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.

G There is a simplistic explanation I've been given which may or may
not be entirely correct, so I'll skip it. However, two points to keep in
mind when under this condition.....
Reduce power.... this will reduce the strain on engines and boat and
even give back some of that speed lost while you sat there at full
throttle, digging a hole.
Watch your wakes, Port and Stbd. If they start to move from a their
normal trailing position to 90 deg off the side, your running out of
water .... this is particularly useful in channels as an aid to stay in
the channel .. i.e. .. if the wake on the stbd side moves up toward 90
deg and the wake on the port side trails normally, move to port. You're
getting too close to the Stbd bank.

otn

Dean wrote:

For the same reason a wave breaks when it nears shore. Your bow wave
is in ground effect, and encounters more resistance....

On Sat, 26 Jul 2003 21:23:48 GMT, Pete Drez
wrote:


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


--
http://ripperd2.dhs.org

Very true, for many reasons

otn

Gould 0738 wrote:

(Yacht to be slowin' down some when you've only got 2- 3 feet of water under
the keel anyway. 10 knots would be fast for my taste under those conditions.)

Thanks to all for the information. On the subject of slowing down at
eight feet of water (three under the keel), I do set my depth alarm at
three feet under the keel. The boat is used most often in the sounds
and rivers of eastern North Carolina, where deep water does not exist.
It is the norm to run most of the time with single digit depths under
the keel. The good news is the depths usually change very gradually,
except for uncharted shoalings, and the bottom tends to be soft sand
or mud.

It is interesting to see how much better the boat runs, both speed and
responsiveness to the helm, if it has ten or more feet of water under
the keel.

Pete Drez

On Sun, 27 Jul 2003 23:20:18 GMT, otnmbrd
wrote:

Very true, for many reasons

otn

Gould 0738 wrote:

(Yacht to be slowin' down some when you've only got 2- 3 feet of water under
the keel anyway. 10 knots would be fast for my taste under those conditions.)

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