Builders' Forum Archives
Re: Small Boat Design Boo
Posted by Mark Camp on Mar 15, 2005
One thing to watch out for. I don't know these specific books, but some experts are very focused on "normal" displacement boats, and forget to mention a very important difference in design of ultralight vessels (like kayaks, especially).
They describe "hull speed" in a way that does not apply much--if at all--to ultralight kayaks. One often reads that the hull speed (1.34 times the square root of the LWL, units of knots and feet) is sort of a barrier between displacement mode and planing mode. And one that requires an enormous increase in power to overcome. It is true for an outboard or a cabin cruiser or a sailboat, and it explains why outboard boat ads never mention "miles per gallon". (It is typically somewhere around 1.5 or 2! or something in that ballpark).
Ultralight kayaks can attain and exceed the calculated "hull speed" without the sudden jump in power required and, as I understand it, without truly planing. I believe that one web article I read (some good soul on this very forum put me onto this news) even said that the slope of the power graph does the opposite at some point, and flattens out!
In Response to: Re: Small Boat Design Boo by Laszlo on Mar 15, 2005