digital era. Plastic and wooden hull models line the walls, stacked on top of each other like coffee mugs crowded in a cabinet, representing decades of industry development. In the center of the lab is a tank that spans the entire length of the building, a model yacht running on its surface. That hull bottom is an exact replica of the custom, 74-foot motoryacht that Bill Prince is designing—at 1:13 scale—and it’s here to demonstrate its seakeeping performance in one of the most storied tank testing facilities in the country.
“Most hulls are not tested,” says Prince, who was last in Hoboken to test a 131-foot aluminum motoryacht that is currently under construction in South Korea. “The owner is going to invest a substantial amount of money on a custom yacht, and he wants to see with his own eyes the physical evidence of how the hull runs.” Today, they are ensuring that a pair of 1,300-hp V8 MAN engines will provide the client with the 18- to 19-knot cruising speed he desires, and they are also testing two different chine designs.
Prince and his client have been at the lab since 9 o’clock this morning. They need to test the light displacement hull at different speed intervals, starting at 12 knots and gradually increasing to 25 knots. Then, every speed interval must be repeated at heavy displacement with a slightly more aft center of gravity. Besides the two students and the research associate helping conduct the tests, we are the only ones here, and we chat casually while congregating around the tank.
“Three… two… one. Go!”
This is a hurry up and wait kind of game, and all conversations stop mid-sentence as the boat begins another trial run, everyone’s focus glued to the model, which is pulled through the water by an arm overhead.
Tank testing is a relatively uncommon step in the yacht design process today, but the methodology dates back to the 1880s, when most tests were done at a fairly large scale. The larger the scale, the higher the cost, so tank testing was normally only commissioned by the navies and nation states of the world. That is, until Ken Davidson, after whom the Davidson Laboratory is named, decided to extend testing to yachts. Using a 20- or 30-foot model to test a 40-foot yacht didn’t make much sense, however, so he needed to figure out how to scale the models down.
“To successfully go up from model scale to full scale you have to scale the two primary types of fluid flow separately,” says Michael DeLorme, senior research associate at the Davidson Laboratory. “Wave making resistance and viscous resistance don’t scale the same.” Davidson also had to figure out a way to simulate turbulence. His solution: gluing sand to the front of the hulls, which at the time were built from wood. Using these innovations, he successfully tested a scale model of Gimcrack, the flagship of the New York Yacht Club, in the Stevens swimming pool and proved that the concept could work. The trial earned him enough funding to build formal testing tanks at the university.
During World War II and into the early 50s, the university’s tanks were operating 24/7. The U.S. Navy’s Bureau of Aeronautics approached Davidson in 1942 about developing seaplanes to support the war effort, recognizing that they did not have the expertise to test small scale models at the high speed needed. Davidson was testing them at the Stevens Institute nine months later.
Today, the Davidson Laboratory is home to the highest-speed tank in the world. The 300-foot tank has the capacity to propel hulls at 100 feet per second, and even though they never actually operate at that speed, the laboratory has conducted tests as fast as 60 feet per second, which DeLorme describes as “frightening.” Because of the tank’s speed capacity, it has tested many America’s Cup hulls over the years.
The lab doesn’t test many sailboat hulls anymore, but they use the same methodologies for testing drag force on motoryachts. The drag balance is tried and true, in use for more than 20 years, and the lab also recently acquired a motion capture system that helps operators analyze rotations and displacements. “These methods are good and very repeatable,” DeLorme says. “It’s more of a mechanical system than an electrical or quasi-electrical system, but it works well.” GoPro cameras—one of the lab’s more modern technologies—are mounted to the apparatus to collect video during the trials.
Today’s test, I’m told, is a fairly simple trial through calm water. Prince is mostly interested in the photos and videos collected from underneath the model. “Primarily what we’re looking for is laminar flow,” he says, “meaning the water that’s nearest the hull is in constant contact with the hull and not breaking loose. For example, a propeller pocket can cause increased resistance and drag. So we’re looking at things like what’s happening around the propeller tunnel.” Prince has designed hundreds of prop tunnels throughout his career, so he already knows that the hull won’t experience any problems on this front, but the benefit of physical testing is that he can collect data to help with future builds. The resistance curve for how much power is needed to achieve different speed intervals can be applied to similarly shaped boats of different sizes.
