design an hull

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by Brett

The full size texts will tell you there are speed penalties for non optimum LCB and Cp<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Hi Brett

They do indeed tell us this, but they’re misleading us a little. Let me explain.

The main studies have taken a bunch of “standard” hulls, nothing too extreme, which are “representative” of the average kind of boat. They’ve determined their main design characteristics – LCB, Cp, Bm, Lwl, and so on. Then they’ve turned them into models and towed 'em, measuring drag as they did so. Finally, they’ve correlated the design characteristics against the drag measurements, and have found interesting relationships. For example, if the hull is towed at a somewhat higher speed, drag is a little less if Cp is a little larger, and vice versa.

Here is a typical table, taken from http://www.tedbrewer.com/yachtdesign.html

Speed/length ratio Cp


1.0 and below .525
1.1 .54
1.2 .58
1.3 .62
1.4 .64

The misleading part is when these studies draw the conclusion that, for higher speed ranges, Cp “should” be higher as well.

For example, we can measure the number of ice creams eaten in London, and relate this to the number of hats worn at the time. There is a clear positive relationship, of course, but we don’t say that you’ll find yourself eating an ice cream when you wear a hat…

Sure, for higher speeds, higher Cp is associated with lower drag, but as far as I can see no one really knows why. There may well be better hull shapes which reduce drag much more than increasing Cp for higher speeds, but no one really knows how to define or generate them.

Lester Gilbert
http://www.iomclass.org/
http://www.onemetre.net/