fuel consumption

Estimate for a semidisplacemnt boat 14 litres per hour over normal engine cycle average assuming when cruising 400rpm below wot.
 
Searush said:
OK, I assumed a semi-displacement/ planing hull, do we actually know which it is yet?

But a blue note an hour, no problem, I could tear them up all day long . . . . :cool: ;)

And for around 150 quid you could go round Anglesey in a day. I usually choose to do it in 3 or 4 depending where I want to stop & play & dodge the adverse tides & the cost would be around a fiver, assuming I had to do a fair bit of motoring to catch the various tide gates. On a good trip I might not use any fuel at all. There is an annual race around the Island that takes around 24 hrs non-stop & none of them are allowed to use engines.

I can't see me switching to a true MoBo any time soon, altho I do appreciate the ability to use mine as a MoBo when it suits.
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Sabre DS uses prop exponent of 3, assuming no right minded person would fit pair of naturally aspirated engines in a performance planing hull with say propeller exponent of around 2.6/2.7. All comes back to propellers move boats engines merely turn them.

If you look at a cruise of 2,300 rpm Sabre 120C is consuming 4-4.1 gallons per hour, if vessel has pair of the same motors propeller demand for the same displacement is far lower for the same cruise speed, Not half but close to, say 4.5 gallons per hour. Oldgit commented about 4 gallons for two Perkins in same displacement node as the Sabre Ford NA motors simple fact is that his his propellers are pulling squat all power out of the motors at 6/7 knots therefore this all sounds about right
 
sorry, this isn't going to be too helpful to OP... but just for interest I think my Hardy is particularly good on fuel consumption, for 2012 total fuel consumption averaged 10lph... I tend to trundle along around 16knots... (yanmar 230hp)
 
Firefly625 said:
sorry, this isn't going to be too helpful to OP... but just for interest I think my Hardy is particularly good on fuel consumption, for 2012 total fuel consumption averaged 10lph... I tend to trundle along around 16knots... (yanmar 230hp)
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MBM ultimate engine test: Volvo D3 220 hp

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View attachment 30050

You've got a great engine/ hull combination.
 
Surely the answer to how many gallons per hour depends how much power is being produced at a particular throttle opening. WOT assume 120bhp from manufacturers figure, and for simplicity assume a flat throttle response, so half throttle would be half the BHP, or 60bhp.

Approx 1 gallon per 20HP per hour produced would seem reasonable. Thus 6pgh at 120bhp or 3gph at 60bhp.

Don't see speed or hull type enters the equation to any great extent. Just my opinion.
 
rustybarge said:
MBM ultimate engine test: Volvo D3 220 hp

Click to enlarge:


View attachment 30050

You've got a great engine/ hull combination.
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I know I have, trouble is I do want to go larger..... but I don't want the fuel bills! I think I improved economy from the previous year by 1.5lph by buying a lighter dinghy and adding 15m of chain, just balanced the boat better. She got a SD hull and as I said that 10lph is average for the entire year....so although 80% of the time we are out of restricted rivers we do sit at a 16knot cruise, if the weathers nice and we are not in a hurry a 6knot potter for a couple of hours is also enjoyed and it gets the average fuel burn down for the year.... at 16knots we are running at 2700rpm, 6 knots around 1200rpm... Oh, and being a die hard sailer I do always try and use the tides, amazing how that helps.
 
Davegriff said:
Surely the answer to how many gallons per hour depends how much power is being produced at a particular throttle opening. WOT assume 120bhp from manufacturers figure, and for simplicity assume a flat throttle response, so half throttle would be half the BHP, or 60bhp.

Approx 1 gallon per 20HP per hour produced would seem reasonable. Thus 6pgh at 120bhp or 3gph at 60bhp.

Don't see speed or hull type enters the equation to any great extent. Just my opinion.
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I am staggered to understand how what appear to be experienced boaters totally fail to understand the very basics of propulsion.

Not for the first time, let me try again!.................Propellers move boats engines merely turn them.

Think of your throttle as cruise control on a car or truck. With properly matched propller rated speed is the only point where propeller demand curves match. Take a look at ANY marine engine data sheet, you will see two curves, one showing full power, the other is the propeller demand curve.

Just to keep it simple, we set throttle(cruise control) on say a SINGLE Sabre 120C at 2,000 rpm. Engine has the POTENTIAL from full load curve to produce 103 hp ' 2,000 rpm, however looking at the propeller load curve based on exponent of 3, propeller is demanding tad less 65 hp..............Where has the 38 hp gone? Quite simply the engine is not producing it! When you set throttle to 2,000 rpm and handed responsibility from your hand to the engine governor to adjust the amout of fuel required to achieve 2,000 rpm.

