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From: "Mark Weaver" <weaver@nospam-corvusdev.com>
Newsgroups: alt.sailing.asa
References: <39D366E9.E52452BD@ou.edu> <970156748.9937.0.nnrp-02.c2de5386@news.demon.co.uk>
Subject: Re: Hull Speed
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Date: Thu, 28 Sep 2000 12:21:49 -0400
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"Donal" <donald@lanode.demon.co.uk> wrote in message
news:970156748.9937.0.nnrp-02.c2de5386@news.demon.co.uk...
>
> Jesse.D.Matlock-1 <Jesse.D.Matlock-1@ou.edu> wrote in message
> news:39D366E9.E52452BD@ou.edu...
> > What is hull speed?
>
> As a boat travels through the water it generates a bow wave just in front
of
> the boat. As speed increases the trailing 'trough' behind this bow wave
> nears the stern. When this trough reaches a point just behind the stern
you
> get a situation where the front of the boat faces resistance from a wall
of
> water, *and* the stern is being sucked down. The whole boat is now trying
> to go uphill. This speed is called hull speed.
>
> The faster the boat goes, the longer the distance between the peak of the
> bow wave and the trough following it. Therefore the longer the boat, the
> faster it can go before this trough reaches the stern.
> To go faster than hull speed, most boats have to plane. So if you watch
> power boats accelerating, you will notice that just before they come up
onto
> the plane, the adopt a very steep angle.
>
> There are formulae that tell you what the hull speed will be for any
length
> of boat. I think that it is about 1.2 multiplied by the square root of
the
> waterline length expressed in feet. The answer that you get is in knots.
>
>

The traditional formula is

1.34 * SQRT(LWL)

However, for semi-planing craft the 1.34 constant is not appropriate but
rather depends on other factors -- displacement in particular. Naval
architect Dave Gerr worked out a formula that estimates the value of the
1.34 'constant' as a function of displacement. Here's a web site that
implements Gerr's formula where you can plug in the numbers for your boat:

http://www.yachts4sail.com/hullspeed.html

Mark