Page 33 – The Right Propeller
There is an awful lot written
about boat propellers.
Unfortunately, they usually only
focus on boats with tens, or even hundreds of horsepower.
Many
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:-
“As
far as efficiency goes, always swing the biggest, lowest RPM prop
you can.”
These are the two
fundamental facts that override all else for our Stirling
boats.
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
hull.
Second. It should also be rotated slowly.
The
reasons – large,
slow propellers are the most efficient.
Do
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.
Two bladed
propellers are more efficient than three blades.
Three blades
are more efficient than four blades. Etc.
But the most
important need is for a large, slowly rotating
propeller.
Propellers can be quite expensive - £400
for a particular diameter and
pitch.
Alternatively
– at a boat jumble sale - £50 or less.
Get
the biggest diameter propeller you can fit.
Then you can set
about the gearing between the engine shaft and the propeller
shaft.
Diameter
and Pitch
The
pitch measures how far the propeller will move through the water
per revolution.
For
our small boats a 12 inch diameter is probably as large as can be
fitted.
Again efficiency is “the king”.
Propellers that have a similar pitch to diameter have a
better efficiency.
But, try 14” pitch, 12”
diameter rather than a higher revving 10” pitch for 12”
diameter.