T&T: Fuel usage

[Robin Brueckner]

"...My 46'  trawler-that-used-to-be-a-sailboat ....cruises 7.5 kts at 1.5
gph (1800 rpm), or 7 kts at about 1.2 gph.  This  year we're cruising at
6.5 kts (1500 rpm) and hoping for 1.0 gph..."

I did not see this before I posted a few moments ago about fuel
usage....This is EXACTLY  the type efficiency I was positing that results
from smaller displacement hulls (sail vs power). Usually this shows as a
reduced beam to length ratio....33,000 lbs in a 46 ft "trawler" is
really,really modest.

In any case, as boat speed is roughly proportional to  the cube of power,
so reducing from 7 to 6.5 knots means a power reduction of about (6.5/7)
cubed or about 0.8, Bingo!!!, just about a 20% reduction, right in line
with the posters hoped for fuel reduction.

But at these speeds, be SURE you travel with prevailing tidal currents
whenever possible as several knots against you vs with your direction of
travel can mean a 50% difference in fuel usage...as say 4.5 knots
opposing vs 8.5 knots with current. In general go into bays on rising
tides, and exit on falling...Eldridge (the Yellow Book) provides
suggestions on how to do this in various  NE states.  In Eastern LI
Sound  and adjacent Fischers Island Sound to Rhode Island Sound, currents
can run three knots (and sometimes more), so plan accordingly....

Also, in general, early morning travel is generally calmer than afternoon
travel which also benefits fuel economy. I have frequently arisen at 4 or
5 AM to travel to/from  Maine with currents and calm water in my favor.
Arriving at your destination mid morning is a nice change of pace.

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