A long distance cruiser can only carry
a small amount of fuel and if he has a long period of no or contrary
wind, he might want to be able to motor for a long time with little
regard for the cost of the small amount of fuel.

the easiest way to make your fuel last longer is to go slower. It takes 2-1/4
times the fuel per hour to go 7.5 knots as it does to go 5 knots. 225% of the
fuel to go 150% the distance.