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Old 27-09-2009, 06:28   #1
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Perkins 4-108 - How Much Fuel Should I Burn an Hour?

At what rpm will I burn the least amount of fuel? How much fuel do I burn per hour approximately?

rgds,

Chris
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Old 27-09-2009, 09:33   #2
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Chris,

It isn't the engine that defines fuel consumption, but the design (size/weight/shape) of the boat, type of propeller and the amount of growth on under water parts of the boat (especially the prop).

For optimal range, think of 70% of hull speed for a good start.

cheers,
Nick.
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Old 27-09-2009, 09:53   #3
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Quote:
Originally Posted by s/v Jedi View Post
Chris,

It isn't the engine that defines fuel consumption, but the design (size/weight/shape) of the boat, type of propeller and the amount of growth on under water parts of the boat (especially the prop).

For optimal range, think of 70% of hull speed for a good start.

cheers,
Nick.

Hey Thanks Nick,
I'm just trying to get close. She's beamy 14'09" 42ft LOA, wieghs 20 tons. bottom is clean. average sea speed w/ calm conditions 5kts. We're not going to win any races. She's a motorsailor. I estimate in a 24hr run I burn 16gallons at 2,200 rpm.

Thanks,
Chris
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Old 27-09-2009, 10:39   #4
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That sounds about right for your boat. My 4-108 in a 44 ft , slightly lighter boat burned about .6 gal per hour. The sweet spot for that engine combo was about 2200-2400 rpm. and the boat would do 7+ knots there.
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Old 03-10-2009, 08:13   #5
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I too have a 4-108 on the boat built in 1985. Its rating is 4000 rpm max with a continuous rating of 3600 however I've never run it that high. Here are some of the stats: 37 ft Tayana design weight 24k and on travel lift around 30000 pounds... 2.1:1 Borg Warner tranny....17 inch 3 blade Max Prop set at a pitch of 14 inches

Cruise at a little over 6 knots @ 1900 rpm
WOT at 2300rpm gives a little over 6.5

For years I've been thinking that I've been operating at a higher rpm since my tach was in error. The above numbers were based on a photo tach.
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Old 03-10-2009, 09:47   #6
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I think something needs to be cleared up here. We seem to be mixing up four different measures of efficiency.

The term used to describe an engines efficiency is called its brake specific fuel consumption (BSFC). The definition is the amount of fuel burned per amount of horsepower generated. An engines specific fuel consumption is determined by the engines design, and nothing else.

A diesel engines specific fuel consumption will varie with its RPM. For a diesel, the best specific fuel consumptions are found in the diesels higher RPM ranges. What this amounts to is that if you want to get the most horsepower for the amount of fuel burned, then run your diesel in its higher RPM ranges.

The second measure of efficiency is the amount of fuel burned per amount of speed generated. This measure of efficiency is determined by the engine, the drivetrain, the hull, the hull speed and environmental factors such as wind and chop. To double the amount of speed a boat goes through the water, you pretty much must cube the horsepower. This means that you can save an enormous amount of fuel by backing down a few knots.

The third measure of efficiency is the amount of fuel burned per unit of distance. Slowing down the boat maximizes this efficiency by a tremendous amount.

Pitching the boat correctly really has nothing to do with any of these efficiencies. To maximize this fourth type of efficiency, you must pitch the boat so that it can reach its maximum governed RPM. Overpitching as well as under pitching will cause less efficiency. Think of this curve as the X-axis being the pitch and the Y-axis as the fuel burned per mile. The curve is a parabola and you want to be at the very peak of the parabola. It works out that you will be at the top of the parabola if you pitch the propeller so you can reach the maximum governed speed, and no more.

What all this amounts to is that with a properly pitched boat, slowing down will minimize your fuel consumption per mile even though your diesel engine is not running at its most efficient RPM on the brake specific fuel consumption curve. The reason is that it takes a lot less energy to push a hull through the water at lower speeds. This difference is a much greater difference than the difference between the BSFC at different engine RPM's.

For practical reasons relating to engine longevity, its a good thing to run your diesel at maximum governed RPM at full load periodically to burn the carbon out of the combustion chamber. Cummins recommends 1/2 hour for 10% of the engines operating time.

I hope this helps to paint a clearer picture of the four different types of efficiencies that are being discussed.

Brake_specific_fuel_consumption-Wikipedia
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Old 03-10-2009, 10:39   #7
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You will burn least at lowest rpms, unloaded engine. But this is not what you want.

What I believe you want is the lowest consumption per mile motored. In such a case, if your engine, train, prop are well matched with boat's displacement then we are talking of rpm in the 50-70% of the max. The speed then will be probably roughly about sqrt of your water length (may vary up and down from there.

To get exact numbers you must take notes - your rpms ver dist sailed ver fuel used (per mile). Some newer engines have dials showing ltr/hour and ltr/mile figures which are great help - esp as the optimum rpm will vary with wind, wave action and whether you carry any sail or not (to name but a few).

b.
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