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Old 10-23-2010
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Which is real world RPM ?

I've a Yanmar 2GM20F. What is the real world rpm for this engine? 3400 or 3600? There's talk of rated rpm, max rpm etc.. all I need to know is which rpm should I pitch/size my prop for?
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Old 10-23-2010
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The continuous rating of 16 hp is at 3400 rpm. You can get 18 hp "peak" out of these engines at 3600 rpm, but only for an hour.

"Real world" is not an operative term I've heard used very much, but I guess it would refer to the continuos rating of the engine, which is 3400 rpm.

All that said, unless you have a 1:1 transmission, your prop won't be spinning at those speeds. These engines usually are fitted with Kanzaki transmissions containing reduction gears in both forward and reverse. There are various reduction options available, so you would need to look at your transmission to see what gear ratio is being used.

EDIT: Example, let's say hypothetically that you have a transmission with a 2:1 reduction in forward. That means at 3400 engine rpm, the prop/shaft will spin at 1700 rpm.

Pulling from memory, I seem to recall reduction options ranging from 2.2-2.8 or so for most Kanzaki sailboat applications. The ratio should be stamped on the transmission i.d. plate.

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Last edited by JohnRPollard; 10-23-2010 at 11:10 AM. Reason: added example
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Old 10-25-2010
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You should prop the boat so that with a clean bottom and no wind you indicate 3400-3600 rpm at full throttle. Many times the tachs are off so you need to check this also
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