One liter per mile - what speed for your RIB?

[HUMBER P4VWL]

ocean pro 6.3, 2.5 V6 175hp opti. 2 up in calm with 140l on board averages at about 24 knots 1:1.

[Channel Ribs]

Zubenelgenubi: 30nm:25L @30kn (90 2/S) = 0.83 lpm 33kn
9D280: 35nm:30L @20kn (25 2/S) = 0.86 lpm 22kn
9D280: 24nm:30L @mixed (55 2/S) = 1.25 lpm 20kn
Leapy: 46nm:26L @20kn (90 4/S) = 0.57 lpm 28kn
Cypman: ?nm:? @30kn (140 4/S) = <1 lpm 35kn
TSM: ?:? @20kn (60 E) = 0.75 lpm 25kn
GordyP: ?:? @40kn (2x 150 Es) = 1.3 lpm 38kn
250kts: ?/? @25kn (115 2/S) = 0.67 lpm 34kn
Matt h: ?/? @? (90 2/S) = 0.84 lpm 29kn
Erin: 270nm/370L @mixed (200 4/S) = 1.37 lpm 25kn
Pablo: ?/? @30kn (90 2/S) = 0.8 lpm 32kn
Noser: ?/? @23kn 23kn
Jargos: ?/? @39kn (250 E) = 1.0 lpm 39kn
Paul: ?/? @? (200 2/S) = 1.0 lpm ?
P4VWL: ?/? @24kn (175 2/S) = 1 lph 24kn

[Leapy]

Originally Posted by malthouse

Zubenelgenubi: 30nm:25L @30kn (90 2/S) = 0.83 lpm 33kn
9D280: 35nm:30L @20kn (25 2/S) = 0.86 lpm 22kn
9D280: 24nm:30L @mixed (55 2/S) = 1.25 lpm 20kn
Leapy: 46nm:26L @20kn (90 4/S) = 0.57 lpm 28kn
Cypman: ?nm:? @30kn (140 4/S) = <1 lpm 35kn
TSM: ?:? @20kn (60 E) = 0.75 lpm 25kn
GordyP: ?:? @40kn (2x 150 Es) = 1.3 lpm 38kn
250kts: ?/? @25kn (115 2/S) = 0.67 lpm 34kn
Matt h: ?/? @? (90 2/S) = 0.84 lpm 29kn
Erin: 270nm/370L @mixed (200 4/S) = 1.37 lpm 25kn
Pablo: ?/? @30kn (90 2/S) = 0.8 lpm 32kn
Noser: ?/? @23kn 23kn
Jargos: ?/? @39kn (250 E) = 1.0 lpm 39kn
Paul: ?/? @? (200 2/S) = 1.0 lpm ?
P4VWL: ?/? @24kn (175 2/S) = 1 lph 24kn

Malthouse...some of those numbers look suspiciously economical

Previous comments relate..."life is full of folks making outlandish fuel consumption claims for boat/car"

[Jorgos]

Thats why I have posted the photos

[GJ0KYZ]

Ribcraft 6.8 with 225 ETec: best fuel economy 1.23 litres per mile at 30 kts with one on board and an empty fuel tank otherwise about 1.5 litres per mile regardless of speed. The other day I was doing 40kts and was using 62 litres per hour.

[BruceB]

5.8m Humber Destroyer with 90 4S Suzi gets 1.3nm/lt at 25knots and drops to 0.9nm/lt at 31knots.
Normal cruising speed 24-25knots in consequence......
This is from quite a few long distances and fuel monitoring meters onboard.

[Channel Ribs]

Zubenelgenubi: 30nm:25L @30kn (90 2/S) = 0.83 lpm 33kn
9D280: 35nm:30L @20kn (25 2/S) = 0.86 lpm 22kn
9D280: 24nm:30L @mixed (55 2/S) = 1.25 lpm 20kn
Leapy: 46nm:26L @20kn (90 4/S) = 0.57 lpm 28kn
Cypman: ?nm:? @30kn (140 4/S) = <1 lpm 35kn
TSM: ?:? @20kn (60 E) = 0.75 lpm 25kn
GordyP: ?:? @40kn (2x 150 Es) = 1.3 lpm 38kn
250kts: ?/? @25kn (115 2/S) = 0.67 lpm 34kn
Matt h: ?/? @? (90 2/S) = 0.84 lpm 29kn
Erin: 270nm/370L @mixed (200 4/S) = 1.37 lpm 25kn
Pablo: ?/? @30kn (90 2/S) = 0.8 lpm 32kn
Noser: ?/? @23kn 23kn
Jargos: ?/? @39kn (250 E) = 1.0 lpm 39kn
Paul: ?/? @? (200 2/S) = 1.0 lpm ?
P4VWL: ?/? @24kn (175 2/S) = 1 lpm 24kn
BruceB: ?/? @31kn (90 4S) = 0.9 lpm 29kn

[martini]

Lucky people - cruising at 37kts I'm getting 2.6 litres per nm!

I believe the previous owners were able to achieve my magical 1:1, but only with assistance from an RAF C130....and a parachute

[Mollers]

Originally Posted by martini

...... cruising at 37kts I'm getting 2.6 litres per nm!

Greedy git.

[Trolli]

Yamaha 150/4ST, less than 1l/m @ about 35K

[Channel Ribs]

Originally Posted by martini

Lucky people - cruising at 37kts I'm getting 2.6 litres per nm!

