BoatBanter.com - View Single Post - Angle of prop shaft

On 06 Jun 2004 17:11:03 GMT, (JAXAshby) wrote:

no matter how deep the prop, the water on the lower side
is still going to be under ambient higher pressure than the water on the
top.

the pressure difference due to water depth is inconsequential. The pressure
change is less than 0.5 pound per foot of depth. the compressibility of water
is near zero. even on a 15" prop, the center of effort difference between top
and bottom blades is less than about a foot. These ar truly miniscule forces
as compared to the force needed to move a multi-thousand pound boat in a
noticeable fashion.

While it's true that the pressure difference is tiny between the top and
bottom of the prop, it is there and does contribute a tiny amount so it
should be factored in along with everything else. One special case that
could occur as a rare condition is when there is cavitation at the top
of the prop and not at the bottom due to the pressure differential.

Steve