What speed should we make under motor ?

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I doubt it - if you are correct then you are saying that there is a 100% discrepancy in efficiency between one engine and another. Not very probable, for modern engines! My figure is from the Yanmar 110hp motor which is turbocharged so might be a bit more efficient than yours, but not a huge amount.

Let's be clear, we are talking about the fuel actually burnt at a given power output and the output is shaft output from the engine/gearbox, not delivered by the prop. I can't follow your arithmetic. What is 0.88T ? To get the figure I gave to litres per hour you need to convert mass (grammes) to volume (litres) so you need the specific gravity of diesel at ambient. The other factor is 'horsepower-hours' which is the actual horsepower being used, not the rated hp of the engine, of course. Does that help?

edit:- ps if your calculation suggests double the fuel actually used then could it be that you normally run at 50% of rated output and you have used the rated figure of 28 when calculating horspower-hours?

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0.88 Tonnes/cu.metre -- still say this is a totally useless formula - how the hell is anyone supposed to know what developed hp they are running at, at any particular time, or on average??
 
I'm sorry you find it 'totally useless' but it is intended for engineering use and is presumably not put into the user manual because most people would not understand how to use it. It is the curve that engine manufacturers supply for professional use. Yanmar publish a curve of specific fuel consumption vs crankshaft RPM which is amazingly(?) flat over the normal revs range one uses.

Among other uses, it allows you to see whether your fuel used is consistent with observed data since unless huge amounts of smoke are being produced, or the engine becomes excessively hot, then the efficiency will not be far from the designed value. Given that you now know the the shaft power you can see whether the vessel is behaving as you would expect, given that shaft power.

You can measure the fuel used and this tells you the shaft power assuming the engine to be within design figures. The other, and main use, is to determine the theoretical fuel consumption given a range of shaft powers. You want to do this when designing a boat since you need to choose the size and capacity of a fuel system long before you build and launch the boat. Without such data, naval architects could not design boats. They would have to put them together by trial and error. So it is far from 'totally useless'!

Why are you quoting a figure of 0.88 tonnes per cubic metre? Is this the density of fuel oil at 20C, or something? What our friend will want to do is to convert the 170 grammes into litres. It isn't rocket science.
 
OK I figure amongst 'most people' then. As you say, we need to convert gms to litres - 0,88t/cu.m = 0.88kg/litre. Maybe there is some use in what you say... /forums/images/graemlins/wink.gif
 
/forums/images/graemlins/smile.gif OK, 880gm per litre. We know that a modern diesel engine should burn 170gm fuel to produce work done of one horsepower-hour. Suppose we go motoring for 10 hours and burn 50 litres of fuel, then our engine has burnt 50 x 880 gm of fuel = 44,000 gm of fuel. Now, each 170 gm produces a horsepower-hour so the total number of horsepower-hours is 44,000 divided by 170 = 259 horsepower-hours. We were out for 10 hours so our engine was producing a shaft horsepower of 25.9 hp on average for that passage.

So, given that a diesel throttle is more of a governor than a throttle, this is the only easy way we have to get any idea of the actual power produced. Most of us have a much better idea of what, say, 26 hp is likely to do than a given quantity of fuel.
 
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