Trawler Fuel Consumption

[oceanasiainc]

Fuel consumed by diesel engines can be calculated at 250gms of diesel per KW/Hour. This applies to all diesels. Hull form will affect the actual speeds achieved. Use the engine maker's graphs to see power curves and extrapolate the number of grams of diesel used per hour (then convert to US gallons).

A good rule of thumb is that a marine diesel will use its HorsePower number in US gallons in a 24hour hour period, running at maximum continuous revs. You can halve that for normal cruising RPM. Divide that by 24 to see the GPH figures

[James S]

Welcome to the forum oceanasiainc.
Thats a useful rule of thumb...thanks.

[BruM]

Modern diesels 4 produce about 20 hp for each gallon of fuel per hour. For technical details look up BSFC, brake specific fuel consumption. That figure works fairly well over the load range. The difficulty folks have in estimating fuel use comes from a misunderstanding of engine makers fuel curves. All diesel makers show max load fuel curve but for boats that curve has no relevance except at WOT. Boats consume power via the prop and are loaded along a propeller load curve not max load. The prop curve is always lower than the max load curve at all points except WOT if the boat is propped properly. The prop curve is shown on data sheets for many marine diesels. For the technically minded it is usually a cubic or 2.5 exponent curve. Estimating fuel use is quite possible from the prop curve IF the engine is loaded to reach full wot.
Some people over prop the boat so that the engine will not reach rated WOT. In that case the fuel at the actual max rpm will be a point on the max fuel curve and below that point will follow the same exponent curve.

[doug86]

Originally Posted by oceanasiainc

A good rule of thumb is that a marine diesel will use its HorsePower number in US gallons in a 24hour hour period, running at maximum continuous revs. You can halve that for normal cruising RPM. Divide that by 24 to see the GPH figures

checking... my Ford Lehman 120hp would use 120gal at WOT in 24hrs, divide by 24 = 5Gal/hr at WOT. Half of that is 2.5gph, which is EXACTLY what my Ford 120 averages at 1700rpm. Most FL owners report consumption rates between 2.5 and 3 gph running between 1700-2000 rpm.

Thanks for that great "rule of thumb" formula.

[chrisjs]

There must be a significant error in the estimation with increasing engine size! Our 65,000lb Selene 47 single screw trawler with 280HP Cummins consumes less then 3GPH cruising at 7-8Kt. Using the rule of thumb approach, would suggest consumption at double that rate. I only mention this to help clarify the situation for those considering larger (over-powered) trawlers, since many (displacement hull) boats (including ours) have larger engines than are needed for cruising speeds.

[lorenzo b]

chrisjs
what is your top rpm and what do you run at to get 7 knts?

[chrisjs]

The Cummins is rated for 2600rpm max. We usually run between 1600 and 1800 rpm.

[BruM]

Chris: I suspect that your engine is not propped to reach 2600 rpm at WOT. If it was your fuel use at 1600 would probably be about ((16 / 26)^2.7) * 14 = 3.77 gph. If propped for 2600 the fuel used at your speed would be the same as now just with a little higher RPM.

[lorenzo b]

What I meant by max rpm is what the engine turns at when you open the throttle all the way while underway. My 8/71 N will turn 1850 and I run it at 1450 burning .8 gals / knt at about 7 knts/hr.

[BruM]

lorenzo: so you are burning 5.6 gph??
I dont have the 8/71 specs at hand what I wot supposed to be? Yours sounds overloaded. Does it smoke?

[lorenzo b]

The 8/71 was rebuilt 200 hrs ago and it runs like a top. I am 62 ft at the waterline with a beam of 20 ft and weigh a good 70 tons with 3000 gals of fuel and 2000 gals of water and I just added 7 tons of ballast.

[BruM]

I guess you missed the question. I dont have the 8/71 specs at hand what I wot supposed to be? Yours sounds overloaded. Does it smoke?

[chrisjs]

Brum:

While we do not often "open her up" we periodically run the engine up to higher RPMs. When purchased in 2006 the boat got to WOT at sea trial and we occasionally run her up to 2300 with no difficulty. So I do not think there is a prop problem. Our fuel consumption has been pretty consistent over the past 4 years. The Cummins also burns hardly any oil.

[BruM]

there was a 2300 rpm 308 hp power rating so as long as you can hit that you are not overproped but you did say 1850 not 2300 "will turn 1850" which is it??

[lorenzo b]

BruM
My prop is a 52/48 five blade which was used to pull shrimp nets off a 12/71
No doubt I could get higher rpms with a smaller prop, but was told that I will get better fuel efficiency with a bigger prop.
On work boats like mine they figure one gallon per cylinder per hour is good