I have a yanmar sb8 which appears to start up and run just fine. If however, I run the engine almost full throttle, soot comes out with exhaust. I have a brand new injector, shaved head, new valves and new exhaust elbow. I have read on some forums that the prop pitch (2 blade) may be a bit too steep. Some forums have suggested having it decreased by about 1/2" pitch angle. This would allow the engine to reach higher rpms with less load? Any thoughts or previous experience appreciated.
opinion on prop pitch/soot from yanmar sb8
- Thread starter pethia
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Get some hard numbers
Your YSB8 is rated @ 3200 RPM max. The prop should allow the engine to reach that figure at full throttle assuming no significant headwind or current etc. If there's too much pitch the engine will be unable to reach that speed and the exhaust will produce a lot of black smoke due to overfueling: the governor is allowing more fuel to be injected but the engine doesn't have enough grunt to rev up to burn the fuel cleanly. Since you didn't mention RPM's, I'm assuming your boat doesn't have a tach for measuring engine speeds. Try & borrow a test tach or something to get some hard data abt. engine speeds. Also, find out what the diameter & pitch of your present prop is. Michigan Wheel has an on-line site for calculating the pitch /diameter for the various props they make: other propellor mfg. probably do too. You have to input data abt. LWL, waterline beam, displacement etc. It's cheaper & easier do do your homework and get some hard numbers together first than just to take a hit & miss approach and get the prop re-pitched hopeing it will cure the problem.
Your YSB8 is rated @ 3200 RPM max. The prop should allow the engine to reach that figure at full throttle assuming no significant headwind or current etc. If there's too much pitch the engine will be unable to reach that speed and the exhaust will produce a lot of black smoke due to overfueling: the governor is allowing more fuel to be injected but the engine doesn't have enough grunt to rev up to burn the fuel cleanly. Since you didn't mention RPM's, I'm assuming your boat doesn't have a tach for measuring engine speeds. Try & borrow a test tach or something to get some hard data abt. engine speeds. Also, find out what the diameter & pitch of your present prop is. Michigan Wheel has an on-line site for calculating the pitch /diameter for the various props they make: other propellor mfg. probably do too. You have to input data abt. LWL, waterline beam, displacement etc. It's cheaper & easier do do your homework and get some hard numbers together first than just to take a hit & miss approach and get the prop re-pitched hopeing it will cure the problem.
TT, thanks. You are correct, the sb8 does not
have a tach. The boat is a 1978 hunter 27'. Lot of boat for a small engine. I will see what the pitch is on the prop.
have a tach. The boat is a 1978 hunter 27'. Lot of boat for a small engine. I will see what the pitch is on the prop.
I had a problem once...
...with my 3HM35 and the mechanic who tuned it and brought it on a sea trial said the same thing (that I might be 1" overpropped). It was the stock prop so I figured to have it tested first. The problem is that one of the two blades was overpitched by 1/2". The prop was repitched and all is now fine.
...with my 3HM35 and the mechanic who tuned it and brought it on a sea trial said the same thing (that I might be 1" overpropped). It was the stock prop so I figured to have it tested first. The problem is that one of the two blades was overpitched by 1/2". The prop was repitched and all is now fine.
Props
Spent some time on a friends Hunter 27 with a Yanmar YSB8 engine - it had a 2 blade Federal Sailer 12" x 8" prop on a 1"shaft. Seemed to work OK and be a fairly good match of prop & engine; no black smoke at WOT & able to turn max. revs etc. I have an ODay 27 with the same engine: a PO had installed a 3 blade 13" x 8" prop and it was far too much wheel for the little Yanmar to turn. Up to abt. 2500 RPM the engine ran beautifuly & the exhaust was clean, but any more throttle didn't produce any increase in engine or boat speed and the black smoke poured out of the exhaust. I wouldn't want to run more that abt. 2500 RPM normally ( that gave me 5 kts, GPS ) but there are times when you may need WOT to buck a current or headwind and heaven knows, 8 HP isn't much by today's standards. If your prop is limiting your RPM as mine was, your engine is only going to produce abt. 5-6 HP tops.
Spent some time on a friends Hunter 27 with a Yanmar YSB8 engine - it had a 2 blade Federal Sailer 12" x 8" prop on a 1"shaft. Seemed to work OK and be a fairly good match of prop & engine; no black smoke at WOT & able to turn max. revs etc. I have an ODay 27 with the same engine: a PO had installed a 3 blade 13" x 8" prop and it was far too much wheel for the little Yanmar to turn. Up to abt. 2500 RPM the engine ran beautifuly & the exhaust was clean, but any more throttle didn't produce any increase in engine or boat speed and the black smoke poured out of the exhaust. I wouldn't want to run more that abt. 2500 RPM normally ( that gave me 5 kts, GPS ) but there are times when you may need WOT to buck a current or headwind and heaven knows, 8 HP isn't much by today's standards. If your prop is limiting your RPM as mine was, your engine is only going to produce abt. 5-6 HP tops.
Clean Bottom and Prop??
It may be too obvious to ask but a prop with a few barnacles can mess up the load. Also a dirty bottom will add load. Finally fishing line etc wrapped around the prop??? My point is that if this is a new problem then the engine/prop pitch match probably isn't the problem. I had a few barnacles on my prop that reduced my speed in half and caused overheating. Once cleaned I got my speed back and overheating was less.
It may be too obvious to ask but a prop with a few barnacles can mess up the load. Also a dirty bottom will add load. Finally fishing line etc wrapped around the prop??? My point is that if this is a new problem then the engine/prop pitch match probably isn't the problem. I had a few barnacles on my prop that reduced my speed in half and caused overheating. Once cleaned I got my speed back and overheating was less.
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