PMY 2002 Performance Wrap-Up Page 2
|PMY 2002 Performance Wrap-Up|
Part 2: Before we conclude, a couple of major caveats are in order.
By George L. Petrie
To see how your boat (or any boat you might be considering) compares, just calculate her transport factor as we have done, then spot it onto the graph above the corresponding speed (or speed-length ratio). Just be sure to do your calculations the same way we did ours to get a true comparison. For details, refer to the sidebar on this page.
You may wonder why we have two graphs, one plotted versus speed and the other versus speed-length ratio. The reason is that the comparison is a two-step process. The first step (using the graph plotted versus speed) helps identify all boats that have a top speed in a particular range of interest, say 20 to 25 knots. The second step (using the graph plotted against speed-length ratio) is a little more complex. For a fair comparison, we have to consider that a 30-foot boat that does 25 knots is very different from an 80-foot yacht that goes the same speed. Thus the second graph provides a truer, apples-to-apples comparison between yachts of differing length.
One of the niceties of graphing the transport factor is that it facilitates comparison between boats of widely different types. Consider the two or three yachts in each speed range that have the highest transport factor within that range. Those define a crescent-shape band on the graph that represents the “most efficient” designs. Points below and/or to the left of that band indicate increasingly “less efficient” hull forms.
Before we conclude, a couple of major caveats are in order. First, consider two hypothetical yachts of identical length and speed, one heavy with big engines and the other light with smaller engines. The two yachts could have the same transport factor, and if so, they would appear as equivalent on our graph. But there are other factors to consider. In terms of fuel cost, the lighter yacht with smaller engines might be preferable, but for canyon running far offshore, the battlewagon would likely get the nod. We offer the transport factor as a useful indicator, not as an absolute measure of performance or efficiency.
Second, and in a similar vein, achieving “most efficient” performance (as we have defined it) is not every designer’s top priority. For some builders the primary focus may be comfort, accommodation, seakeeping, and/or other subjective goals. Each owner’s priorities may differ as well. In the end, we present the transport factor as just one more piece of information to help you make the most informed buying decisions possible.
George L. Petrie is a professor of naval architecture at Webb Institute and provides maritime consulting services. His Web site is www.maritimeanalysis.com.
This article originally appeared in the January 2003 issue of Power & Motoryacht magazine.