# How to Determine Auxiliary Diesel Range?



## Winderlust (Jan 26, 2009)

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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## MoonSailer (Jun 1, 2007)

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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## PBzeer (Nov 11, 2002)

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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## Bene505 (Jul 31, 2008)

Great post. Thanks. Rep to you.



PBzeer said:


> 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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## Winderlust (Jan 26, 2009)

*This I can use*

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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## Rockter (Sep 11, 2006)

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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## sailingdog (Mar 19, 2006)

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.



Rockter said:


> 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.
> 
> ...


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## Rockter (Sep 11, 2006)

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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## boatpoker (Jul 21, 2008)

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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## Valiente (Jun 16, 2006)

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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## sailingdog (Mar 19, 2006)

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. 



Rockter said:


> My motor does not have a return line.
> 
> It has never had a return line.
> 
> ...


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## Joesaila (May 19, 2007)

*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 

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


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## Omatako (Sep 14, 2003)

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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## boatpoker (Jul 21, 2008)

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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## Rockter (Sep 11, 2006)

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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## boatpoker (Jul 21, 2008)

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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## jrd22 (Nov 14, 2000)

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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## Rockter (Sep 11, 2006)

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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## MoonSailer (Jun 1, 2007)

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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## seabreeze_97 (Apr 30, 2006)

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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## Winderlust (Jan 26, 2009)

*My original question was how to determine range.*

I feel stupid and naive for my original post.

I have no boat. I'm looking for a cruiser.

When I look in YW part of my criteria is range. I was trying to get a "rule of thumb" to compare yacht ranges, based on engine size, tank capacity, cruising speed, etc.

I am concerned about tankage, additional tankage and placement of such. It seems that not all offshore cruising boats are set up for it. Tankage is part of the issue. Other issues are other issues.

Sorry I wasn't more clear.


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## chucklesR (Sep 17, 2007)

Winderlust,
Given the above what you really need to know then is what range you should be looking for given your intended use of the boat - for example if you intend blue water sailing you really shouldn't be considering boats that are under tanked. Having jerry cans strapped to the rail is not good in rough weather, having a dozen of them strapped on is just ugly and not, IMHO safe.


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## Winderlust (Jan 26, 2009)

chuck,

I know I want longer range vs shorter range, in my future boat. I was asking for an easy way to estimate boat ranges, all things being equal: seas, wind, etc. This helps me in my boat search. Someone already provided a link that said each 18 hp roughly burns .5 gal per hour. So if my engine is producing 36 hp at cruising rpm, then it will burn 1 gal per hour, at cruising speed. From there I can determine average range based on tankage and cruising speed. Does this not sound reasonable?

Now I'm interested in the fuel return line or no return line controversy.

W.


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## xort (Aug 4, 2006)

Charlie Wing wrote an intersting book called the Livaboard Report. He did a bunch of research on every aspect of living aboard, poling cruisers on what they had, what they used, etc.

One question was "how many nautical miles per gallon do you average under power"? and then he graphed "estimated fuel efficiency vs. length".
30' to 33' averaged about 11 kmpg. 36' averaged about 10 kmpg and 39 to 42 averaged around 8.5 kmpg. Obviously there will be ranges within each category depending on engine size, hull config, etc. But this is an interesting estimate.
He also graphed estimated HP of different boats. 30' to 33' had 22hp, 36' had 35hp, 39' had 40hp and 42 had 60hp.

One item not mentioned in all this is the prop. Variations there can have a big effect.


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## Rockter (Sep 11, 2006)

One problem is knowing how many horsepower you are using. Checking revs is a very rough guide as it's the governor position that determines the fuel delivery rate, and not the revs, really. Into a stiff headwind and chop, the governor is delivering far more fuel to maintain the revs than in flat calm, and downwind, the motor will use much less fuel to maintain the given rev.

This is why I liked the fuel delivery rate check, measuring exactly how much is used, calibrating in flat calm.

That 36 ft class stated above was covering about 10 km/USgal, or about 6.25 road mile/USgal, or 5.44 nautical mile/USgal, 6.5 nautical mile/imp gal.

