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Old 22 February 2010, 16:02   #71
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Quote:
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Lucky people - cruising at 37kts I'm getting 2.6 litres per nm!
Your boat is uniquely configured though.
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Old 23 February 2010, 04:59   #72
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Boat weight makes a big difference

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.
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Old 23 February 2010, 05:05   #73
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Should have mentioned...

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Originally Posted by jwalker View Post
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.
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Old 23 February 2010, 06:12   #74
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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?
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Old 23 February 2010, 14:57   #75
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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!
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Old 24 February 2010, 03:57   #76
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Quote:
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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...
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Old 24 February 2010, 04:45   #77
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Quote:
Originally Posted by Erin View Post
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
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Old 24 February 2010, 05:04   #78
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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.
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Old 24 February 2010, 15:04   #79
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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.

Quote:
Originally Posted by Polwart View Post
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.
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Old 25 February 2010, 05:44   #80
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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 . . .
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