Definitely somethingw WRONG here ... A 2Gm should be using 1/2 GPH
Blocked exhaust pipe at the water injection elbow will do this....

Make chart of actual boatspeed vs. rpm. Plot various points of
operation in kts. vs. rpm ... from slow to wide open throttle
operation.
The data points (well below hull speed) will describe a straight line
..... then as the bow begins to rise, the data points will 'knuckle' and
the straight line will begin to curve upwards.
If you operate anywhere in the range of the straight line the fuel
economy will be at its best, if you operate in the curved portions, the
fuel economy will be worse, if you operate where the tha curve starts
to go straight up ..... all you're doing is using energy to lift the
bow and develop huge bow and stern waves .... and you wont go any
faster than being down the flat section of the plot.

Go to your engine manual and look at the power curve HP vs. rpm. vs.
GPH.

In article . com,
wrote:

Another thread discusses sailboats motoring "too much" but I think most
sailboats will use their engines to supplement sail but we still worry
about fuel economy. Even motoring alone, my yanmar 2GM seems to burn
only about 1-1.5 gals/hr at 6 kts in my 28' S2 but still I wonder about
the best way to conserve fuel in very light air.
Normally, in light air, I start with sails and no engine and eventually
get impatient with going only 3 kts and start the engine and put us up
to 5 and then eventually 6 kts or more. It is this last little bit
that I think burns the most fuel because she is most efficient at
slower speeds but as we get closer to hull speed fuel use rises
sharply.
An alternative strategy that would burn less fuel but would go a little
slower might to be ALWAYS run the engine at sufficient rpm to get to
3.5 kts and then use the sails to supplement that.