If I nurse the motor, I can do better than that. I have to hunt the revs for a low rev that the motor seems happy with.

So much is dependent on how much water you are disturbing. Ease that throttle if you are low on fuel and it will go a long way.
.


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## chucklesR (Sep 17, 2007)

Winderlust said:


> chuck,
> 
> I know I want longer range vs shorter range, in my future boat. I was asking for an easy way to estimate boat ranges, all things being equal: seas, wind, etc. This helps me in my boat search. Someone already provided a link that said each 18 hp roughly burns .5 gal per hour. So if my engine is producing 36 hp at cruising rpm, then it will burn 1 gal per hour, at cruising speed. From there I can determine average range based on tankage and cruising speed. Does this not sound reasonable?
> 
> ...


I have a Westerbeke 30b, which is really 27hp not 30, go figure. 
It's got an electric fuel pump that puts out 20 gph, no governor, always on, so it is always putting 20gph through the system. I have an observed and measured fuel burn rate of .75 gph at 3000 rpm.

That means that 19.25 gph are being filtered and returned to my tank per hour. I've never seen a diesel that doesn't have a return.

Max RPM is 3600, so I'd be taking and educated guess, but still guessing to say at 3000 rpm I'm making 22hp or so, and putting about 18 of that to the prop (alternator, transmission, drive leg losses siphon off the rest).

I can switch my supply and return paths independently between my two 18 gallon tanks, so I can take from tank 1 and return to tank 2 - in effect polishing (filtering) the fuel. Of course in 1 hour or less I run out of fuel in tank 1, and I better hope tank 2 is empty when I start 

I do that sometimes just to filter the fuel, the engine doesn't have to be running, just the fuel pump turned on.

The Volvo's return is just called a leak off I guess, and maybe it's just 'leaked off' to the supply side line. I'd be curious how they do that with different pressures being involved.

As to range - 3000 rpm on mostly flat water gets me about 6.8 kts, if I can use all 36 gals (unlikely level of efficiency there) and could actually find enough flat water to run for 48 hours (36/.75) I could in theory make 326 miles as my range. I typically run at 2800 rpm, 6.5 kts in flat water meaning 6 kts or less in the choppy Chesapeake Bay. The bay takes off .5 kts going north, adds it back going south due to current.

Being conservative I figure my range is 250 miles, I can double that with interior, ventilated jerry can storage (my aft lazerettes are huge) but I'd have to put 250+ pounds of fuel right where I want it least.

I can make that range infinite by simply raising the sails.


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## jrd22 (Nov 14, 2000)

Winder- In your last post you make an important distinction. An 18 hp engine will use app, .5gph , if you are using all 18hp. As many have stated here a 27-30hp diesel will use around .5gph at cruise because you are only using about 18-20hp at your cruise rpm. Although there are lot's of variables, it all comes down to mpg. If you use water line length to determine hull speed and assume a conservative cruise speed of 80% of hull speed you can determine your anticipated app. range based on tankage. We carry 190 gallons of fuel on Laurie Anne and burn an averge of 1gph so 190 hours. Cruise speed is 6.5 knots so our maximum range is 1235nm which means we only have to stop once for fuel on the way to Hawaii ). Allowing for a 20% reserve that would give us app. 1000nm range. As modern boats have maximized living space the tankage seems to have suffered.


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## Valiente (Jun 16, 2006)

Winderlust said:


> I feel stupid and naive for my original post.
> 
> I have no boat. I'm looking for a cruiser.
> 
> ...


Outside of motor vessels like trawlers (which can have enough fuel capacity to cross the Atlantic at eight knots), few sailboats have the fuel reserves to motor for days at a time, hence the terms "auxiliary", as the motor is auxiliary to the sails, which by definition are the prime means of moving a sailboat.

If you are looking for a cruiser with motor-sailer characteristics, by which I mean greater fuel capacity in this context, you might want to consider either purpose-built motor-sailers or existing cruisers that are structurally able to incorporate further diesel tankage, by which I mean deeper bilges, usually.

