What does this fuel graph mean?

Agree with Hadenough, but if you want an accurate figure, you need the propeller power/rpm curve. Read the consumption off the BSFC graph at the chosen rpm and hence power. Multiply SFC by that power to give g/hr.
Hope that's right!!
 
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Looks as if you just read the top graph for a given RPM e.g. 212 gm/kWh at 1,400 RPM. Lower left graph shows about 45 kWh obtained at 1,400 RPM so that means you'd use about 45 x .212kg at 1,400 RPM and that's 9.54kg. I think specific gravity of diesel is around .820 - .950 so average 0.89 makes consumption about 10.7 litres per hour at 1,400 (& 45kW output, approx. 60HP).

You can just repeat the calculation for any given RPM. The upper graph is pretty flat below 1,400 RPM and the lower left isn't miles away from being linear in that region. So it wouldn't too far out to just estimate 8 l/h for 45HP.

I assume that's with everything perfectly setup and correct prop. etc.

I don't have details of your boat e.g. Displacement, waterline length, propeller type etc. so difficult to know HP you'd expect to use most of the time.

If you had a lot of experience with your previous engine and knew RPM you normally used that might be useful. If you could get similar graphs for old engine then it would be simple to get a reasonable estimate of HP you normally used and could use that figure to get likely cruising RPM for your new engine. You could also compare predicted fuel consumption with figure you actually achieved and adjust new prediction accordingly.
 
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Mistroma has it

I used a figure of 2000 rpm and 0,835 for the density of diesel and arrived at approx 16 litres per hour
 
While the calculations given so far are theoretically correct, they assume you will be drawing all the potential HP at any given revs. The reality is that fuel burned is a direct function of the amount of power the propeller demands. Specific consumption varies very little from engine to engine and you can expect very similar average consumption to your existing. You an also scale from smaller or larger installations. So on my boat with 30hp I never use more than 20 for Cruising and average 2l an hour. I expect your boat will use between 4-5l an hour. It will of course rise rapidly if you wish to apply more power to go faster or for example if you have a dirty bottom and need to use more power to get your speed.

Check your records for your existing engine and determine the range you would like. That will determine your tank capacity. I would think 100 hours would be reasonable, so 500 litres. My boat has 150l so 75 hours, but many smaller boats have much smaller tanks, partly for space reasons. However given a typical annual usage of a sail boat auxiliary as 150 hours pa a range of 40-50 hours is not unreasonable.
 
Thanks for doing the math. So far with a 70hp engine we've cruised at 1700rpm out of a 2300rpm maximum. The existing engine is a slack tolerance 40 year old lump that does about 6l per hour at 1700 and about 15l per hour at maximum. I was hoping the new one being a modern design would be better :o
 
It is unlikely you will need a huge tank. However considerations like ability to clean it out and to be able to drain the dregs from the bottom and the available space will dictate size and shape. good luck olewill
 
What Tranona say is correct, the specific energy curves assume that the engine is loaded to it's maximum, throughout its operational rev range.

However, the power the propeller can use is governed by the equation:

image099.gif


It looks complicated and indeed whole text books have been written on the subject, but if you look at the last two terms in the equations, n cubed and D to the 5th power, you'll get the gist of it.

n is the rotational speed of the prop' think of it as shaft rpm (it's actually revs per second)
and
D is the diameter of the prop'

The prop' diameter is normally fixed, so unless you are changing the prop' don't worry about it, but because its to the 5th power, very small changes in prop diameter make a very big difference to the power you need to spin it. (and that's one reason why it's often difficult to match a prop to an engine)

The rpm is to the 3rd power, so that increases the power required very quickly as the rpm increases.

If you increase the engine revs from say 2000 rpm to 3000rpm, you need about 3.375 times more power.

Most yachts (and small displacement vessel) have propulsion systems designed so that the maximum engine power, i.e. maximum rpm, is used at about the theoretical hull speed. This is the only point that the prop' is absorbing all the power the engine can produce, so the graph will be correct.

At less than full hull speed and full rpm, the governor will be throttled back to maintain the set rpm, and so use less fuel than the graph would suggest.

In the example above, approximately 1/3rd the fuel the graph would suggest at 2000 rpm.

So from my own boat, I know this to be about right. At full power, (with a clean hull, no wind, flat water) I get hull speed of about 9 knots and use about 10L per hour. At cruising revs, 2000 rpm I get about 7knots and use about 3L per hour.

If you want to get better fuel consumption, in a like for like situation, then you need to go for a more sophisticated engine, turbocharged, direct injection and full electronic control. The Yanmar on my boat is "agricultural", 4 cylinder, non turbo, totally mechanical, 2.2L lump which produces 50 odd bhp on a good day. My car has a 2L turbocharged diesel, with common rail injection and full engine mapping electronic control and produces 150 odd bhp. A great car engine, much more efficient than the Yanmar, but I wouldn't want it in my boats!

Hope this helps.
 
Fascinating information. I don't think I've seen anything raised to the 5th power in ordinary use before. The nearest was flow in a tube, which is related to the 4th power of the radius, I think.
 
Thanks Ian, I understand the concept and see why the graph looks the way it does now. It gives me a real world 4lph cruising benchmark and a pushing hard but not WOT of 8.5lph. 250l tank it is then, down from the current 2x375l but that was insane, it was based on the feeling that you couldn't have too much fuel on board.
 
In case it helps, my Southerly 46RS has a 400L diesel tank, I generally cruise under sail whenever I can, but I run a 5kVa generator for at least 1 hour most day and a Eberspacer Hydronic 10, for I guess on average 2 hours a day, both run at around 1L per hour when running flat out, but considerably less at normal loads. I cruise for between 2 and 3 weeks at a time, sometime 4 weeks. I've never had to put more than 150L in to fill it at the end of a cruise, and that was in a period of very calm weather, when my crew had a fixed time table, so we had to motor a lot more than usual.
Your 250L tank would work OK for me.
 
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As the boat is a motor sailor I would suggest a bigger tank than which you propose. If you work on the reasonable principle that she will use 6-7 litres per hour (approx 1.5 imperial gallons) then a 200 litre tank will only give 28 hours of cruising. That's Ipswich to Dover and Dover to Eastbourne. By then your tank is possibly sucking air. Obviously it would be more than adequate if you are in your cruising ground and pottering but it would make a journey to the Baltic one of searching for fuel berths along the way.
 
Surely it depends on how much you intend to use the boat and, importantly, where. If you are going to be pootling between marinas, where fuel is readily available, a small tank will suffice. If you intend going further afield, then obviously you need to be able to carry more fuel. Your call.
 
Sinbad1951 said:
As the boat is a motor sailor I would suggest a bigger tank than which you propose. If you work on the reasonable principle that she will use 6-7 litres per hour (approx 1.5 imperial gallons) then a 200 litre tank will only give 28 hours of cruising. That's Ipswich to Dover and Dover to Eastbourne. By then your tank is possibly sucking air. Obviously it would be more than adequate if you are in your cruising ground and pottering but it would make a journey to the Baltic one of searching for fuel berths along the way.
Click to expand...

True but we sail rather well and reckon on sailing 60 or 70% of the miles on passage.
 
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