Engine Torque-v-Fuel Consumption

[peasea]

Does motoring at the r.p.m. at which the maximum torque is developed,give the minimum fuel consumption for a marine diesel engine?

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[PeteMcK]

Not necessarily, but most diesels (non-turbo, that is) give a fairly constant torque across their whole working range of rpm. You could fill a book with the details of the various factors involved making analytical efficiency predictions (using peak temperatures and pressures, cut-off ratios, pumping losses, etc., just for a start), but the easy way to predict is to look up the tables or charts of actual test-bed performance that most makers provide.

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[qsiv]

I'd say motoring at just under hull speed would be more efficient, irrespective of engine. Many yachts have engines fitted that will allow them to get close (and in some cases exceed) their natural hull speed. It is remarkable how little power is actually needed to reach lower speeds. The extra power fitted isn't necessarily wasted, as it comes into play when manouevering and when punching into wind and waves.

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[PeteMcK]

I'd go along with that, if that's what the question was really about. For my own boat, motoring at 6 knots rather than 7 (hull spd about 7.2) my fuel consumption per hr is 50% lower (that's for a 14% speed reduction) and that's all to do with hull resistance. My engines's data sheets give an almost exactly linear relationship between power output and fuel consumption. Hull resistance increases almost linearly with speed until you get near to the so-called hull speed which, in reality, is not a clear cut step. Going slower than say 80% of hull speed isn't going to produce such noticeable fuel savings per mile compared to that top 20%.

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[Chris_Robb]

I usually motor at 1600 RPM (Perkins 4236). When it is flat calm the govenor only gives enough deisel to the engine to provide the torque for 1600 revs in flat calm. If it blowing 7 on the nose - 1600 rpm will be using a much greater amount of fuel as the torque required to turn the prop is considerably more.

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[PeteMcK]

All of my above assumed flat calm apart from the linear power v. consumption comment. Punching into F7 at 5 knots I use 1.3 gph and 2200 rpm and something which sounds very much like full torque: at 6 in flat calm I use 0.5 gph and 1400 rpm and the noise being more like a fast idle (Thornycroft T90) . As a rule of thumb, a diesel will use about 1 gph for every 20 horsepower it's churning out, virtually regardless of the rpm it's being run at. Torque times rpm = power = gallons per hour.

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[Chris_Robb]

That explains why at the end of a voyage from Chichester to Ushant into a steady 8 we burnt nearly 3 galls per hour - couldn't believe how much we had used. under normal circumstances and 7 knots we burn about 1gph.

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