If fuel flow data are handy (dedicated sensor or from an electronically controlled engine) it's easy to create a speed/fuel flow chart. Just go out on a dead calm day and write down the numbers at evenly spaced engine RPM. Do a run against any current, and then do another in the exact opposite direction. Average the two.
If an engine curve and a prop curve are available for the hull, do the same thing but record engine rpm and speed. Then use the two curves to extract ballpark fuel burn at each rpm/speed. The prop curve for our boat is surprisingly accurate....at least when compared to cumulative real world fuel burn at our normal cruise speed.
The same methodology can be used to establish comparative data for single versus twin engine efficiency gains. It's about 5% for our 44 when operating at the same slow cruise speed of 8.0 knots. (The fuel burn improvement would increase at slower speeds where rudder angle would be much less).
Bottom line...get the engine and prop charts for your hull, or install a device that measures fuel flow on at least one engine. If an engine and prop chart are not available for your hull, it says something about the boat builder....
BTW, a Grand Banks is not a displacement or a "by design" semi-displacement hull, nor are the typical Taiwan Trawlers. They're planing hulls...typically poor performers at higher speeds with too small engines, and awful performers if they have engines big enough to get over the hump.
If an engine curve and a prop curve are available for the hull, do the same thing but record engine rpm and speed. Then use the two curves to extract ballpark fuel burn at each rpm/speed. The prop curve for our boat is surprisingly accurate....at least when compared to cumulative real world fuel burn at our normal cruise speed.
The same methodology can be used to establish comparative data for single versus twin engine efficiency gains. It's about 5% for our 44 when operating at the same slow cruise speed of 8.0 knots. (The fuel burn improvement would increase at slower speeds where rudder angle would be much less).
Bottom line...get the engine and prop charts for your hull, or install a device that measures fuel flow on at least one engine. If an engine and prop chart are not available for your hull, it says something about the boat builder....
BTW, a Grand Banks is not a displacement or a "by design" semi-displacement hull, nor are the typical Taiwan Trawlers. They're planing hulls...typically poor performers at higher speeds with too small engines, and awful performers if they have engines big enough to get over the hump.
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