Your boat is uniquely configured though.

[ecoXR24]

For planing boats, the all-up weight is probably the most important factor. Length, hull shape, prop and engine efficiency all play a part of course. Power: weight ratio is the main thing that affects speed, so for two similar boats using the same power, the lighter one will go faster or use less fuel. If people can have a stab at the all-up weight of their boat, Malthouse's table will be more useful, e.g. my hull should be ..., carrying 100 litres of fuel, 4 people and one small dog, or whatever.

This means almost any RIB can probably do 1:1, but heavy ones will do it much more slowly.

We have a graph of power versus speed for various all-up weights on our website: http://www.eco-marine.co.uk/xr-range.html The figures are from a well known text book and apply to boats in general, not just RIBs. Fuel consumption is more-or-less proportional to power, so where the graph shows a heavy boat needs twice as much power as a light one for the same speed, it will use twice as much fuel. If we can make some assumptions about typical outboard efficiency and prop efficiency, we could do a graph of weight versus 1:1 speed.

[ecoXR24]

Originally Posted by jwalker

My boats have done about 1lt a mile no matter what engine was on the back and even my current boat weighing in at about 3tons with a diesel still does about 1lt per mile.

Diesels will be a lot more economical of course: a typical 4 stroke petrol engine is about 25% efficient and a typical diesel is 40% efficient - a diesel should use about 5/8 the fuel of a petrol engine, all other things being equal.

[Mollers]

Originally Posted by ecoXR24

Diesels will be a lot more economical of course: a typical 4 stroke petrol engine is about 25% efficient and a typical diesel is 40% efficient - a diesel should use about 5/8 the fuel of a petrol engine, all other things being equal.

How exactly are you calculating these efficiency figures?

[Erin]

Hmmmm. I've tried to work this one out mathmatically. Petrol provides 35,500KJ of energy per litre (48,000kJ/kg). Which at 100% efficiency should move a 1500kg boat some 2.4km through the water assuming no resistance (1J = 1Nm).

Where I'm struggling however, is to convert this to reality. Say we use 1 litre to go 1 mile (1.6km at 30kts) Then that suggests an efficiency of 66%. That just ain't right. It should be far less!

[Poly]

Originally Posted by Erin

Hmmmm. I've tried to work this one out mathmatically. Petrol provides 35,500KJ of energy per litre (48,000kJ/kg). Which at 100% efficiency should move a 1500kg boat some 2.4km through the water assuming no resistance (1J = 1Nm).

Where I'm struggling however, is to convert this to reality. Say we use 1 litre to go 1 mile (1.6km at 30kts) Then that suggests an efficiency of 66%. That just ain't right. It should be far less!

I'm not sure what you've done - but you need to look at "resistance" otherwise once the boat is moving it just keeps on moving forever and so 1L of fuel would get you round the world / to the moon / sun etc...

[John Kennett]

Originally Posted by Erin

Hmmmm. I've tried to work this one out mathmatically. Petrol provides 35,500KJ of energy per litre (48,000kJ/kg). Which at 100% efficiency should move a 1500kg boat some 2.4km through the water assuming no resistance (1J = 1Nm).

I don't know what the answer is, but I'm pretty confident that you're barking up the wrong tree. Wrong equations, and some decimals in the wrong place too.

Using your figures, 1 litre of petrol would have sufficient energy to lift your boat vertically approximately 2.4 metres.

1500Kg ~ 14700N
35500KJ / 14700N = 2.4m

[Poly]

Erin,

When talking about "efficiency" of an engine. Do people not usually mean the amount of energy put in compared to the amount of useful engergy coming out.

So if we assume that a reasonably fuel efficent 20 HP engine uses 1L of fuel every 10 minutes (6L / hr which would be about right for a 20HP 4st at full throttle). You tell us that 1L of fuel has 35,000 kJ of energy.

If all of that energy were converted to useful power then the engine would put out

35000 (kJ) / 60 (s/min) x 10 (min) = 59.2 (kJ/s) = 59.2 kW of power.
20 HP = 14.9 kW

so actually the engine is only 100x(14.9/59.2)= 25% efficient.

[Erin]

Originally Posted by John Kennett

Using your figures, 1 litre of petrol would have sufficient energy to lift your boat vertically approximately 2.4 metres.

1500Kg ~ 14700N
35500KJ / 14700N = 2.4m

Are you sure? 35,500,000J / 14,700N looks like 2,400m to me or 2.4km.

Originally Posted by Polwart

Erin,

When talking about "efficiency" of an engine. Do people not usually mean the amount of energy put in compared to the amount of useful engergy coming out.

So if we assume that a reasonably fuel efficent 20 HP engine uses 1L of fuel every 10 minutes (6L / hr which would be about right for a 20HP 4st at full throttle). You tell us that 1L of fuel has 35,000 kJ of energy.

If all of that energy were converted to useful power then the engine would put out

35000 (kJ) / 60 (s/min) x 10 (min) = 59.2 (kJ/s) = 59.2 kW of power.
20 HP = 14.9 kW

so actually the engine is only 100x(14.9/59.2)= 25% efficient.

I started to confuse myself with the time element. Turning Joules into Watt Seconds.

[John Kennett]

Originally Posted by Erin

Are you sure? 35,500,000J / 14,700N looks like 2,400m to me or 2.4km.

See, I told you I didn't know what the answer was!

I don't think you're going to lift the boat vertically 2.4Km using one litre of petrol though . . .