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Old 08-25-2019, 11:36 AM   #1
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Perkins 4.108 WOT RPM

Anyone know what RPM the Perkins 4.108 should hit under load at WOT? The Perkins manual says 3400 (or 3600 cant recall). The mechanic who just did my survey says 4,000.

Right now it only hits 3,000 underway. I need to get a different pitch but don’t want to go smaller than necessary.

As a side note it is a Krogen Manatee. I called the Krogen headquarters the original specs show 4,000 RPM at WOT with a 22” prop but don’t show the pitch. Anybody with a manatee and the 4.108? Apparently this is the engine they installed with the first few before they switched to the volvo.
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Old 08-25-2019, 01:16 PM   #2
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You should stay with the continuous rating. The commercial rating is higher because the engine is usually run at various rpms. And the pleasure rating is higher because most pleasure boats run under 100 hours per year and will last enough years.
Changing the prop and running at 4000 rpm will shorten the life of the engine and probably not make much difference in speed.



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Old 08-25-2019, 02:44 PM   #3
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I agree w Lepke,
I’m fairly well acquainted w the 107/8. Had one in the Willard before we repowered. All the Willard owners that I know including myself propped them for 3000. They ran in taxi cabs in England to 4000 but not in boats.
My Perkins was overpropped when I bought her 14 or so years ago and Would only make 2750. Took out an inch of pitch and she rose up to 3000 and sounded and felt less loaded ... and she was of course.

Repowered w same BW gear and run the Mitsubishi at 3000 rpm also. Same bore and stroke on both the engines. The Mitsu seems just a modern version of the Perkins.
So can’tsitstill I’d prop to 3000 to 3100. Since it has a history of running higher I’d favor the high end of 3100.
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Old 08-25-2019, 06:42 PM   #4
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Quote:
Originally Posted by Lepke View Post
You should stay with the continuous rating. The commercial rating is higher because the engine is usually run at various rpms. And the pleasure rating is higher because most pleasure boats run under 100 hours per year and will last enough years.
Changing the prop and running at 4000 rpm will shorten the life of the engine and probably not make much difference in speed.


Ok thx!
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Old 08-26-2019, 05:09 AM   #5
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"Right now it only hits 3,000 underway. I need to get a different pitch but don’t want to go smaller than necessary."

If the boat is a displacement boat , what speed does 3000 give, and what is the waterline length?

If you are happy with what ever speed you obtain at 2700 and below your current setup is fine.

If the boat climbs on top of the water and you tow skiers , a prop change would be a good idea.


The 4-108 is not an industrial duty engine , a constant load of much over 2 GPH may reduce the engine life .
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