Am I overpropped? Will it damage my engine?

[Ziya Gun]

Hello again BB, Thanks for the fast replies to my previous posting, here goes again, I had my 15 hp YANMAR rebuilt and replaced my cracked 2 blade prop with a 3 blade prop and installed a tachomemeter. Now I can see my rpm and I top out at 2000, at 2100 black smoke pours out. In neutral, engine revves at 3600 rpm max. Based on what I read on BB, I must have the wrong prop. Is there a correct 3 blade prop for 80 model 15 hp YANMAR 2Q??? and will I damage my engine if I use this current prop below the max speed?Thanks

 

[Tim]

You're way over propped.

What are you spinning? The prop size is dependendent on your boat lwl and displacement. On my Yanmar 2gm (very similiar) on an H30 I used a 13x10 3 blade.

 

[Steve Dion]

Who knows?

Ziya:No one can tell you if you are REALLY overpropped unless you know what prop you are trying to spin. You need to get these specs first. My guess is that your 3 blade prop should be a 14 x 11. Get these specs and report back. You are probably not going to damage your engine but should consider that you could be causing coking of the elbow and possible overheating/loading of the engine. The additional fuel is going to pollute your engine oil too.Who put the prop on your boat? Maybe they can tell you what is going on. Otherwise just have a diver pull the prop and take it to a prop shop. This is probably cheaper than a pull.

 

[Alex]

You didn't mentioned prop size..

..but from what you wrote it looks yoursis oversize.For a 27 foot LWL/4.5 ton displ(very loaded..) boat like my 'H-29.5' a max of 15 by 10 3blade is suggested , altough I found that a little overpowered and I use a slightly smaller one , for a my 18 HP yanmar.For your 33 foot Hunter (which I don;t know , but I assume is heavier, and about same LWL), I would think a 15HP engine is definitely too small to begin with..Have a proffesional advise prop size (incl blade design , not all are the same).Hopefully , when you went from a 2 blade to 3 blade , your pitch is 1.5-2 inch less , otherwise it is too big.Definitely motoring at 2000 only on a 3600 RPM Yanmar , is not healthy to your engine.