Where's my fuel gone?

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Adetheheat

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For a small 7.5m motor cruiser with 150 hp outboard is 2 litres per mile reasonable? It seems to gobble fuel. Its a Beneteau Antares.
 
Martxer said:
Does sound a lot for a single 150 . My display was reading 2 litres per nm yesterday at 22 knots with twin 200s
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That equates to about 2.25 gallons per nautical mile and sounds a bit low. By comparison, the boat in my profile photo is 6.9 metres long, 200 verado installed. Boat 1700 kg on its own so probably around 2300 kg all up.

The Antares you have is 2000 kg alone so add engine fuel etc is probably 2,500 kg.

Additionally it has quite a large cross sectional wind resistance plays a part.

My economical cruise was 18.75 knots and I averaged 2.9 nm/gallon.

I did extensive tests to get the correct prop for the engine, perhaps more than most people. But then again, I would do Hamble to Brixham trips of 100 miles so efficiency counted...

What speed and load do you have when on the water?
 
My boat uses 60 litres /hr at 23kts (I don't have a fuel use readout so that's approximate and on calm water).
but it has 2 engines

So that's 30 litres for 23 Nm per engine
= about 1.3litres per Nm per engines (170hp engines)
Its inboard diesel engines on outdrives but older technology - mechanical fuel injection

A friend has newer 260hp common rail diesel engines that use fuel at the same rate in a bigger boat !

I believe modern petrol out boards are almost as fuel efficient as inboard diesels ?
 
Adetheheat said:
Mostly 20 knots cruising speed. Trim was about half way so no I wasn't planing
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I would expect you to plane at 20 knots. The hull looks more semi displacement though, so maybe you don't have a definite transition from displacement to planing.

Can you achieve WOT on the outboard? What propeller is fitted?
 
To add a bit more detail boat is 9.1 meters and around 3500kg. At 4000rpm doing 25knots and burning 60 litres across 2 engines so 30 litres per side roughly 2 miles per gallon
 
Hot Property said:
That equates to about 2.25 gallons per nautical mile and sounds a bit low. By comparison, the boat in my profile photo is 6.9 metres long, 200 verado installed. Boat 1700 kg on its own so probably around 2300 kg all up.

The Antares you have is 2000 kg alone so add engine fuel etc is probably 2,500 kg.

Additionally it has quite a large cross sectional wind resistance plays a part.

My economical cruise was 18.75 knots and I averaged 2.9 nm/gallon.

I did extensive tests to get the correct prop for the engine, perhaps more than most people. But then again, I would do Hamble to Brixham trips of 100 miles so efficiency counted...

What speed and load do you have when on the water?
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you've got that the wrong way round, not 2.25 gallons per mile, but 2.25 miles per gallon.

I get between 1.8 to 2.6 nm per gallon at cruising speed in my boat - 9 metre , single 300hp outboard, the fuel economy varies based on sea state, wind direction, amount of trim tab I'm using etc. I probably average about 2.25nm per gallon, similar to the op.
 
julians said:
you've got that the wrong way round, not 2.25 gallons per mile, but 2.25 miles per gallon.

I get between 1.8 to 2.6 nm per gallon at cruising speed in my boat - 9 metre , single 300hp outboard, the fuel economy varies based on sea state, wind direction, amount of trim tab I'm using etc. I probably average about 2.25nm per gallon, similar to the op.
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Thanks for the correction - 2.25 galls per mile is sub optimal!
 
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As a point of reference, we have a Scorpion 6.5m rib (6.8m) with a 150hp Optimax outboard.

It burns 27-30 litres per hour at economic cruise (25-26knots, 4,000 rpm). - about 4mpg

It burns 66 litres per hour at WOT (42-44knots, 5,350 rpm). - about 2mpg

The boat is dry stacked and has a super clean bottom, balanced prop and a slender deep-v hull.

(calculations based on SmartCraft gauges, which seem pretty accurate +/- 5%)
 
Sounds a bit high.
I have an Antares 8 with Yamaha F200 outboard. On a trip with 4 people on board, both fuel tank and fresh water full, our travel weight is around 3 100 kg. At 25 kn (4 300 rpm) we manage 1,4 litres per nautical mile.
What engine & propeller combination do you have?
 
For a planing boat the key points are ready-to-go weight and speed desire.
Remove any dead weight you can and get free range & performance ;)

150 hp on Antares 750 as tested by Honda

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You'll notice that Litri/Ora (liter per hour) kicks up if in a hurry which is absolutely normal. Up to 8 and a tad above 20 knot seems to get you the best economy.
 
What engine does she have. If it’s anything recent from merc for example, I’d expect about a litre a mile cruising at that speed perhaps.
I know my mercury 115 CT on a 19ft deep v cuddy, will use about 1.6/1.7 litres per mile at about that speed. Well propped is the key. That would be around 3600 rpm or close either way, from memory.
 
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