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Old 30-09-2011, 10:59   #16
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Re: Gallons per Horsepower-Hour

burn rate. The same right? IN the PNW we have a lot of high current situations at times. It always seems like the engine is working harder in these situations, prop vibrations occur, occassionally the temp gauge goes up just a touch from normal. But the logic says it's no different than just motoring in calm water. I guess it is a little differnt as the currents are not always a smooth flow past the hull like the water is in calm water....
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Old 30-09-2011, 11:10   #17
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Re: Gallons per Horsepower-Hour

Jeff, we have a 2003 in a 31 ft twin keel yacht which weighs 5100 kgs in cruising form.

We average 2000 - 2200 revs (5.5 knots) and use 0.5 litres per mile. Engine sounds a bit harsh at any higher revs above 2600 although it should go to 3000 we don't.

A friend uses 1.7 litres per hour in the same type of yacht, so similar values.

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Old 30-09-2011, 11:42   #18
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Re: Gallons per Horsepower-Hour

Would like to see a Gallon Per hour per Knot at a given displacement instead of per HP.

I get 1 GPH when I run at 2300 RPM on a 3,300 max engine and make ~6.8 knots. If I ran at full throttle (3,300) I would go to ~7.2 and probably burn 2GPH. That last HP is very inneficient.
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Old 30-09-2011, 12:23   #19
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Re: Gallons per Horsepower-Hour

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Originally Posted by barnakiel View Post
The ratio is not fixed but pretty close to 200 grams per hp per hour for most engines in their typical use. The hp per revs can be read for each engine's specs.

b.

Most engine specs show hp versus rpm for a propeller which is sized to match maximum rpm at WOT. They also may show fuel consumption at various rpm assuming a typical boat motoring in flat water with THAT propeller. Your own experience may vary widely.
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Old 30-09-2011, 12:34   #20
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Re: Gallons per Horsepower-Hour

Cheech, you are correct.

The gallons per hour you calculate using specific fuel consumption in gal/HP-hr times HP, is based on the HP expended, not the rated HP. So, one must make an estimate of the HP expended. You would need other data, either a boat mfg estimate of HP to push the boat at desired speed, or a calculated value for HP, summing up the loads for the propeller a certain speed, and taking into account cutlass bearing (losses), shaft seal losses, transmission losses, and gear box losses.

As a worst case fuel consumption, you could simply use rated engine power, and that will give an estimate of fuel consumption, albeit on the high side.

I was just answering his question regarding how to mathematically convert the value.
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Old 30-09-2011, 13:14   #21
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Re: Gallons per Horsepower-Hour

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Here's a teezer for the group!: If a boat is motoring at hull speed (not SOG)against 5 knots of current, will it burn more fuel?
A related question to what you mentioned is what is the WOT rpm under various current flow. For example at the dock when the boat is stationary my WOT number is some 200 rpm lower than when the boat is free to move. With that observation, I suspect that current flow plays a roll in the WOT rpm and if that is indeed the case then the answer to your question may not be the obvious one.
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Old 01-10-2011, 09:59   #22
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Re: Gallons per Horsepower-Hour

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But isnt gal per hp per hr, based on the HP you are USING rather than the rated HP of the engine? (...)
One would think this is obvious.

If you are reving very slow (say at the idling speed) you are using only some hp of the max hp of your engine.

The fuel (unit) / 1 (!) hp ratio is nearly fixed, but not fixed, at approx 200 grams per 1 hp per 1 hour.

Another issue is that to go X faster we need to generate more than the proportional amt of hp - because of how hull physics work.

Then again there is the fact that the prop will be optimised for only one speed (unless we have a variable pitch one).

Here:

http://www.hallberg-rassy.com/HR310/HR310propeller.pdf

is a very fine pdf by HR where one can read and judge how the various factors of speed / prop choice / etc. affect how far we can drive on one tank. It can be read vis-a-vis the engine manufacturer's engine curves.

Cheers,
b.
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Old 01-10-2011, 22:14   #23
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Re: Gallons per Horsepower-Hour

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Marine diesel engines range from about 15 hp per gph for a NA diesel operating at the lower ends of its operating range (down to about 50% rpm) to 20 hp per gph for a modern, common rail, turbocharged engine operating heavily loaded.

My friend, you know your stuff. I have some real word numbers that will drive your point home. On my last "vessel" a dynamic positioned drilling rig. I would hold position with (6) 5000 hp electric thrusters and (7) 4.6 Mw gensets, so total production of 32 Mw. Hotel and drilling loads were about 1 Mw, the remainder to be used to hold position against environmental loading. A 4 kt current combined with 50 kt winds had me close to being maxed out and a possible shear and pinch off the drill string and disconnect of the riser. This procedure costs the drilling company a bit over $1 mil, so they hope their deckofficers can old position. I was running 25 Mw just for thrusters and had a fuel consumption of over 1,000 gal per hour. By your numbers, works out close, but in our application we were doing about 26 hp per gph. But that would make sense because the most efficient diesels are the big inlines we have aboard container ships, 60,000+ hp @ 118 rpm. Those numbers are a lot better than the drill ship's 900 rpm medium speed diesels.
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Old 02-10-2011, 02:21   #24
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Re: Gallons per Horsepower-Hour

Fuel Consumption: Information from Answers.com
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Old 02-10-2011, 02:46   #25
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Re: Gallons per Horsepower-Hour

I always work on the general rule of

1/3 liter per hp hour for two stroke outboards
1/4 liter per hour for four stroke petrol inboards or outboards
1/5 liter per hour for modern inboard diesels.

Check your engines rev curve to find out hp for the revs you are using.

Voila, fuel consumption rate.
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