A quote from Task Force from this thread:
http://www.subsim.com/radioroom/show...fficient+speed
"If you go slower than the standard bell [the boat's most efficient speed] you use up more fule cause your motors are useing fule at a higher rate than you are moveing."
So, the sweet spot for the fleet boats is about 9.5-10 knots. S-Boats (using RFB) is 2/3 bell which will push you about 6knots.