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.