I was under the impression that 1.0 or 1.1 was the best recommended(efficient) speed for a FD boat....according to Mr. Buelher.
Every boat has it's own number based on it's design, how it's built and whether it's sitting on it's designed lines or not...
here are some highlights of the same old stuff I have learned and used for 45 years+...many opinions on this forum are only tastes of the whole concept of boat design. I'm getting ready to sell my library of design books if anyone is interested...keep an eye in the classified section.
http://www.sponbergyachtdesign.com/THE%20DESIGN%20RATIOS.pdf
Nevertheless, what this tells you is that most displacement boats travel most of the time
at Speed/Length ratios of at least 1.0 and slightly above, so you need enough volume to
support the hull at those speeds. If volume is either too much or too low—that is if Cp is
too big or too small—your hull drag is going to go up. Either the boat is going to have to
push too much water out of the way (Cp too big) or it is going to sink into its own waves
(Cp too small).
Speed/Length ratio Cp
1.0 0.52
1.1 0.54
1.2 0.58
1.3 0.62
1.4 0.64
1.5 0.66
1.6 0.68
1.7 0.69
1.8 0.69
1.9 0.70
2.0 0.70
see how the s/l ratio is tied to the prismatic coefficient....the first 2 numbers are the s/l ratio...followed by the
Prismatic coefficient
Definition: Speed-length ratio is the speed of the vessel in knots divided by the square
root of the vessel’s waterline length in feet = V/Lwl^0.5. At speed-length ratios less than
1.34, the vessel is in displacement-mode motion—that is, the hull is simply moving the
water out of the way as it moves forward. When speed-length ratio is between 1.34 and
2.5, the vessel is in the semi-displacement or semi-planing mode—that is, it is trying to
rise up over its own bow wave to get onto plane. Some boats are designed to operate
at these speeds. Above speed-length ratio of 2.0 to 2.5, the vessel is planing and relies
on dynamic lift to raise and hold it out of the water so that it can skim along the surface
of the sea.