The optimum propeller diameter is dependent on shaft power and rpm, and can be calculated if the specific working condition is known.

Mostly this is the full throttle full rpm condition, as the engine manufacturer wants the propeller to suit this condition, and thereby prevent overloading of the engine.

If you give engine particulars and reduction ratio I can work out which diameter would be preferable.

Do you have speed/rpm readings including full throttle with the present propeller?

Kind regards

Lars

Thank you for the reduction ratio, this confirms that the 20 degree setting is correct with 16" diameter

Calculation suggests that at cruising rpms there is only a small gain with 17" diameter, about 2%, but at full throttle the bigger propeller provides considerably higher thrust provided the pitch setting is 16 degrees. This choice would be particularly good in a strong headwind.

18 degrees is a little too much for the engine and would prevent it from achieving full rpm and thrust in a blow, but be fine for cruising as long as the engine is lightly loaded. The difference in cruising fuel consumption with one step larger or smaller pitch is, however, not significant.

Hope this enables you to make a decision.

Would be interested to hear what Max-Prop suggest

Lars