Only formula I am aware of is the speed formula.

rpm x pitch
------------------ = theoritical speed x efficiency = actual speed
1056 x gear ratio

1056 is a conversion factor to get from inches of pitch and rev per minute
into miles per hour.

gear ratio will be 1.5 or 1.86 or 2.0 depending on your motor usually.

--
Tony
my boats and autos - http://t.thomas.home.mchsi.com


"Capt. Frank Hopkins" wrote in message
nk.net...
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?R²(0.66)=19.0250?R²(0.80) it doesn't work out exactly.

If the formula doesn't display in the newsgroup, I will try a different
venue.

Thanks,

Capt Frank



Tony Thomas wrote:
First of all, normally a 4 blade prop will have a smaller diameter than
the
3 blade to compensate for the extra blade surface.
You want to go with the same pitch prop. The 18 will be way too small a
prop and will travel less and be less efficient than the 21. Stay with
same
pitch which is inches of travel forward per revolution (in theory not
compensating for slip). A 4 blade will have less slip at lower rpms
than a
3 blade which is why it will be more efficient at cruise.