Navman make a good, in fact very good, affordable fuel
measurement system. About the most accurate you will get for the "amature" market.
fuel use should be measured in a x?/hr bases. A boat has to many variables working with or against it to measure fuel usage based on distance.
As already stated, the manufacturer will or should have a fuel curve published. If the manufacturer was really good, they will have several curves on that Graph. That is because there are several different standards in use. For instance, Hp! There is Brake Hp, Shaft Hp, metric Hp, SAE Hp. I think that's all. Then there are two main loads applied that vary. The bench tested load is pretty much linear. But on a boat, you have a Propellor law curve. This means a non-linear load. Of course to make things even more difficult, that law varies with water
temp and Salinity.
Then the are variables to the engine
bench test. A specific weight of oil
is used for lubrication. The fuel oil
has to be a specific temperature. Temp has a big variance on volume of fuel. It goes on.
This is one reason why up untill now, fuel usage computers
were for the proffesional market. It was vary expensive to accurately calculate Fuel use because of something as simple as temperature. Navman have made a very affordable unit that gets around this problem.
But aside from all the above, for all Diesel
engines, it is ruffly accurate enough that a given dose of fuel will result in producing a given "bang". I have said this before, but I need to dig out my old theory papers and find the exact quantities of fuel dose. I have the numbers for each injector squirt and per Hp and Per Revs and so on. It is interesting to see that no matter what engine it is, all of them are very close to the same. However, my theory papers are before the days of Electronic controlled injection/high pressure rails. So I can't comment on them, accurately at least.
Unlike a Petrol(gasoline) engine, load has a lot less influence on the fuel usage. About a 20% increase between loaded and uloaded. Petrol can be as much as 80%. The newer computer controlled engines are a little more efficient, because they have been able to produce a more steady HP/torque range over a wider variance of RPM. Thus giving a better power drive to the water
over a greater RPM.