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.

There are a couple of simple things you can do that will help you better
understand what the optimum RPM range is for your boat for fuel
conservation/consumption.

What you need a

a. a chart showing your engine's RPM vs. fuel consumption/hr. - this is
available in the Yanmar shop manual. If you don't have this, it's a
great resource and worth buying from your local Yanmar dealer. The
charts you're interested in should be somewhere in the front of the
manual (if memory serves me).

b. another chart showing engine RPM vs. boat speed through the water -
this is something you can easily create on your own. "Chapman's"
describes how to do this, but essentially you'll run a measured mile on
a calm day at various RPM's, clean bottom, clean prop, full tanks.

Armed with these two charts, you'll soon see that there is an optimum
range of RPM's (where both curves start to "flatten out") for fuel
consumption vs. speed made good.

The next step is to compare what the charts say "should be" versus what
your boat actually does by maintaining a log, monitoring your fuel
consumption, and comparing this information with designer specs and what
other similar boats are getting.

In our case (an Express 37 with a 3GM) a quick call to the designer and
a few fellow E37 owners revealed that although our fuel consumption was
about right, our boat speed was about 1.5 knots less than norm. We
later tracked this down to the prop which had been replaced by a
previous owner and the pitch was not as recommended.

Hope this helps,

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