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Date Posted: - Friday - 06/30/06 - 11:59pm
Author: Mike Cox
Author Host/IP: NoHost / 72.24.202.31
Subject: Recommended Cruising RPM
In reply to: Will Holoman 's message, "Symptoms" on - Friday - 06/30/06 - 5:04pm

Will,
You will find varying opinions from diesel mechanics and other "experts" on the recommended cruising RPM for the Perkins 4-107/4-108. These opinions will be in the range of about 2300 to as high as 2900. In practice, I think you will find a lot of sailors cruising 4-107/108's between 2200 and 2500. The power curve graph for the 4-107 shows that with a typical propeller load the engine should top out with a continuous rating at 3000 RPM and be putting out about 36 SHP. The Average Fuel Consumption chart for the 4-107 shows .5 GPH @ 1500 RPM, .66 GPH @ 2000, .75 GPH @ 2250, .82 GPH @ 2500, 1 GPH @ 2750, and 1.3 GPH @ 3000. An overpitched or oversized prop will not be able to reach the max continuous rating of 3000 RPM. If a 4-107/108's actual max RPM is very far from 3000 RPM due to a prop mismatch, it will not be delivering anywhere near it's rated continuous output of 36 Shaft Horse Power. If a 4-107/108 is topping out at 2000 RPM, then it is really only able to put out about 15 to 20 SHP before it will overload. A lot of sailors only focus on the max hull speed. If their engine is getting them close to hull speed they're satisfied without regards to RPM, but it may not be good for the engine.
Mike

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Replies:

[> [> [> [> Thanks for the comments -- Lawrence Killingsworth, - Wednesday - 07/ 5/06 - 5:16pm (NoHost/198.185.135.7)

Thanks for the comments, everyone. I don't have any of those power curve graphs, etc., so when the former owner said "Run her at 1500 RPM," that's what I did. I may just rev her up a bit more next weekend.

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