Max Cruising RPMs for Yanmar - Page 4 - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #31  
Old 04-10-2008
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One of the important factors which nobody has so far mentioned is whether the max rpms are no-load or loaded. The max rpms that you get from the engine spec are no-load. Loaded you can reduce that number by around 300 rpms, then take 80-85% of that as your cruising speed. It is a good idea to know that your tach is accurately reflecting the rpms of your engine. Be aware that changing your alternator can change the tach reading as well.
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Old 04-10-2008
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Quote:
Originally Posted by petegingras View Post
jimmalkin,
White smoke i think fuel contaminant, i.e. water, tank growth.
more likely blown head gasket. enough water in the fuel to cause white smoke means your racor is full and it won't be blowing white smoke for long! If you see black smoke you need to throttle down, and if it doesn't go away something is askew (wrong prop, back pressure being most likely). The "older diesels smoke more" theory doesn't hold up. Ideally, the exhaust of a 1 cyl 8hp all the way up to a supertanker should be clear under normal operating conditions.
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Old 04-11-2008
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Quote:
Originally Posted by ebs001 View Post
One of the important factors which nobody has so far mentioned is whether the max rpms are no-load or loaded. The max rpms that you get from the engine spec are no-load. Loaded you can reduce that number by around 300 rpms, then take 80-85% of that as your cruising speed.
Not sure if this is correct. My engine handbook states maximum output of 4,000 RPM and does'nt specify whether it is load or unload. However in the detailed engine manual it states maximum no-load at between 4,500-4,600.

I think you have research your particular engine to work out what your engine specs are assumming
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This was advice I got from a mechanic and we looked at the specs on a number of engines. You can test it out yourself. Put the engine in neutral and take it to max. Then do it under load. If you are loosing much more than 300 RPMs then you are over propped and loosing less under propped ie too much or too little pitch.
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Old 04-11-2008
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Quote:
Originally Posted by ebs001 View Post
This was advice I got from a mechanic and we looked at the specs on a number of engines. You can test it out yourself. Put the engine in neutral and take it to max. Then do it under load. If you are loosing much more than 300 RPMs then you are over propped and loosing less under propped ie too much or too little pitch.
For some reason I feel the strong urge not to find the max RPM. Never done it on my boat and don't want to. Reminds me of working on a car when I was in HS. I reved up the engine from the throttle body, and the gas pedal stuck! Tach was pegged at 8K by the time I got her shut down. It lived, but that was the last time I will ever redline an engine, unless of course I'm trying to destroy the thing


Now here is a question: If I throttle up, say to 2800 RPM, with no way on the boat, a dead stop the engine quickly reaches a specific RPM. As the speed of the boat sloooowly increases and reaches its equilibrium (about 5 knots at 2800) the RPM has not changed. Is this a result of the governor? If so, why is it that I loose about 200 RPM when I put a discharged battery bank online? Am I under-propped?

Last edited by sailboy21; 04-11-2008 at 09:35 PM.
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Old 04-11-2008
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Sailboy, if you're asking why you loose RPMs when you are charging your batteries the answer is that it takes energy to operate your alternator especially a high output one operating in the bulk phase. In my experience the rpms are reached more slowly than you suggest and so a boat at 0 velocity is not reving at the same rate as it is when it is doing 5 knots. The difference between say 3 and 5 are negligible when reading an anologue tach. Are you under propped? If you don't want to test the max on your engine than you'll have to go to the manufacturer's specs to find out.
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