"Charles Momsen" wrote in message
...

"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?

Don't think so. Drag increases to a greater extent than lift. If this
weren't the case your average airplane or boat propeller would never
cavitate.


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

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

Excellent! That is one static psi difference in a foot. Take a three-bladed
prop with a surface area of about 600 square inches and you're dragging the
blades of the damned thing through 600 psi on the bottom half? Correct?

What kind of pressure do you have on the top half? 500PSI? Makes your mere
..98 psi seem suddenly more significant, doesn't it?

Wilbur Hubbard

So you drag the top of the prop through