Yes, hull type does matter because propeller demand is characteristic of hull shape, displacement hulls come in around 3, planing hulls typically 2.6/2.7.

I assumed this stuff is taught at basic powerboat training levels.
 
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Latestarter1 said:
I am staggered to understand how what appear to be experienced boaters totally fail to understand the very basics of propulsion.

I assumed this stuff is taught at basic powerboat training levels.
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Leave it out. Powerboat training is about tides and hitting or should I say not hitting the hard bits.

Anything in the direction you are showing is done at engineering level.

Mind you most of us power boaters can open bottles and cans with the best of them ;)

Just to throw another spanner in the works, my boat has a 135hp Lehman and I normally run around 1700-1800revs at that she burns less that 2 gallons per hour.

Tom.
 
LittleShip said:
Leave it out. Powerboat training is about tides and hitting or should I say not hitting the hard bits.

Anything in the direction you are showing is done at engineering level.

Mind you most of us power boaters can open bottles and cans with the best of them ;)

Just to throw another spanner in the works, my boat has a 135hp Lehman and I normally run around 1700-1800revs at that she burns less that 2 gallons per hour.

Tom.
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Hmmm...Let me see, I'm going to guess 5 kts.:p
 
LittleShip said:
Leave it out. Powerboat training is about tides and hitting or should I say not hitting the hard bits.

Anything in the direction you are showing is done at engineering level.

Mind you most of us power boaters can open bottles and cans with the best of them ;)

Just to throw another spanner in the works, my boat has a 135hp Lehman and I normally run around 1700-1800revs at that she burns less that 2 gallons per hour.

Tom.
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Tom,

How is it throwing a spanner in the works???

Say GB 32 is around 14 tonnes with LWL of 29ft??

Your Lehman Super 135 is exact same motor as Sabre 120C.

GB hull form is slightly unusual, flat sections, displacement, but capable a being bit harder than hull speed, my hull speed SWAG would be about 8.5 knots. Setting throttle at 1,800 rpm gives you POTENTIAL power of around 95 hp. However based on 14 tonnes and cruise speed of 7 knots your propeller is only DEMANDING about 55hp, therefore 2 gallons per hour sounds perfectly feasable consumption.
 
Latestarter1 said:
Tom,

How is it throwing a spanner in the works???

Say GB 32 is around 14 tonnes with LWL of 29ft??

Your Lehman Super 135 is exact same motor as Sabre 120C.

GB hull form is slightly unusual, flat sections, displacement, but capable a being bit harder than hull speed, my hull speed SWAG would be about 8.5 knots. Setting throttle at 1,800 rpm gives you POTENTIAL power of around 95 hp. However based on 14 tonnes and cruise speed of 7 knots your propeller is only DEMANDING about 55hp, therefore 2 gallons per hour sounds perfectly feasable consumption.
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Sounds about the right kind of fuel burn, my perkins at 1800 rpm are doing 2.5 gallons each and giving 20 knots, equats to 4mpg.
 
Latestarter1 said:
I am staggered to understand how what appear to be experienced boaters totally fail to understand the very basics of propulsion.

Not for the first time, let me try again!.................Propellers move boats engines merely turn them.

Think of your throttle as cruise control on a car or truck. With properly matched propller rated speed is the only point where propeller demand curves match. Take a look at ANY marine engine data sheet, you will see two curves, one showing full power, the other is the propeller demand curve.

Just to keep it simple, we set throttle(cruise control) on say a SINGLE Sabre 120C at 2,000 rpm. Engine has the POTENTIAL from full load curve to produce 103 hp ' 2,000 rpm, however looking at the propeller load curve based on exponent of 3, propeller is demanding tad less 65 hp..............Where has the 38 hp gone? Quite simply the engine is not producing it! When you set throttle to 2,000 rpm and handed responsibility from your hand to the engine governor to adjust the amout of fuel required to achieve 2,000 rpm.

Yes, hull type does matter because propeller demand is characteristic of hull shape, displacement hulls come in around 3, planing hulls typically 2.6/2.7.

I assumed this stuff is taught at basic powerboat training levels.
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Call me thick, but I don't see your argument. Power demanded by the prop or power derived from engine means the same thing. Fuel consumption is dependent on power produced/absorbed whatever the shape or speed - surely we agree?
 
Davegriff said:
Call me thick, but I don't see your argument.
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Thick.
...sorry, couldn't resist. :D
Jokes aside, yes, indeed fuel burn depends on power absorbed.
But the point LS is making is that for any given engine/rpm/prop combination, the power absorbed can be different, depending on the hull.
 
So, whats my 1984 Scand 32 Atlantic going to guzzle with a pair of TAMD40b's on shafts?

Alleged top speed was originally 28kts with it's semi planing hull, certainly leaps up to 20 in no time at all!
 
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