|PMY 2002 Performance Wrap-Up|
Our exclusive comparison of all the boats we tested this year.
By George L. Petrie
When you read the boat test articles in PMY, do you ever wish there were a way to compare one model to one another? Sure, our test results show you boat speed and fuel consumption, but wouldn’t it be interesting to compare test results for two or more boats of different size, weight, horsepower, and speed?
Can such a thing be done fairly, and why would you want to make such a comparison anyway? To begin with, PMY can’t possibly test all models from all manufacturers. But say we’ve tested a 38-foot express cruiser from Brand X and a 42-footer from Brand Y, and you want to see how the two stack up. To do that, you’d need some way to compare them on an apples-to-apples basis. Or better yet, say you want to see how your boat compares with today’s newest offerings. We think we’ve found a way to do just that.
Inherent in most fields of engineering is the need to compare competing alternatives as a basis for making design decisions. Accordingly, naval architects have developed several methods of comparing vessels of widely different configurations. One of the simplest methods is to compare each vessel on the basis of its “transport factor.” Simply stated, the transport factor is a quantitative measure of three performance characteristics basic to a vessel’s design: how much it can carry, how fast it can go, and how much power it needs to do the job. Designers usually strive to maximize the first two and minimize the third, so we define transport factor as the vessel’s weight times its speed divided by total horsepower, with higher values indicating more efficient performance. Simple enough.
Actually, a little too simple, because some hulls such as trawlers are characteristically heavy and slow, while others like express cruisers are designed for speed. So we can’t simply compare on the basis of transport factor alone, but we can graph the transport factor against speed, which is one of its main parameters, and speed-length ratio, which distinguishes between boats of similar speed but widely differing length. When you do that, you get a graph like the one on page 68, which summarizes the results of each boat test PMY performed during 2002. Selected spots on the graph are keyed to a table showing each yacht’s principal dimensions and power, and the top speed we measured during each test.
Why have we chosen to identify only a few from the 52 boat tests we published in 2002? Ah, there’s the rub. You see, a valid transport efficiency number requires not only accurate speed and fuel consumption data, which we record, but also accurate measurements of displacement and waterline length, for which we are dependent on each boat’s manufacturer. In plotting data from all our boat tests, we found the results for some boats well outside the norm because, we feel, one or both of these two supplied pieces of data were suspect. In fact, in some cases we had to estimate values for waterline length because the manufacturer was unable to provide that information. From now on, we’re telling all manufacturers that we’d like to plot transport factors for the boats we test in 2003, so it is incumbent upon them to supply reliable displacement and waterline length figures. Of course, the same applies to you if you decide to plot your boat on the graph.
Even though we’ve identified only a few boats, you can still make some interesting comparisons from the graph. You’ll notice that yachts in the slow-speed range generally have higher transport factors than faster yachts. That confirms what we already know: Hulls moving at slow, full-displacement speeds can carry much more weight with far less horsepower than faster semidisplacement and planing hull forms. The more important comparison is, within any particular top-speed range, how does each boat stack up? Which has the highest transport factor in the 30- to 35-knot range, or whatever speed range you might be interested in?.
This article originally appeared in the January 2003 issue of Power & Motoryacht magazine.