"Wilbur Hubbard" wrote in message
anews.com...

"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


Wouldn't lift increase in the same proportion as drag?

1 foot of depth gives a pressure differential of .06 atmospheres or .98 psi.

http://www.youtube.com/watch?v=u5KKcBQen-Y