There is an awful lot written
about boat propellers.
Unfortunately, they usually only
focus on boats with tens, or even hundreds of horsepower.
focus on planing boats. Boats that rise up out of the water and
skim across the surface.
But browsing boat owner forums
on the subject of propellers, the repeated comment is:-
far as efficiency goes, always swing the biggest, lowest RPM prop
These are the two
fundamental facts that override all else for our Stirling
First. The diameter of the propeller should be
as large as possible.
i.e. as large
as can be fitted under the boat without striking the
Second. It should also be rotated slowly.
reasons – large,
slow propellers are the most efficient.
not waste the precious Stirling wattage on an inefficient, small
outboard motor propeller.
They are designed for far higher
revs and are just plain inefficient.
propellers are more efficient than three blades.
are more efficient than four blades. Etc.
But the most
important need is for a large, slowly rotating
Propellers can be quite expensive - £400
for a particular diameter and
– at a boat jumble sale - £50 or less.
the biggest diameter propeller you can fit.
Then you can set
about the gearing between the engine shaft and the propeller
pitch measures how far the propeller will move through the water
our small boats a 12 inch diameter is probably as large as can be
Again efficiency is “the king”.
Propellers that have a similar pitch to diameter have a
But, try 14” pitch, 12”
diameter rather than a higher revving 10” pitch for 12”