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Old 15-02-2010, 13:54   #1
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Fuel Economy

I see boat listings that say the engine does 5 knots at 0.6gal/hr. Last year I figure my Yanmar 44 HP used about 1.1 gal/hr averaged out during the year. Now I admit that I motored it hard lots times (like half the hours) due to needing to make time so wasn't running it at the most efficient speed. So I don't know what it would use if I ran it slower at the best efficient speed. I think my first 3 days on the boat when I was bringing it up from where I bought that I got only about 1.25 gal/hr. But wonder what others get for their fuel usage.

And I'm not talking about motor sailor boats. Thanks!
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Old 15-02-2010, 14:24   #2
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Diesel engines need an amount of fuel for each hp produced per hour.
A typical figure is some 175 gram/hp/hr.

A 44 hp running at full load would need 8 kg or well over 2 gallons per hour. I guess you weren't running it flat out. It all depends on how hard you run your engine and how much boat needs to be moved.

When I was really worried about fuel consumption it was with figures like 40 tonnes per 24 hours (make well over 10.000 hp and pushing 20.000 tons on ship).
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Old 15-02-2010, 16:55   #3
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Slowing down will make a big difference. The reistance of the hull through the water increases as a square as the boat speed increases. As sigmasailor says, hp hours are directly related to fuel burn rate. In addition, the rpm will be higher which means there will be more energy wasted in spinning the engine.

Running a 40hp 4108, I am used to getting 3/4gal/hour but we don't run it that hard.
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Old 15-02-2010, 17:14   #4
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62 hp Volvo, 24,000 lb. disp., 6-6.5 knots we average 1.0 gph.
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Old 15-02-2010, 18:35   #5
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So many variables and finding the sweetspot has to do with how she was set up unless you have a variable pitch prop. As stated in a different language , diesels burn about .45 lbs per hp/hr (varying some) used. Fixed propellers can only give max power and max speed for one rpm. for the given application. However, as pointed out above, the faster you go, the more friction is being faught by engine and boat. Then there are other efficiencies having to do with rpms that the engine was designed for. And of course limitations on hull speeds, speed/length, wave period. In the example above, jrd22 is using about 15 hp.(at .45lbs/hp and a 1gph burn) Acording to my book and if he is set up efficiently, he is about 35 foot waterline at a little over 10 T. If that's the case, at 20 hp used he will get a little over 7 kts and only 8 kts at 35 hp used and I would imagine the numbers would be lower because he probably doesn't have a variable pitch prop of course...
But, the basic idea for fuel economy that you have some real control over is speed. The slower you go, the better you'll do. Just keep her in the operating range specified by the engine manufacturer for the pitch prop you have for the boat. Unloaded and low engine speed can cause "slobber" and premature cyl wear as well as carbon buildup.
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Old 16-02-2010, 04:52   #6
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Wasn't really looking for various engineering whys, just how much people's engine use per hour as comparison.
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Old 16-02-2010, 05:47   #7
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Don,

We use .72 gals/hr (US gallons) running at speed = 1 x sqr(lwl) . . . (sqr=square root)

If we are running engine full out, we can go 1.34 x sqr(lwl) but we then use 3 gal/hr.

That last 34% costs you a ton in terms of fuel economy.

This is a 75hp engine on a 47' boat.
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