As was said, this is a tricky question.
But, for consideration, a well-sailed cruising boat on passage will make about 75-80% of her hull-speed potential over a 24-hour period.
Waterline length: 34'
Hull speed: 5.8*1.4 = 8.2 knots
80% of hull speed: 6.56 knots
24-hour run: 157.4 nm
Mine does a bit better, averaging 6.7 knots or 160 nm per day on offshore passages on a 34.2' waterline (27K lbs displacement, 42' LOA).
Obviously, there are many variables. But, the "80% rule" often comes close, I've found.
Last edited by btrayfors; 01-09-2009 at 03:38 PM.