Propellor Pitch Choices

[Drascomber]

This is the situation.

I have a choice of 6", 7" or 8" pitch propellors all of the same diameter and design.

With the 8" prop the engine maxes out at 4,470 rpm.
With the 7" prop the engine maxes out at 4,800 rpm.
With the 6" prop the engine maxes out at 5,150 rpm.

In each case the corresponding maximum boat speed is 6.1 knots - whichever prop is used.

The outboard manufacturer's recommended full throttle speed range is 5,000 to 6,000 rpm

The shop manual says "If full throttle operation is below the recommended range, the propellor MUST BE changed (his capital letters) for one with lower pitch to prevent loss of performance and possible engine damage".

On this basis I should undoubtedly use the 6" prop. However, one of the higher pitch alternatives would give a lower engine speed which would be less noisy and possibly more economical.

As there is manifestly no loss of performance, can anyone please explain to me why a lower engine speed could be detrimental?

By the way 1. - I usually cruise at 3,500 rpm.
By the way 2. - This engine has fixed ignition timing.
By the way 3. - We are talking about a single cylinder four stroke. (The latest models of which have varying ignition timing)

 

[Pye_End]

Presumably these are speeds 'on the flat', and that in a bit of a sea things are different? You have an easily driven hull, but you probably only stress engine/prop in difficult conditions.

With a more substantial keel cavitation may well become an issue with larger pitches when the boat speed becomes low (eg due to waves), so to improve acceleration the lower pitch is useful in these conditions. This is presumably less of an issue on a boat like yours?

Also, they say that if an engine cannot achieve near max revs then it is not using its full power potentail.

If it were mine I would take the advice of the manufacturer.

If you think that you have too much power available and want to be more economical then maybe a smaller engine....?

 

[VicS]

Based on the info given the 6" pitch prop is the one to use .

A finer than standard pich prop is the one to choose for a displacement hull. The coarsest one would be for a planing hull and the middle one, which is presumably what is supplied as standard, is a compromise.

Only by fitting a prop that allows the engine to reach its max revs range at the maximum hull speed will you get it to develop full power but since you operate well below that there is not a great deal to gain from changing from the standard one. As you say it will be noisier and may well use more fuel.

You have more or less proved that 6.1 knots is the max hull speed but you could calculate a figure from 1.35 times the square root of waterline length in feet.

The reason for saying that the correct prop MUST be fitted becomes important with a light, high speed planing hull because if a fine pitch prop were fitted it could allow the engine to over rev.