Page 33The Right Propeller

Previous page______ Index______ Next page

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.