"Charles Momsen" wrote in
:


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

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


http://www.windows.ucar.edu/tour/lin...ity.html&edu=h
igh

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?


Yes, you are absolutely correct.

blondie