Which of two fuel consumption formulas?

[Buddy_Y]

Hi All:

I am contemplating purchasing a new boat (a 32-foot fibreglass trawler with a 6-cylinder, 175 horsepower Isuzu engine), and I have a question regarding diesel fuel consumption. I’ve been told of two different formulas to calculate a boat’s potential fuel consumption: one based on horsepower, one based on cylinders.

The first diesel fuel consumption formula that I’ve seen in several contexts is to divide the horsepower by 20 to generate the number of gallons per hour used. (The figure ‘20’ is up for debate; some people say 18, and I’ve seen the figure 16.75 used in a post on this site.) Using this formula, the 175 hp engine would burn 8.75 gallons per hour. This figure would be less at cruising, of course—let’s say 5 gallons if the boat were only drawing upon 100 hp of its potential.

Yet, other people have mentioned to me the rule of thumb of one quart per cylinder per hour. Since the boat has 6 cylinders, this would mean 1.5 gallons an hour.

Which formula is right?

p.s. In case it matters, the boat is listed as 11600 pds displacement, with a cruising speed of 8 knots and a top speed of 12.

Thanks!

[GordMay]

Since it’s a “new” boat, the manufacturer should be able to give you a Fuel Burn (consumption), RPM, Speed chart, for that engine, as installed in that boat.
Although these marketing figures may somewhat idealized & optimistic, they will give you a much better idea than will any rule of thumb formula.

[Pelagic]

I agree with Gord, the builder/architect should be able to give you better power/performance curves for a fuel consumption/RPM/Speed table than a basic formula that does not take into account loading and fuel rack settings.

Just add on 20% to that sales number to give you realistic budgetary costs.

Many Owners get carried away putting flow meters on their feed and return lines and then connect to a fuel consumption display.

The problem is that data is very suspect because the flow meters should be adjusted for variable fuel temperature to give an accurate reading and they don’t do that.

Replacing the fuel you’ve burned after logging time and rpm is the best method of getting to know your basic consumption. However, also learn to apply various seaway % depending on your experience in different weather conditions to help you with long trip planning.

[GordMay]

Noted Naval Architect Dave Gerr, suggests a rough but conservative fuel burn estimate, using the formula:
Gallons per Hour = 0.054 x H.P.*
* HP from propeller HP curve

See Chapter 6 of Gerr’s “Boat Mechanical Systems Handbook” at:
Boat Mechanical Systems Handbook - Google Book Search

[skipmac]

First, I don't see how the forumula using number of cylinders could be accurate since it does not account for the size of the cylinder and the reculting size of the combustion chamber to be fill wit air/fuel mixture. Obviosly the same formula would not work with a model airplane cylinder with 1/4" diameter and a large commercial engine with a 24" diameter.

Regarding fuel flow meters I don't understand how a temp adjustment is needed. These meters measure volume fairly accurately and a gallon is a gallon regardless of temp. There is some change in the density of diesel with a change in temp, meaning that a gallon will contain more or fewer molecules of fuel (and then more or less energy) but that change is pretty small within the range that the fuel wil be used. Would have to look up the figures but would guess no more than 1-2% which for purposes of calculating mileage is more than adequate.

[DeepFrz]

There is a discussion going on now in the Nordhavn Dreamers mailing list and it seems like modern (Tier 2) engines can use a figure of around 20 (between 19 - 20) to divide horsepower to get the fuel burn per hr. This does not tell you what your boat will get because it says nothing about how easily your hull is driven through the water. Finding owners of the particular boat and talking to them will get you a better idea of fuel burn at particular hull speeds.

[GordMay]

Dividing by 19 is equal to multiplying by 0.0526
Dividing by 20 is equal to multiplying by 0.050
FWIW: Both figures are fairly similar to Gerr’s approximation of multiplying by 0.054 (÷ by 18.518).

Originally Posted by DeepFrz

There is a discussion going on now in the Nordhavn Dreamers mailing list and it seems like modern (Tier 2) engines can use a figure of around 20 (between 19 - 20) to divide horsepower to get the fuel burn per hr. This does not tell you what your boat will get because it says nothing about how easily your hull is driven through the water. Finding owners of the particular boat and talking to them will get you a better idea of fuel burn at particular hull speeds.

