How to Determine Auxiliary Diesel Range? - SailNet Community

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Old 01-30-2009
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Question How to Determine Auxiliary Diesel Range?

Is there a rule of thumb that I can use to estimate auxiliary diesel range. If I know the cruising speed and engine horse power, can I get a reasonable burn rate (mpg). In Yachtworld they usually mention tank capacity, so with mpg I can get range. I assume the miles in mpg, for a cruiser, is in nautical miles. I tried searching old threads but gave up on that.
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Old 01-30-2009
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For my boat I figure 1/2 gallon/hr at 5 knots with a 20 gallon tank that gives a range of 200 miles. This is a conservative estimate. My boat is 32' 12500#s with a 20 hp diesel. Of course everything changes depending upon conditions. That is why I use conservative estimates. Speed is a huge factor as speed really sucks up the fuel.
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Old 01-30-2009
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Marine fuel is generally determined in gallons per hour, rather than miles per gallon. This link should get you on the right track: All About Fuel and Your Boat - Nautical Know How
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Old 01-31-2009
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Originally Posted by PBzeer View Post
Marine fuel is generally determined in gallons per hour, rather than miles per gallon. This link should get you on the right track: All About Fuel and Your Boat - Nautical Know How
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PBzeer this is a good answer; thanks.

Quote from your link:

"Diesel engines consume about 1 gallon per hour for every 18 hp used. You can estimate the number of gallons consumed per hour by multiplying horsepower used by 0.055.

Note: An engine at cruising speed usually uses only about two-thirds of its maximum available horsepower. Most marine engines are designed to run continuously at between 60 and 75 percent of maximum speed. Diesels tend to be more toward the top of the range."

So the horsepower factor is .055.

Can I say that, in calm/flat, a 54 hp diesel running at 75% rpm cruising speed ((54*.75=40.5)*.055), burns 2.23 gph. And if the boat's cruising speed is 8nts and its fuel capacity is 50 gallons, then its range should be ((50/ 2.23)*8) or 179 nautical miles!?

I think I got it.
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My 10 ton ship, long-keeled, driven at about 5.5 kt has a consumption of about 9 nautical miles per UK gallon, or about 7.5 nautical miles per US gal, in flat calm.

The motor is a 35 hp diesel, when used flat out, can reach 8.5 kt. It's a while since I did that.

Work the engine hard, and you'll think there is another hole in the tank.

Get someone to help you. Choose a flat-calm day, with no tide. Pull the fuel hose off the tank fitting. Get a wee calibrated jug and fill it up with diesel.
Get your stopwatch and your calculator. Run your motor at various speeds and calculate the fuel consumption yourself.

It must be flat calm. Into a headwind it will alter. Into a chop the ship really drinks fuel. That 5.5 kt can become 1 kt into a wicked chop.

Write your figures down, and you can plot a wee graph to work out your optimum cruise.

To save fuel, run the ship slowly. It can save enormous amounts of fuel. The state of the hull surface also matters. In a wee sea trial about a decade ago, at Stonehaven Scotland, 1700 rpm with a wee thin weed growth meant a speed of 5.5 kt. Next morning, same conditions, with the weed film removed, that 1700 rpm became 6.6 kt.

That's quite a hit.

I simply cannot keep all the weed off the ship all the time though. It's nothing like the same problem in fresh water though, well, here at least.
.

Last edited by Rockter; 01-31-2009 at 07:15 AM.
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The only problem with this technique is that it won't work, since most diesel engines have a fuel return line, and unless your wee calibrated jug has the fuel return line run in to it as well, you'll just see how much the engine sucked up, but not what it actually USED.

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Originally Posted by Rockter View Post
My 10 ton ship, long-keeled, driven at about 5.5 kt has a consumption of about 9 nautical miles per UK gallon, or about 7.5 nautical miles per US gal, in flat calm.

The motor is a 35 hp diesel, when used flat out, can reach 8.5 kt. It's a while since I did that.

Work the engine hard, and you'll think there is another hole in the tank.

Get someone to help you. Choose a flat-calm day, with no tide. Pull the fuel hose off the tank fitting. Get a wee calibrated jug and fill it up with diesel.
Get your stopwatch and your calculator. Run your motor at various speeds and calculate the fuel consumption yourself.

It must be flat calm. Into a headwind it will alter. Into a chop the ship really drinks fuel. That 5.5 kt can become 1 kt into a wicked chop.

Write your figures down, and you can plot a wee graph to work out your optimum cruise.

To save fuel, run the ship slowly. It can save enormous amounts of fuel. The state of the hull surface also matters. In a wee sea trial about a decade ago, at Stonehaven Scotland, 1700 rpm with a wee thin weed growth meant a speed of 5.5 kt. Next morning, same conditions, with the weed film removed, that 1700 rpm became 6.6 kt.

That's quite a hit.

I simply cannot keep all the weed off the ship all the time though. It's nothing like the same problem in fresh water though, well, here at least.
.
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Last edited by sailingdog; 01-31-2009 at 01:38 PM.
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My motor does not have a return line.

It has never had a return line.

Sailingdog, do you think that I spend my time making this stuff up? Does it ever occur to you that perhaps I have tried it and it works?

Does that ever enter your head?

Or do you just know that it doesn't work, from 3500 miles away? Did I make up the figures about the 5.5 and 6.6 kt too? Are they fiction also? Maybe Stonehaven is fiction too...?...

Stonehaven Harbour Webcam


If his motor has a return line... mine doesn't... then stick a wee rubber hose on the end of the return line and route it to the jug then (it does not even have to be the same jug, you can use another container, then pour it into the jug, or use two jugs and subtraction). Either way, with a tall jug, well calibrated, and a stopwatch it is very accurate indeed. A calculator helps, but it's not essential.

Oh, and warm the motor first.

.

Last edited by Rockter; 01-31-2009 at 10:33 AM.
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I don't doubt your veracity but I have never heard of a diesel engine without a return fuel line (new common rail type excepted) and don't understand how that would work given the pressures involved. What make/model is it ? I'd love to learn about it.
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I haven't explored this methodically, but I know from the literature supplied in the manual and my own records of time run and speed attained that my 52 hp can push my boat at 7 knots expensively or 5 knots parsimoniously...so I do 5 knots and if there's nine-10 knots of wind from the right direction, setting all the sails can buy me another knot or so. With no engine and 10 knots of wind, I might do 4.5 knots of speed, so it's worth it to motorsail at times.

The point is that range is extremely hard to discern when chop, revs and speed are so interlinked. My solution is to add a third tank for "daytank" use, giving me 40 + 50 + 50 = 140 gallons. In a dead calm at 4 knots, that's nearly a week's worth of motoring or 600 NM of range...but when is it going to be a dead calm for nearly a week? Better I should sail!
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