Determining the best Propeller for Boat Engines

I was talking to a colleague who loves to fish. He travels up to 20km
from shore with friends. It is a very dangerous hobby.

Luckily he had just purchased a GPS which helped him economise on his
petrol consumption. Setting his propeller pitch propeller will allow
him to save petrol even more.

Propeller is just like a gear. Low pitch means that the boat will
travel at shorter distances for a single engine revolution making it
slow but able to carry heavy loads. It is just like driving at first
gear.

Although powerful, it means that it does not save petrol if you travel
at long distances.

http://www.propline.com/Propeller-General-Information/Propeller_Sizing.htm

Selecting the Correct Propeller For Your Boat and Engine
You may also want to view:
Propeller Installation, Determining RPM, Propeller Terminology,
Frequently Asked Questions

The best propeller size for your boat and engine combination is based
on the recommended operating range at wide open throttle (w.o.t.) for
your engine, which you will find in your operator's manual. This will
be expressed in terms of a certain horsepower at a certain RPM
(revolutions per minute).

The goal in prop selection is to determine what propeller style and
size will maximize performance for your boat, while allowing your
engine to operate in the recommended RPM range. The correct propeller
will prevent the engine from over-revving, yet allow it to reach the
minimum RPM where maximum horsepower is produced.

Run the boat/motor at w.o.t. under normal operating load to determine
the maximum RPM you are able to obtain. A tachometer is necessary for
this test. Adjust the motor trim angle for the optimum performance. If
during this test, you begin to exceed the maximum rated RPM of the
engine, reduce throttle setting to a position where maximum RPM is not
exceeded.

If your test results in your being able to over-rev the engine, you
need to increase the pitch of the propeller. Increasing the pitch
increment by 1" will result in approximately 200 RPM drop. If your
testing shows, however, that you are only able to obtain a RPM
somewhat lower than the maximum rating given by your engine
manufacturer, you would need to decrease pitch. Decreasing pitch would
increase your RPM.

Example:

Operating Range =

5000-5600 RPM

Top End of Operating Range =

5600 RPM

Tachometer Reading =

4800 RPM

Difference =

800 RPM

For every 1" of pitch size, the effect will be approximately 200 RPM.
Knowing this, take the difference in the above example at 800 and
divide it by 200. The result is 4. The prop to use will be 4" in pitch
less than the prop that was used.

Switching from an uncupped to a cupped propeller will also reduce your
RPM. The cupped propeller of the same pitch and diameter will
typically reduce your RPM by approximately 200.

Once your wide open throttle RPM falls within the recommended range of
the engine manufacturer, you have a propeller that is suited correctly
for your boat with respect to RPM. If you use your boat for fishing,
cruising and skiing, one prop probably won't do all three things
equally well. It is best in circumstances like this to have two
propellers. One to accommodate one set of circumstances and the other
to perform best under the different load. It is imperative that the
wide open throttle RPM fall within the range specified by your engine
manufacturer.