How to Determine Auxiliary Diesel Range? - Page 3 - SailNet Community
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post #21 of 34 Old 02-04-2009 Thread Starter
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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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post #22 of 34 Old 02-04-2009
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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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post #23 of 34 Old 02-04-2009 Thread Starter
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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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post #24 of 34 Old 02-04-2009
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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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post #25 of 34 Old 02-04-2009
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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.
.

Last edited by Rockter; 02-04-2009 at 09:35 AM.
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post #26 of 34 Old 02-04-2009
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Quote:
Originally Posted by Winderlust View Post
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.

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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post #27 of 34 Old 02-04-2009
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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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Quote:
Originally Posted by Winderlust View Post
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.
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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post #29 of 34 Old 02-04-2009
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Quote:
Originally Posted by jrd22 View Post
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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post #30 of 34 Old 02-04-2009 Thread Starter
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Thanks guys and gals,

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