On Sun, 02 Nov 2003 05:16:31 GMT, "Capt. Frank Hopkins"
wrote:
Ok, Tony,
That makes sense. Is there a formula for calculating a prop's thrust?
rpm vs hp? Given 21" pitch and the current 3 blade a 21" diameter
(stainless steel)
21?Rē(0.66)=19.0250?Rē(0.80) it doesn't work out exactly.
If the formula doesn't display in the newsgroup, I will try a different
venue.
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Capt Frank,
Recommend that you get a copy of Dave Gerr's excellent book:
"Propeller Handbook"
http://tinyurl.com/te13
I bought a copy earlier this year after pondering many of the same
questions and issues. Excellent reading and very thorough coverage
of a complex subject.
To make a long story short, Gerr says that prop slip is basically a
function of blade area. Three blades are more efficient if you can
swing a big enough prop to get the required area. Many boats however
have insufficient prop aperature space for a large prop without
compromising blade tip clearance between prop and hull. It turns out
that insufficient tip clearance leads to vibration because of
asymetric blade loading. For those boats with insufficient clearance,
a 4 bladed prop offers a way to increase blade area without increasing
the prop diameter (and decerasing clearance).
Hope that helps but I still recommend the book for the complete story,
and many, many, design equations and charts.