T&T: Props

[Mark Richter]

Ken,
You didn't give us much to go on.  Why do you want new props in the first
place?  Is your boat a planning or displacement boat?  If intentionally
over-propping the boat for fuel economy, will you be careful not to push too
hard and possibly overload the engine?  Tell us everything about your boat,
engines, props and useage, and you will get more helpful answers.  Or buy
Dave Gerr's "Propellor Handbook" and learn everything you want to know from
a reliable soruce, but much of it is pretty technical (math required).

There are two basic ways to recommend a new prop:
1.  Calculate the needed prop based on basic boat characteristics such as
displacement, engine horsepower and RPM, transmission ratio, and prop
aperature clearance (max prop diameter possible).  This method is used for
new boat designs, since there is no real performance data available, but
it's not terribly accurate, especially for planning boats.  This method is
what most prop calculators do, like the one from Michigan Wheel.

2. Determine the needed prop by comparison with the existing prop and
existing boat performance.  We need to know the existing prop diameter and
pitch, number of blades, prop blade area (if available), and engine RPM at
wide open throttle, as well as the engines designed max rpm at rated
horsepower.  Hopefully, this performance data was taken when the boat is in
"average" condition of bottom cleanliness and weight (liquids and junk
aboard), not bare and pristine from the boat dealer.  Performance data based
prop sizing is far more reliable.  Many new boat designs use method 1 for
the first set of props, then use method 2 for the final set of props.

Mark Richter, M.E., m/v Winnie the Pooh
homeport Ortona, FL, lying Stuart, FL