I suspect that many of you don't understand the significance of the link to the Perkins data sheet that was posted above which contains a fuel consumption curve for that engine. Let me explain:
The curve uses a theoretical relationship between engine rpm vs hp required WHILE DRIVING A PROP. At wot it is the same as the full load power/fuel consumption curve but at any lesser rpm the hp required is less than what the engine can put out at wot.
The manufacturer then applies his knowledge of the fuel consumption required to produce that hp at each rpm to produce the fuel consumption curve. That curve is approximate because it is based on theoretical hp since it would be impossible to measure each boat and propeller's hp requirement, but it is pretty close, particularly at or below displacement speeds.
So for your own boat first run the engine up to the speed you want to know the fuel consumption for and note the rpm. Then go to the curve the bottom one of several on that data sheet, and read over to the right for the fuel required. Note that Perkins being British, that gph is in Imperial gallons per hour. An Imperial gallon is 20% more than a US gallon.
For example, at 1,600 rpm a good moderate rpm for that engine, it burns about 1.7 Imperial gph or about 2 US gph. That is consistent with the individual boat data reported above.
And if you want to go fast, say maybe 1012 kts at 2,200 rpm then the engine will burn 3.8 Imperial gph or about 4.5 US gph. And of course multiply by 2 for the total fuel consumption for a twin.
David
