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.