[David M]

One cannot make an accurate estimate of fuel consumption based on the number of cylinders because cylinders vary in displacement and therefore burn a different amount of fuel per stroke, in addition to other factors.

Go right to the engine manufacturers website and look up what is called the "Brake specific fuel consumption" for your engine. This is a measurement of how much mechanical energy is produced per unit of fuel.

Going to the manufacturers website will give you the facts as opposed to making estimates.

Only after you know your engines specific fuel consumption can you make an estimate of its efficiency...that's if you are going to do it all on paper.

Really the best way to get the numbers right is to do a practical test of running the boat over a measured distance. Do this multiple times at different speeds to generate a speed curve. Having a fuel meter helps. Run a smooth line through the different points and there you have it.

Be sure to check out the tables in the Wiki page. The different efficiencies of different engines I think are quite interesting.

http://en.wikipedia.org/wiki/Brake_s...el_consumption

[Bash]

...to determine my fuel consumption rate at 80% WOT, and it turned out to be precisely what Yanmar said it would be.

In other words, there's an easy way to do this.

[Buddy_Y]

Hi All:

Thanks for the advice! It seems the cylinder estimation approach to fuel consumption is just plain off. Maybe it's the kind of 'wisdom' that boat brokers invented to reassure buyers.

I'd love to be able to just consult the engine's manual, but it's not really a 'new' boat--it's just new to me. (I expressed that confusingly in my original post.) The boat per se is early eighties, and the engine is original.

So, based upon the horsepower formula, this trawler would be burning 5+ gallons an hour even at cruise. That'll put a dent in my ambition to move up from my present boat.

Thanks again!

[Cheechako]

The HP method sounds way off. A 32 ft trawler shouldnt burn anywhere near 5-6 gph unless you're trying to plane it! My 40,000 lb sailboat with a 90 hp diesel burned less than a gallon per hour. My 32 ft Trawler, ~12000 lbs burns less than 1.25 gph (Perkins 4-236). Nordic Tug's 32 (18500 lb) with 280 hp is rated at about 3.3 gph at 8 knots.

[DeepFrz]

It all comes down to speed. 100 hp to drive a 32' boat at cruise sounds like you are trying to cruise to fast. I don't know what the displacement of your boat is or the wl length but I would guess that you will use far less than 100 hp to drive it at a reasonable trawler cruise.

At 7 knots, I'm burning about 3.5gph, so that translates into less than 70 hp at your specific fuel burn numbers.

The quote is from a fellow running a Nordhavn 55 with a 330 hp engine. He will have a much longer wl length but also much more weight.

[Cheechako]

Yea true. On the other hand be careful about using the old traditional Sailboat recommendations, sometimes the theory aint the reality. 30 years ago I built a 31 ft cutter and used the traditional method to size HP. (you know, in those days the diesel "auxilliary" was just to get you up to the dock or out of the marina!) That boat had a 13hp diesel in it, and flat calm it would move at hull speed, but it was a real dog....

[stevensc]

I left Annapolis Nov. 23, 2008 bringing our, new to us, 2003 Monk 36 trawler home to Louisiana, a trip a bit over 2000 miles. We left with full tanks and our last fill up was in Dauphin Island Alabama @ 1871 miles (we travelled along the ICW so all miles mentioned here are Statute miles)
Some of the data for the Annapolis to Dauphin Island portion are :
consumed 558 gallons of diesel
put 257 hours on the 210 hp Cummins 6BT 5.9 diesel, plus 95 hours on the 9 kw generator.
1871 miles cruised
35 travel days, the shortest travel day 28 miles, longest non stop leg Tarpon Springs to Appalachicola 184 miles in 23 hrs
The Monk 36 is a semi displacement boat, 20,500 lbs dry displacement, LWL 33' beam 13', draft 4'
I hope these figures will be of interest, if you have any questions please let me know.
Steve