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Discussion Starter #1
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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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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Discussion Starter #5
This I can use

PBzeer this is a good answer; thanks. :cool:

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.:cool:

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!?:rolleyes:

I think I got it.:cool:
 

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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.
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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.

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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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.

.
 

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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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Rockter—

While yours may not have a return line, what works on your engine obviously won't work on any that do, and the majority of small sailboat marine diesels that I've seen have fuel return lines. It isn't my fault that you're trying to apply your fairly unique situation to cases where your assumptions fail miserably.

BTW, I never stated that your engine had a fuel return line...you made that assumption, and we all know what that makes you. :rolleyes:

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.

.
 

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Whose thumb?

As a rule of thumb my 27hp Yanmar burns about .4 gallons per hour. I have an 18 gallon tank. And, if I pay careful attention and 'use the same thumb everytime' I won't run out of fuel....again :confused:

The boat is a 30' shoal keel, 9,500 lbs.
 

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So here's the summary of the above:

Some info is correct
Some info is partially correct
Some info is not correct but . . . .

Given that the OP asked about the range on his fuel tank and distilling all of the above info down, the range cannot be determined other than by experience and record keeping.

Because if you know you consumption at 73.663 % RPM because you think that's the optimum throttle setting and you've found a way to use a jug and a flow meter successfully, you still have wind, current, tide, chop, air temperature, all variable and all of them will affect your RANGE positively or negatively.

Range and fuel consumption are two different concepts.

So start recording your engine hours, trip distances (not from your paddle wheel log) and fuel used and build up your own record of what you range is likely to be under given conditions.
 

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Hey Rockter, I checked out your stone haven lnk. I was raised just up the road from you in Fraserburgh. The name of the boat in my avatar is "Isle of Skye". Still like to know what kind of engine you have.
 

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Here it is... the Volvo MD17C...

http://www.bluemoment.com/manuals/VolvoMD11C_D_17C_D.pdf

...and it most definitely does not have a fuel return line.

In 1997 I had to tear the top off the fuel tank to fix a fuel tank leak. There are two lines feeding off the tank, one for the motor, and one for the heater.
There are no other lines in the tank as I had to cut the top off it, seal it, have someone weld the top back on, then I had to drill the top to allow the motor fuel take-off and the heater fuel take off. The repair took me weeks, so I would have seen a return line.

The fuel metering trick works with my olde motor.

For anyone that wants the volume conversions, or indeed how it's done, with or without a return line, I will help.


Rockter.
 

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Rockter, thanks for the manual. Photos 2A and 2B on page 12 show a line connected to the bottom of the injectors and it is refered to as a "leak-off pipe'" What does this line do and where does it go ?
 

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Our old boat had a 27hp Yanmar, 13000 disp. 34' and over 1000+ hrs we averaged .6gph. We ran the engine conservatively, 22-2600rpm. This produced a cruise speed of close to 6K, which is almost hull speed so there was no need to run the engine faster (autoprop). Your fuel consumption will vary tremendously depending on how hard you push the boat. You can burn almost twice as much fuel per hour by using horsepower to exceed hull speed, or in some cases just reaching it. You can find your most efficient speed with a little experimentation. On a calm day with no current start at a low rpm, about 25% of your rpm range, and note your speed on the gps. Increase rpm until your speed goes up 1/2 knot. Keep repeating this until it starts taking more rpm to increase by the 1/2 knot, your most efficient speed will be just before it starts requiring more rpm for the same gain in speed. I usually run at one notch above this so the engine is under a bit of load.
 

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boatpoker :

Probably back to the injection pump. I will look next time. I might manage a picture or two.

It does not return to the tank though, as there is no return line to the tank, of that I am sure.

.
 

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My volvo MD11C has no fuel return line!!!! I use 0.5 gallon/hr and 200 mile range for planning purposes only. There are too many varibles to make a precise estimate of fuel consumption during a given trip. I almost ran out of fuel motoring into a strong wind one day. I was burning at least twice the amount of fuel as usual. A dirty bottom or a barnacle on your prop changes everything.
 

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Most engine manufacturers list their fuel burn rate. On an older model it may take some digging. At least then you can have an idea of run time per tank. Don't forget to factor in run time just to recharge batteries, etc.
 
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