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

if you learn to listen and feel for it, it is better than a depth finder keeps
you from running aground.

So from reading the other posts, and thinking about it, what happens is that
at a certain speed there will be a noticeable build up of water in front of
the displacement type hull since the water has no place to go ... so the
boat becomes a water plow and slows down.

"Pete Drez" wrote in message
...
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

Perhaps. Another thing apparently about shallow water is that boats turn
more sluggishly, but even more interesting is that they sink lower into the
water and increase their draft - only if moving. And whether moving at
trawler speed is enough to do this I do not know, but it has something to do
with the venturi effect: with so little space under the boat, water has to
flow out of the way faster under the hull than in deeper water, and faster
flowing fluids have lower pressure, so hey presto! the boat settles in a bit
lower. (The physical explanation for the venturi effect appears to have
changed a bit since I took physics, but the phenomenon is very real.)

====

Charles T. Low
- remove "UN"
www.boatdocking.com
www.ctlow.ca/Trojan26 - my boat

====

"bowgus" wrote in message
le.rogers.com...
So from reading the other posts, and thinking about it, what happens is
that
at a certain speed there will be a noticeable build up of water in front
of
the displacement type hull since the water has no place to go ... so the
boat becomes a water plow and slows down.

So from reading the other posts, and thinking about it, what happens is that
at a certain speed there will be a noticeable build up of water in front of
the displacement type hull since the water has no place to go ... so the
boat becomes a water plow and slows down.

"Pete Drez" wrote in message
...
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