I have a 1988 Legend 37. Last year I had a major tuneup done on the Yanmar 3HM35F (elbow, injectors, valves adjusted) and the engine runs great. When the mechanic took it for sea trials, he got some dark smoke at the high end of the throttle and believes that it it slightly overloaded. He recommended dropping an inch of pitch from the factory prop which is a 17x10 to a 16x10. Has anybody else encountered this situation? What did you do?
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don't drop an inch
Paul,before you start changing props ask yourself did the engine smoke at high end before the work was done? If not I would ask the mechanic to recheck his work then maybe consider a prop change. More often then not the factory prop is the best set up and hard to bet unless you are changing number of blades on the prop. I would also consult with a good prop shop as to what they recomend for you boat based on engine and trans. The key here is did it smoke before the work was done !
Paul,before you start changing props ask yourself did the engine smoke at high end before the work was done? If not I would ask the mechanic to recheck his work then maybe consider a prop change. More often then not the factory prop is the best set up and hard to bet unless you are changing number of blades on the prop. I would also consult with a good prop shop as to what they recomend for you boat based on engine and trans. The key here is did it smoke before the work was done !
Agree with Mechanic
Yes, I encountered the same situation when I repropped several years agowith the added downside that the stern got all sooted up. Black smoke at the higher RPMs is a good sign that the load on the engine doesn't match the load on the prop, this much is for sure. He is probably right about reducing the pitch; however, I don't remember if I increased or decreased the pitch on mine to fix it. I THINK the pitch was increased because it motors faster than before. I put on a Max Prop and it has gear teeth so the pitch can be easily adjusted in increments when the boat is out of the water. Since you'll have to take it to a prop shop to repitch it I'd suggest asking them for another opinion. As long as it's being worked on for the pitch it would be a good time to have it balanced too.
Yes, I encountered the same situation when I repropped several years agowith the added downside that the stern got all sooted up. Black smoke at the higher RPMs is a good sign that the load on the engine doesn't match the load on the prop, this much is for sure. He is probably right about reducing the pitch; however, I don't remember if I increased or decreased the pitch on mine to fix it. I THINK the pitch was increased because it motors faster than before. I put on a Max Prop and it has gear teeth so the pitch can be easily adjusted in increments when the boat is out of the water. Since you'll have to take it to a prop shop to repitch it I'd suggest asking them for another opinion. As long as it's being worked on for the pitch it would be a good time to have it balanced too.
Check with a prop shop
First, on a calm day, make a log of boat speed vs rpm. Pick a full range of speeds. Take this information, along with prop, engine, gear box, and boat data to a good prop shop. They will tell you if the prop is correct, or not.
First, on a calm day, make a log of boat speed vs rpm. Pick a full range of speeds. Take this information, along with prop, engine, gear box, and boat data to a good prop shop. They will tell you if the prop is correct, or not.
Possible drop.
Hello, In my opinion black smoke could be a few related problems. He is suggesting a reduction of pitch or diameter due to a possible excessive over loading due to the prop or the addition of some other engine driven item. If this is the case when you have the boat away from the dock in forward gear with full throttle, you will not have the ability of obtaining the rated 3,600 Rpm’s. For example 3,100 were the maximum achievable Rpm’s current, this can be caused because you may have installed a 150amp alternator, or some other engine driven item, which now drags the engine down. Not all engine over loadings are a result of the propeller. We find that the propeller is the easiest thing, however, to modify in order to reduce the overall load being put on the engine. If the Rpm test showed us that the engine is capable of obtaining the recommended Rpm’s you may be suffering from past carbon build ups. If this were the case I would purchase a de-carbonizer, fuel additive, and run it through my system. This will help to shed the motor of carbon deposits faster. But I do agree with the mechanic, that if the load to the engine is to high I would reduce that load by reducing the propellers pitch if no other problem is found. Good luck Kevin
Hello, In my opinion black smoke could be a few related problems. He is suggesting a reduction of pitch or diameter due to a possible excessive over loading due to the prop or the addition of some other engine driven item. If this is the case when you have the boat away from the dock in forward gear with full throttle, you will not have the ability of obtaining the rated 3,600 Rpm’s. For example 3,100 were the maximum achievable Rpm’s current, this can be caused because you may have installed a 150amp alternator, or some other engine driven item, which now drags the engine down. Not all engine over loadings are a result of the propeller. We find that the propeller is the easiest thing, however, to modify in order to reduce the overall load being put on the engine. If the Rpm test showed us that the engine is capable of obtaining the recommended Rpm’s you may be suffering from past carbon build ups. If this were the case I would purchase a de-carbonizer, fuel additive, and run it through my system. This will help to shed the motor of carbon deposits faster. But I do agree with the mechanic, that if the load to the engine is to high I would reduce that load by reducing the propellers pitch if no other problem is found. Good luck Kevin
Pitch or Diameter
You mentioned your prop is a 17 x 10. I assume that corresponds to a 17 inch diameter with a 10 inch pitch, and if it is the original prop, then it probably is a two blade prop. I would also expect that if the 17 inch diameter is an acceptable one, that is, if there is enough clearance, then you would need to change the pitch from 10 in. to 9 in. if the engine cannot reach its max rpm. My gues is that the 10 inch is about right in your case, but then one never knows what other loading is on the engine besides the prop. Would be nice to hear from owners with the same gear as yours for reference. Ernie
You mentioned your prop is a 17 x 10. I assume that corresponds to a 17 inch diameter with a 10 inch pitch, and if it is the original prop, then it probably is a two blade prop. I would also expect that if the 17 inch diameter is an acceptable one, that is, if there is enough clearance, then you would need to change the pitch from 10 in. to 9 in. if the engine cannot reach its max rpm. My gues is that the 10 inch is about right in your case, but then one never knows what other loading is on the engine besides the prop. Would be nice to hear from owners with the same gear as yours for reference. Ernie
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