I have a trawler that draws 5 feet of water. Nornal cruising speed is
8.5 knots, with 10 feet of water or more under the keel. As the depth
under the keel decreases the speed starts to slow, down to about 7
knots when there is 3 to 4 feet under the keel.

I have heard that boats will slow in shallow water and am interested
in the physics of why this happens. I would appreciate a link to
information on this subject or an explanation of it if someone
understand the subject.

Thanks

Pete Drez