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over powering your vessel

Posted: Thu Feb 23, 2017 11:39 am
by Scot2640
I have heard various things about using a larger than necessary diesel engine to power a vessel. specifically i'm referring to a full displacement trawler (21 ton) operating on a single screw at low RPM utilizing a less than a 100 HP engine (as the designer recommends).

setting aside the extra cost of a larger engine than needed and the lost fuel economy, is it bad for a modern diesel engine to be run under loaded?

i have heard it both ways. yes- the engine needs to be properly loaded to run efficiently and maintain its well being and avoid internal build up.

I have heard no- modern engines, particularly turbos, suffer no ill effects being run continually at low RPM.

As I shop for used engines, i find very few for sale in the 100hp range. however, there seem to be many 6bt's and comparable lehmans in the 200+ range at good prices.

what say you?

Re: over powering your vessel

Posted: Thu Feb 23, 2017 11:50 am
by hoodman
Could the extra power be useful in heavy winds or does the 100hp already account for that?

Re: over powering your vessel

Posted: Thu Feb 23, 2017 12:20 pm
by sproggy
Any engine (diesel or petrol, marine or otherwise) has an optimal operating rev range and load - the conditions under which it operates most efficiently, where efficiently means economically and cleanly. With modern fuels and modern engine designs you are unlikely to cause any engine damage or 'internal build-up' by running a more powerful engine at lower load as long as other factors like the cooling system allow it to reach its design operating temperature.

So other than paying more for the engine, it likely weighing more and taking up more space and the worse fuel consumption you are unlikely to do it any harm by running it more lightly loaded. In fact, properly maintained, it may last longer than a less powerful engine running closer to its peak output.

You mentioned turbo engines, though. If you are running an over-sized turbo engine at well below its normal load you will suffer low turbo boost (because that relies upon exhaust flow) and a far bigger loss of efficiency when running under-loaded than you would with a normally aspirated diesel engine. So if you want to use an over-sized turbo diesel you might want to consider changing the turbo(s) for a smaller unit which will give higher boost at lower loads but that's a complex route to take unless you've got a tame turbo engineer on hand.

Re: over powering your vessel

Posted: Sun Aug 13, 2017 5:08 am
by North
I know this is an old post... the Cummins 4BT (3.9M) is normally 150hp, very common and easily rebuilt- or available as rebuilt from several sources.

may not be relevant for a displacement vessel, but one thing I did for my boat was found my engine first, 4bt 150hp, and then decided to stretch the boat 10% as I had lots of power and the engine was at the top end of the designer's weight allowance, figuring this would allow it to carry the extra weight better.
Had I found a good deal on a smaller engine, say 100- 120hp, I may not have stretched the boat, or would have kept the hull lighter, rather than using thicker hull material.

Re: over powering your vessel

Posted: Sun Aug 13, 2017 10:44 am
by kens
The Cummins 4BT without the turbo is a 4B.
The 4B was rated I believe 80hp.
If my memory is correct, the Cummins 6B (no turbo) is 120hp.