Prince has spent almost four months developing renderings for this custom project. First, he develops displacement estimates and determines the hull’s longitudinal center of gravity, which should be 58 percent aft of the bow on this pilothouse yacht. Once his estimates are developed, he shapes the hull so the center of buoyancy matches the center of gravity, and then he designs it in 3-D. The model that’s now running in the tank is made from engineered plastic by a
company called Accurate Pattern in Milwaukee, and it costs about $100 an inch to build.
Prince’s client comes from the commercial construction industry, so he has an affinity for seeing physical evidence with his own eyes, and because he had an exact idea of what he wants out of his custom boat, he requested the hull be tank tested before putting the design out to bid at different shipbuilding yards. Today, sophisticated software and a library of empirical hull forms make it easy for designers to calculate speeds given a particular hull form. Tank testing has been regulated to one-off custom builds where speed is critical, or utilized by large production builders with plans to build upwards of 80 hulls over the course of a 10-year production run. However, for some clients, digital technology just doesn’t compare to physical results.
“It’s not that he didn’t trust our advanced software or our experience,” says Prince. “He just decided that given the relatively small cost of doing it—it’s about $20,000 to build the hull model and do the testing—is a percentage of the multi-million-dollar build cost of the boat, he wanted to go ahead and do it.”
It’s that same practical thinking that has defined other choices in his client’s life. As the hull model prepares to take one of its final light-displacement runs before a break to transition to heavy displacement, he pulls out his phone to record the test—an iPhone 5. Yes, he is commissioning a multi-million-dollar yacht while still using a device that some might consider antiquated. It’s a better size than the newer phones, he justifies, plus it still works perfectly well. Much like tank testing, the iPhone 5 has largely been replaced by more-modern technology, but it still holds value, especially to those who value basic function over the extravagent bells and whistles added to each new model. “And it’s sturdy,” he says as he proceeds to throw it across the room. I hold my breath, but the phone emerges unscathed. Maybe he’s onto something.
Three months after the model was put through its paces in the test tank, Prince and his client are pushing ahead with the project, even amidst an unexpected global pandemic. “The tank test validated what we had calculated,” Prince reports. “It was a success because we didn’t change anything.”
Prince’s client is a knowledgeable boat owner who used to do a lot of extended cruising on his 57-foot pilothouse motoryacht. Over the years, he developed a clear idea of exactly what he wanted in his next yacht, and he brought the concept to Prince to design the final product. “What’s very satisfying to me is to meet someone who comes to us and says ‘Look, I’ve been boating for my entire life, I’ve been thinking about my custom boat for a long time, and here’s four years’ worth of notes that I’ve taken that I want to share with you so you can bring my dream to life,’” Prince says. “It’s a lot of fun to have that kind of detailed road map from a client who’s passionate about their life dream of owning a real custom yacht.”
One of his most important requests was a very dry ride and a hull that’s able to deflect as much spray and as many head-sea conditions as possible. The hull’s performance in the tank helped them decide on the proper chine design to deliver on this request. Prince had already built a number of spray-mitigating features into the boat, including the regular chine, the increased freeboard and the Portuguese bridge, but the client wanted to test an alternate chine detail as additional support for the spray deflectors. So, Prince designed a wider chine that fared into the bow and the existing chine amidships to increase the original chine’s width by 40 percent.
Besides making the final decision on the chines, they refined some of the interior details, and then the design was ready to go out to bid at two North American yards. Once the client receives pricing, he will choose a builder and move forward with the build process.
Tank testing is not a necessity in the digital age, and it is far from the norm, but the mechanical procedures are practical and repeatable enough to stand the test of time, and they cater to the mindset of practical clients who want to protect their investments as much as possible. Plus, Prince says, “he enjoyed it and had a good time watching the process.” That’s one thing digitalization can never replace: the opportunity to watch your new hull design physically kiss the water for the first time, long before its ready for you to step aboard.
