Prop advice for outboard

[greeny]

I'v just fitted a s/hand Yamaha 4HP 4 stroke long shaft to my Sun 2000 Trailer sailer to replace an old 2 stroke.
The prop on it looks non standard,it could even be a prop off a 2.5 HP from the looks of it.
In use it drives the boat at tickover or just above but when revs are increased the prop just races in the water and cavitates providing no drive at all.
Clearly it needs re propping but which to chose is the question.
I have a choice of 3 from Yamaha.
7.5" dia pitch 8".
7. 5" dia pitch 7"
7.5" dia pitch 6.5"
Which one would be best for a 1.25 tn boat.

 

[MikeCC]

Probably the smallest pitch.

 

[Chiara’s slave]

I recently re propped our 2 ton, 15 hp outboard powered boat, with a 5” pitch, and she's fast and easily driven. Having the wrong prop isn’t causing cavitation on it's own, you are ventilating from the surface probably. That wasnt one of the symptoms on ours.

 

[greeny]

I recently re propped our 2 ton, 15 hp outboard powered boat, with a 5” pitch, and she's fast and easily driven. Having the wrong prop isn’t causing cavitation on it's own, you are ventilating from the surface probably. That wasnt one of the symptoms on ours.

Thanks for that suggestion. I suspect that is maybe part of the problem but first want to try the "correct" prop because the one fitted right now is not helping the problem. If that doesn't work then I have to look for other options. Getting the outboard lower in the water is difficult because it's transom mounted. Would need a bracket and there's not a lot of room and will move the outboard controls out of easy reach.

 

[Chiara’s slave]

Just a ventilation plate might well help. Our prop is by polastorm, via ebay. Theres lots of prop calculators on the net, ours is good for about 12kn theoretically, the boat will get up to about 8.5 now, previous prop is managed 6. Well worth getting this one right.

 

[LONG_KEELER]

I'm surprised that 2.5 hp prop that you have fits your new outboard unless it has been modified in some way.
The prop diameter is the most significant factor with regard to power . Bit like the luff length on a sail or wing length on a glider. Could you measure the current prop's diameter and let us know ? This fact would probably help to clear up the cavitation issue.

 

[greeny]

Just a ventilation plate might well help. Our prop is by polastorm, via ebay. Theres lots of prop calculators on the net, ours is good for about 12kn theoretically, the boat will get up to about 8.5 now, previous prop is managed 6. Well worth getting this one right.

I'm surprised that 2.5 hp prop that you have fits your new outboard unless it has been modified in some way.
The prop diameter is the most significant factor with regard to power . Bit like the luff length on a sail or wing length on a glider. Could you measure the current prop's diameter and let us know ? This fact would probably help to clear up the cavitation issue.

Prop diameter of the existing prop is approx 6.25 inches but difficult to measure in situ on the boat. The blades have an exagerated swept back shape that is similar to one I've seen on a 2.5 Ymaha on another boat but of course mine may be from another outboard with 9 spline drive shaft. Not much surface area in the blades either. Certainly doesn't look to be as big in area as the listed prop blades. I'm inclined to order the 6.5" prop as the lowest pitch listed and try it. If that fails then it may need to be lowered in the water or do something with an anti cavitation plate addition/modification.

 

[Chiara’s slave]

Ours looks like this. Lots of blade area, no camber, low pitch

 

[ChromeDome]

Check prop sizes for dedicated sail boat outboards (commonly referred to as 'sail power').

Until you fix issues like too high mounting (surface piercing prop blades) you can forget improvement by dia/pitch changes.

4 hp isn't exactly a racing motor so props designed to run in an air/water mix wouldn't be available.

 

[greeny]

Update on the prop. In the end I went for a 7.5" dia pitch 6.5" . The performance is excellent compared to the old prop. No prop slipping, spinning out and over revving. Much quicker boat response when accelerating or going into reverse to manoeuvre .
So the biggest diameter available and the lowest pitch worked. Came up the river estuary today (flattish water) into 20 knot headwind at 5 knots plus. What a difference. Thanks to all who helped me make the choice.