"Charles Momsen" wrote in message
...
This graph:

http://www.windows.ucar.edu/tour/lin....html&edu=high

Shows the density of water as a function of depth. Water density changes
from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1%
in 250 ft. Since that portion of the curve is linear, one can estimate
that water density would change .0004% over the diameter (tip to tip) of
a 12 inch propeller. So is a .0004% change in water density (in the
vertical plane no less) going to walk a boat sideways? Don't think so.


Think, Momsen, think! There are very large forces at work when a propeller
is turning at speed. You are stuck on static in your thinking. Picture it
this way. Let's say you were riding a bicycle at 1mph and you had a ten mile
per hour headwind. You would experience an 11mph head wind. Now, if you
aren't a girly-man you should be able to sprint up to 35mph. You would then
experience a 45mph headwind. Suddenly your inconsequential wind has great
consequence.

It's the same way with a propeller and the lift vs.drag coefficient. Even a
very small density difference results in a significant drag difference
between the top half of the prop and the bottom half of the prop. But there
is another thing that has a greater effect than density causing density to
be only part of the equation. Water density does not vary greatly due to the
fact that it doesn't compress easily. What does change significantly with
depth is water pressure (divers say 1 atmosphere for every 15 feet?) The
more pressure = the more drag for the propeller. I hope this helps.

Wilbur Hubbard