We just completed sea trials for what will likely become our new boat. One of the items that the surveyor pointed out was that the 27hp Yanmar did not reach maximum rated RPM under load (while under way)and was concerned that the prop was oversized. We saw some blue smoke while motoring at max RPM. We turned about 3400 RPM at max throtle under load and about 3900 RPM in neutral. This was confirmed with a strobe. The boat is equipped with a three blade prop, that I am told is the standard for that boat. The boat is a 1998 model. The motor is clean and looks to be in good condition. Can anyone tell me if this is normal or if you have had similar problems? Thanks, Larry
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what is the prop size?
What is the prop size? You might be just a hair overpitched but need more info for a good evaluation.Bases on what you said I would not be overly concerned (as to void the sale) but would try to get to the bottom of it.A good prop shop can run the numbers and get you a good size for comparasion.
What is the prop size? You might be just a hair overpitched but need more info for a good evaluation.Bases on what you said I would not be overly concerned (as to void the sale) but would try to get to the bottom of it.A good prop shop can run the numbers and get you a good size for comparasion.
Larry, you're actually pretty close-3400 rpm is close to max rpm under load, typicall you should be propped to hit hull speed around 80-85% of max load speed. at that point, the engine will last a lot longer and run quieter. most 3gm30's are run around 2500-2800 rpm. You should also watch the stern as you raise rpms, when you start to dig in that's as fast as the boat will go.
Don't think it was standard......
Larry: I do not think that a three blade prop was standard on the boat, but that really does not make any difference. You should go to a good prop shop and have them size a prop for your boat. I would guess that all you need is to have the prop re-pitched, probably 1 deg of pitch will make all the difference in the world.
Larry: I do not think that a three blade prop was standard on the boat, but that really does not make any difference. You should go to a good prop shop and have them size a prop for your boat. I would guess that all you need is to have the prop re-pitched, probably 1 deg of pitch will make all the difference in the world.
Prop sizing
Most engine manufactures publish a maximum rpm and a continous run rpm (usually about 80% of max). The govenor is usually set at 10% above max. Assuming that 3900 is govenor limit, then 2800 should be close to continous run rpm. If you can run 2800 without the engine smoking, then you should be propped about right.
Most engine manufactures publish a maximum rpm and a continous run rpm (usually about 80% of max). The govenor is usually set at 10% above max. Assuming that 3900 is govenor limit, then 2800 should be close to continous run rpm. If you can run 2800 without the engine smoking, then you should be propped about right.
Prop Size
YOur boat seems to be slightly overpropped, ie the pitch is a bit too course. A properly propped cruising sailboat should hit or slightly exceed the engine's maximum horsepower rpm in flat water with no headwind. For the Yanmar 3gm30f this is 3,600 rpm for a maximum of 2 hours or 3,400 rpm continuous. I believe that you should be slightly underpropped, so that when you really need all that the engine and prop can deliver- while powering against a strong head wind/sea the engine will run up to its maximum continuous rpm. Against a head wind/sea you will be moving much slower over the water and therefore your maximum rpm will be slightly below what you can do at WOT in flat water. If you are underpropped, you will be able to get the most out of the engine in this situation. If you start out overpropped in flat water, your engine will bog down and smoke while powering against a head wind/sea.
YOur boat seems to be slightly overpropped, ie the pitch is a bit too course. A properly propped cruising sailboat should hit or slightly exceed the engine's maximum horsepower rpm in flat water with no headwind. For the Yanmar 3gm30f this is 3,600 rpm for a maximum of 2 hours or 3,400 rpm continuous. I believe that you should be slightly underpropped, so that when you really need all that the engine and prop can deliver- while powering against a strong head wind/sea the engine will run up to its maximum continuous rpm. Against a head wind/sea you will be moving much slower over the water and therefore your maximum rpm will be slightly below what you can do at WOT in flat water. If you are underpropped, you will be able to get the most out of the engine in this situation. If you start out overpropped in flat water, your engine will bog down and smoke while powering against a head wind/sea.
Thanks
Just a quick note to say thanks to everyone who offered advice. I have been very thankful of all the help that I have received these past months from all of the great people on this website. Thanks again!
Just a quick note to say thanks to everyone who offered advice. I have been very thankful of all the help that I have received these past months from all of the great people on this website. Thanks again!
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