Floating a Design Page 3
|Floating a Design|
Part 3: The last two of types of tests can be called detail tests.
By Chris Martin — February 2003
Recording accelerations gives the naval architect actual numbers that describe the motion, which he can use not only to compare this design objectively to his previous ones, but also to international standards for ergonomics. Or he can use them to optimize the structural design, which is especially important for weight control in higher performance craft.
The last two of types of tests can be called detail tests, although not everyone we interviewed deems them necessary. One detail test that Sarin uses involves determining the ideal spot to position underwater components, such as bow thrusters, rudders, and propeller tunnels. Wool tufts are attached to the hull near the component of interest, and the boat is towed through the water while underwater observations are made of both the direction and motion of the tufts. Sarin says, "The objective is to eliminate any `surprises' that could be detrimental to the overall performance; i.e. irregular flow patterns resulting in additional resistance and/or `dirty' water flow into the props, etc."
The last test is the wake wheel test. For this, a series of special, different-size propellers, called wake wheels, are installed in place of the standard props. The wheels' rotational speed is measured as the model is towed, which gives an estimate of how fast the water is at the location of the propeller. Because it is usually slower than the speed of the boat, this is a factor in correctly pitching the propeller.
So what does all this cost, and what is it relative to fixing the problems it can prevent? According to Stensgaard, model testing can range from $20,000 to $30,000, depending on the extent of the trials, with a large portion of that being construction of the model. We also asked Ed Hageman for the usual fixes for these problems, and then asked yards like Rybovich Spencer and Knight & Carver for ballpark figures for doing the work. If the boat doesn't reach the desired speed, or if there are spray or ride problems, hull modifications can be attempted. This might mean, from simplest to most complicated, adding ballast, modifying the spray rails, adding a wedge or extending the bottom to change the running trim, adding a bulb to trim the bow up, or extending the cockpit to trim the bow down. Prices can range from $5,000 to $200,000 and higher, depending on the size of the boat and complexity of work. Increasing the speed by empowering with larger engines runs in the $30,000 to $100,000 range, plus $200,000 and up for new machinery, including larger shafts, gears, etc.
Of course, none of this takes into account cost overruns, which are common, and frustration. Hageman bluntly concludes, "Often, the problem is so fundamental that it is economic nonsense to make the change. In which case, live with it or sell it."
So it would seem cost-effective to model test once the cost of repairs equals the cost of testing. Sarin sums up, "It is the builder's and owner's cheapest form of insurance that the yacht they are building will perform up to their expectations."
Chris Martin is an associate member of the Society for Naval Architects and Marine Engineers.
This article originally appeared in the January 2003 issue of Power & Motoryacht magazine.