Floating a Design Page 2
|Floating a Design|
Part 2: The second type of test is for seakeeping.
By Chris Martin — February 2003
Once a designer or architect has the test results, the task is to properly interpret them. "One test is valuable, but being able to compare it to other tests enhances the value," says Gerry Stensgaard, manager of the Ocean Engineering Centre (OEC) at B.C. Research, a test tank in Vancouver, Canada. "Designers who do model testing a lot build up their own database to compare the results of one given test to many previous ones." Other resources for comparison include computer program predictions and databases of similar boats that have been built.
While computer programs are often used before and sometimes instead of model tests, Jim Leishman of Pacific Asian Enterprises, builder of Nordhavn boats, explains an advantage all respondents cite for tow tanks: "While laying on the rail cart that propels our model down the tank--just a couple feet from the test model--we can see how the spray is being deflected, and we can hear the tiny concussions that in real scale could create a significant amount of noise."
The last method of comparison is via databases of similar boats, and the OEC offers designers and builders a peer-review chart that shows how a hull stands up to similar ones. Monk says he took advantage of this and recently completely redesigned a hull "because the resistance figures from the first model test didn't meet our expectations, even though the boat ran beautifully to the eye." Sarin understands the reaction: "We are constantly learning and improving on past designs. We like to think that each new hull is an advancement over its predecessor."
There are three to four tests that those interviewed for this article commonly perform. The first is a resistance test, during which a model is moved through the water by a "towing carriage," and the required force--its drag--is measured. When Monk has this test performed, he says, he requests smooth-water resistance runs at cruise- and full-displacement speeds and in both bow- and stern-down conditions, for example. "This gives the naval architect the ability to correct performance and propeller predictions for possible future situations when the boat varies from the original weight and trim estimates, a common occurrence," he explains.
From the drag forces measured, engineering calculations are used to estimate the horsepower that will be required to move the real boat through the water, known as the Effective Horse Power. These numbers won't provide you with the size of engines directly; the designer must first account for propellers and drive-train inefficiencies. During this test, designers can also observe the hull-to-water interaction to keep an eye out for side wetting and excessive spray coming aboard and to ensure wake waves are regular and smooth.
The second type of test is for seakeeping, to see how a boat will run in various sea conditions. A series of waves that are statistically identical to what happens in the open ocean are scaled to the model size, their height and period specified by the sea state, which is similar to the Beaufort scale. What's measured depends on what information the naval architect is looking for. At the simplest level just visual observations of the model are made with photos and videotape. For more detailed information, vertical accelerations are measured at the bow, the center of gravity, and the stern. Either way, Stensgaard says the naval architect can see "whether the motion is smooth or an abrupt, violent motion" and whether spray rails are working properly.
This article originally appeared in the January 2003 issue of Power & Motoryacht magazine.