Help with Prop Scoring

[rfinlay98]

Hello all. I am a new (well at least new to me) owner of a Starcraft Superfisherman 170, with a 2003 75hp 4 stroke Yamaha. I have grown up with boats in my family my whole life, but this is my first crack at ownership. I am extremely pleased with how everything runs so far. About 35mph at 5300RPM However, after my last trip I was a little concerned when I looked at my prop when I got home. The marina I bought it from had put a new prop on for me, and gave me the old one. After a half dozen trips and about 20 hours of use, the paint has started to wear away in spots, and somehow something has started to score/bore into the blades on the prop. Apologies, I do not know the pitch of the prop, however it is a 4 blade...looking at the old prop (3 blade), it doesnt seem to have the same wear. What is the cause, and what can be done to fix this problem?? Any assistance is greatly appreciated! Apologies, the site wont let me post pictures.

 

[fhhuber]

You'll need 3 posts to post pictures and we need pictures.

There's a good possibility you have cavitation issues and that can eat a prop, especially an aluminum prop.

Cavitation is essentially the prop turning so much faster than it can push the boat forward that it is creating extreme low pressure ( in front of the blades) and causing the water to boil due to the low pressure. When the bubbles collect on the prop and collapse the incoming water hits the blade like a tiny bomb going off.

Severe cavitation can eat the prop blades right off the prop very fast.

It will look like the prop has been hit with many shotgun blasts of tiny bird shot in a pattern that shows where the bubbles are collapsing on the blades.

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Your boat speed sounds low to me... and that would support the idea of a cavitation issue. with the prop pitch and someone who knows that engine, they can calculate the speed it SHOULD be able to go if that is the correct prop.

 

[jimmbo]

35mph for a 17 ft with a 75 sounds reasonable to me.
The cavitation burn on the prop may be the result a prop design issue, a disturbance in the water flow over the gear housing, gearcase being a larger dia than the prop hub, excessive trim angles

 

[fhhuber]

kind of depends if its aluminum or fiberglass.

Aluminum 17 ft and a 75 should cook along nicely.
I didn't look it up first, but assumed the Starcraft was aluminum.

 

[ahicks]

My bet is you're spending a lot of/too much time with the trim out a hair too far - resulting in cavitation.

 

[fhhuber]

Could be that simple...

Could be excess prop pitch with not enough blade area.

Could be lot of things.