Perkins 4.108

boatmike

boatmike

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This might seem a question that can easily be answered on t'internet, but I am looking at a boat with a P 4.108 which the owner insists is rated at 36HP. On line all I can see is that the 4.108M was rated at 45HP at 3600 RPM for pleasure craft which is 36KW
not 36HP. Am I right that he is confusing KW with HP or was there ever a downrated version???
 
neil1967 said:
I have a Perkins 4108 - I have seen them variously quoted at 48, 49 or 50HP, but not 36 - I suspect you might be correct that the owner is slightly confused.
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Mine was rated @ max 3k RPM which i think is around 36>37 hp. The 4108 would give 48hp @ 4.5k rpm
 
I have a Perkins 4 108 and I'm sure it is rated at 37 hp @ 3000rpm and an intermittent rating of 51 hp @ 4000rpm

I just came across a Workshop Manual for the 4 107 and 4 108 which looks very much like my manual (which I don't have with me at the moment)

If you look at page 11 (Part B) of the manual you will see the rating details.
 
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The propeller expert wanted to know what the recommended cruising rev was for a 4 108 was and I told him 3000 rpm so he did his calculations and supplied a prop with that in mind. But I'll be happy to sit on 2500 rpm at cruise....
 
neil1967 said:
I stand corrected :) I wonder how many people get 3000 or 4000 RPM out of their 4108 - I don't think I can get mine past about 2600.
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My propeller expert would suggest it is "over propped" if that is the cas

Are you over-propped? - Ocean Navigator - January/February 2017
www.oceannavigator.com › January-February 2017

Dec 29, 2016 - Overpitched and oversized propellers (over prop) are common .... When a boat is over-propped it means that the engine is not working at its at its optimal torque range.

http://www.oceannavigator.com/January-February-2017/Are-you-over-propped/
 
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Interesting article. I don't have any other signs of over propping - temperature is OK, no sooty exhaust, etc. I have a maxprop, so I can change the pitch of the propeller. I had to change the gearbox a few years ago and couldn't get the exact same ratio - the new box was 2.05:1 rather than 1.86:1, so on the advice of darglow I increased the propeller pitch from 18 to 20 degrees, which restored my cruising speed. I would like to be able to play around with the pitch, but it would mean a lift each time, which would get expensive!
 
neil1967 said:
I had to change the gearbox a few years ago and couldn't get the exact same ratio - the new box was 2.05:1 rather than 1.86:1, so on the advice of darglow I increased the propeller pitch from 18 to 20 degrees, which restored my cruising speed. I would like to be able to play around with the pitch, but it would mean a lift each time, which would get expensive!
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My Borg Warner gear box is 2.9:1 with a 21 inch diameter propeller and the pitch is 15 in. There's a magical figure of 87%(?) that the propeller experts aim for and mine is close to that according to him.

After discussions with the prop expert I have come to the conclusion that the motor has to be matched to the prop in the same way as a tractor engine must be matched to the wheels. Not much point in having a powerful tractor engine if the rear wheels are bicycle wheels!
 
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