Most modern designs are "go fast", more or less flat bottomed boats with long fin keels, and there is a limit to how much tankage they tend to have.

By contrast, my boat is a full-keeled motor sailer, and adding 40% more diesel tankage is as easy as simply putting a daytank beneath the existing engine. Conceivably, I could converting the existing SS water tanks to diesel and add 200 more gallons, pushing my range close to 1,500 NM, but hey, it's still a sailboat, not a trawler with a mast.


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## Valiente (Jun 16, 2006)

jrd22 said:


> Allowing for a 20% reserve that would give us app. 1000nm range. As modern boats have maximized living space the tankage seems to have suffered.


Our boats aren't wildly dissimilar, and your numbers sound good. Prudence, however, means a "straight-line" range isn't usually used. Think how far you could go if someone, say, sustained a compound fracture in a part of the world without a SAR service, and a windless high pressure system had just moved in. The useful range, when obliged to motor, becomes 500 NM until the hypothetical accident, and 500 NM back, with a 20% reserve. It's only a straight line if it is shorter to continue to more land (with, in this case, appropriate medical facilities) than to return to the last equipped port you left.

If this sounds overly dramatic, there are plenty of popular destinations in the Pacific with no SAR facility whatsoever. I believe I was reading the other day that Fiji has nothing, and unless you are close to merchant shipping or an outlier of the NZ or Australian navies, you are on your own.

Anyway, my point with engine ranges is the idea that it is illustrative to think of one's boat travelling not as a series of point-to-point passages, but as a moving "circle of autonomy" across the ocean, within which occasionally will fall islands and mainland coasts, and within which you can move without refuelling or resupply in any direction. The radius of that circle is your working range, and if your passage plans or comfort level mean it is too small, your fuel provisions need revision.

Ideally, as has been said, you want to keep your deck-top fuel supply a very small proportion of your range considerations, because of weight considerations, safety and the often impracticalities of refuelling at sea. My Atomic 4 on my 33 foot IOR-style racer gives me only 80 NM or so of range, and if I motor, as I have during protracted calms, all day, I have had to throw five gallons of gasoline into the tank five miles out into Lake Ontario. Even that, even in a calm, can be tricky, not to mention potentially polluting/dangerous.


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## Winderlust (Jan 26, 2009)

Thanks guys and gals,

These have been very useful posts to my thinking about tankage.


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## MoonSailer (Jun 1, 2007)

Sailboats often travel by raising their sails and using the wind for propulsion. It seems crazy to me to carry enough fuel to motor across an ocean in a sailboat. Motoring is a power boat thing. A sailboat makes a very poor powerboat. Way to much drag from the keel and way to much windage from the mast. Instead of worrying about carrying a lot of fuel it might be better to carry more food and water. What would you do if you had engine trouble???


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## Winderlust (Jan 26, 2009)

MoonSailer,

I agree with half your premise, that is sail when you can... pretty obvious though. 

However, maybe one's passage plan to use the trades or to cross between gyres pans out poorly (stuff happens). Bring lots of food and water... Ok. Good advise. Additionally, how about a cruiser with tankage that gives it great auxiliary range when the wind is adequate, infinite range when the wind is pure and emergency range when the wind is nothing or hard from the wrong quarter. How would that sound to an offshore sailor? This all assumes the food and water were not pushed overboard to make room for the fuel.

Just asking.


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## Valiente (Jun 16, 2006)

The engine is also used for maneuvering, power generation, and either motor-sailing to make three knots turn into five, or to make a ride in leftover slop easier. It's also possible to use the motor aggressively in heavy weather to keep a boat oriented the right way down or up waves.

Lastly, some places the diesel is like it's from a First-World refinery, other times it's hand-siphoned from a filthy barrel. It's preferable to carry more of the first type and less of the second when possible.


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## Rockter (Sep 11, 2006)

I remember being impressed with the DownEaster 33, capable of 1000 miles under motor, one tank, nursing the motor to conserve fuel.
.


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