fuel consumption

[BC Mike]

Thanks Joli.
You confirmed one of the dicrepencies that I reffered to, the amount of fuel per hp. My formula from the book was 6/10 per hp per hour, and the number you posted was 4/10. I suspected that my diesel and likely many others are more efficient than the ones the author was referring to.
Also we use imperial gallons in the rest of the world, so if I say my car does 40 mpg that may be only 33 per US gallon.
The fuel pumps now measure in liters here, but some of us still like to refer back to the old system.
At 4/10 pound of fuel per hp per hour it is possible for me to get 20 miles per imperial gallon, if I limit the speed and only use 6hp.
My boat like so many others is easy to drive up to 5 knots, is a bit more work to hit six, but from there on is where the real power is consumed. 6.5 is possible but a real waste of energy.
I have the charts for my motor but can not desipher the fuel consumption graph. The hp to revs graph makes sense with what I have written.
Michael

[Joli]

Happy to help Mike.

[swagman]

We've 46 foot overall, 41 on waterline and are 11.9 tons.

The Yanmar 75hp turbo via saildrive to 19 inch Max Prop consumes 1.6 US Gallons (6 litres) per hour at 2,500 revs - and makes 7.5 knots.
We carry 250 litres in main tank and 3 x 20 litres as back up.
Cheers
JOHN

[Christopher Chapman]

I bought a 51 ft Piltohouse Motorsailor with a Perkins 4.236 engine. Seller claimed it burns 1/2 gal / hr at 1500 RPM. I left San Diego for Victoria BC, wind on the nose and 2 meter swells fine on the bow all the way. So I powered all 1200 odd miles home with it singlehandledly. Actual fuel consumption was 1.6 gals / hr average at 1500 RPM. Stopped for fuel in San Franciso, then once more in Oregon. Was a great trip - too bad my new sailboat turned into a powerboat but then again try running any powerboat for 1200 miles at 1.6 gals/hr !

[cruiser6003]

Realize this is a VERY old thread but one more data point for the original poster. I have a Columbia 45, MOTOR/Sailor, 27,000 lbs (prob more like 30,000 with gear) LWL of 34'10". Max theoretical hull speed 7.9. She has a Perkins 4-236 (85 hp).

For motoring only, in calm waters the numbers are:

1000 rpm, 6 kts at .75gph fuel burn = 8nm/gal
1200 rpm, 6.5 kts at .9 gph =7.2 nm/gal
1500 rpm, 7kts at 1.3 gph =5.3
1800 rpm, 7.5 kts 1.8 gph =4.16
2200 rpm 8 kts 2.5 gph =3.2 nm/gal

I carry 130 gallons, so at max cruise, I have 1040 nm range under power alone. With sail assist I run the engine at 1000 rpm (with run ups to 1800 every hour) and get an extra 1.5 kts.

Hope this helps.

[stevensuf]

mine is 43 foot 10,400kg yanmar 55hp, in calm i can get close to 12mpg at 5 knots 2000rpm, motoring into a swell/ headwind knock that down to 3-4 mpg

[stevensuf]

nb staying about 30-40% away from hull speed will hugely increase your mpg

[dustman]

Seeing this conversation about weight vs power requirement made me want to do this:


So I went to the Psycho Snail website and punched in some numbers in the calculator. It uses Gerr's formula.



1) 10,000lbs, 40' LWL, at 7kts = 11.04 HP


2) 20,000lbs, 40' LWL, at 7kts = 22.07 HP


It would seem that in a general sense power requirements for a given LWL are directly proportional to displacement(weight). However, assuming you proportion your hull to have minimal surface area for the given length the wetted surface area does not go up proportionally to the weight. But! The heavier boat would have to be much fatter at the given length so its form drag would be greater along with having to push aside(or more accurately, up) exactly twice as much water at the given rate(speed). My conclusion is that the lighter boat would be *relatively* even more efficient than the heavier boat at all but the slowest speeds where friction is the dominant factor.

[Sojourner]

About 4 liters an hour in our 53 foot amel with perkins prima m80t engine..

[Talbot]

Dependent on ship type and design, wind resistance has a major effect as well. Yes I know - sailing boats are supposed to sail, but there are times when this is not feasible. My last boat could be pushed through the water at max speed easily with a 9hp outboard saildrive. However against a reasonable headwind this was impossible. My boat had a 27hp diesel outboard, and was able to push against a 42 knot headwind, despite not having an optimum propeller.