any propeller experts? need new prop for yamaha 15

[lenten]

i bought a yamaha 15 hp 4 stroke for a 13 foot dell quay dory --it would just about plane with 2 adults---i have now changed it for a fat open displacement boat--15 feet x 6 feet---bare boat weight 350 kg +engine+fuel+2 adults+odds and sods adds up to about 600 kg---wife has a knack of always finding a patch of shingle to stir up so the prop is a bit ragged ---this year i am treating the engine to a new one----the existing is 9 1/4 by 10---i presume this is a general purpose prop for lighter faster boats---can anyone advise what size prop for a heavier boat---thanks

 

[MapisM]

You have three ways to find the optimal prop: test, test and test.
That said, I'd start with a 3 or 4 inches shorter pitch.
Ideally, you should also go for a larger diameter, but I don't think you can, due to the distance between the shaft and the cavitation plate.

 

[VicS]

As above. A smaller pitch and if possible a larger diameter.

You dont say what year but looking at various parts lists props available were
all three blade 9¼" diameter by 8", 9", 9¾", 10", 10½", 11" and 12" pitch depending on exact model.

10" then pretty well in the middle.

Downside of fitting a smaller pitch is that for the same boat speed the engine is running faster ... if you need the higher revs to get more power then thats fair enough but a boat that size is not likely to need more power. A 15hp engine should be more than adequate for a displacement hull that length. You cannot make it go any faster than the waterline length allows.

Try it. I doubt you will need to change it. You might even decide a smaller engine would be more appropriate.

 

[lenten]

thanks to you both for replies---i bought the engine new in 2005 and i don t really change it as i know how well i have looked after it---the only prop that has a larger diameter is a 9 3/4 by 8 dualthrust (whatever dualthrust means)---at the moment i can get max speed at 1/2 to 2/3 throttle---the only time i have needed more was when there was 6 adults onboard---i was hoping that a well matched prop would give me slightly better fuel consumption because i sometimes do long days but if i understand you right vic less pitch equals higher revs which would give higher fuel consumption??---regards lenten

 

[moonraker 36]

and if you keep dinging the prop.fit a prop guard.

 

[lenten]

my wife dings the prop--she likes to steer but doesn t read the water too well ---she is getting better--------------i hope?---regards lenten