Diameter is usually the single biggest determinant of efficiency of a propeller
and subsequently the entire power train of an installation. Except for high speed
vessels, the larger the diameter, the more efficient an installation. Typically,
diameter is limited by draft and underwater appendages but it is best to fit the
largest diameter propeller with appropriate clearance. Besides efficiency, for a
given SHP, the larger the diameter of a propeller the slower the RPM’s required.

For example, let’s look at a typical sail boat using the
boating speed calculator we punch in LWL of 30, displacement of 21000 and
play around with the speed till we get 25 HP. We determine that it’ll go just
over 6 knots with a 27 HP diesel and a stock three blade prop. We have 25 HP available
at the shaft (SHP). Let’s see what happens to efficiency by changing the RPM
at the shaft with different gear ratios and the diameter of the propeller.

500 RPM would require around 29 inch diameter but would yield and efficiency of
around 57%

3000 RPM would require around 10 inch diameter but would yield an efficiency of
around 33%

That’s 14.25 HP vs. 8.25HP converted into forward motion, quite a big difference
of 6 HP. At the extreme, a ten foot diameter prop will give us upwards
of 79% efficiency, but dragging a prop that size, under sail, will not be fun, not
to mention figuring out where to put it.

Determining Diameter

A propellers diameter is sized according to two factors:

Shaft Horsepower (SHP). This is the horse power at the after losses
from the transmission/gearbox, ancillaries, exhaust backpressure etc.

RPM at the shaft. i.e. after reduction by a transmission/gearbox.

The formula used to determine diameter is:

D = (632.7*SHP^0.2)/RPM^0.6

So in or example D = (632.7*25^0.2)/500^0.6

Or 29 inches.

Outboard motors usually have high RPM’s which is fine for go fast planing
vessels, but for displacement vessels this is a problem due to a large loss in efficiency
with the high RPM’s and small diameter. Some outboard manufacturers do make
“high thrust” versions, with a bigger reduction gear ratio. This will
give better outboard propeller efficiency, but even these, typically have
higher RPM’s than desirable for a low speed displacement boat.