Power to weight ratio
The displacement, sail area and waterline length of any well designed boat will give a rough, but ready guide to its likely performance
Why multihulls have more speed potential
Some simple measurements can show clearly what generally makes a faster boat, be it monohull or cat or tri. They also show why multihulls have more speed potential.
Obviously more sail area will produce more power to push the boat faster. But more sail needs to be supported by a bigger, and therefore heavier, hull. Heavier hulls have more resistance and tend to slow the boat down. The amount of sail that a boat can carry depends on the static stability or 'stiffness' of the hull. Therefore a designer will try to get more stability (so that the boat can carry more sail) with less weight (so that there will be less resistance in the water).
Stability from ballast or beam?
More static stability, or stiffness, in a monohull comes from more beam and more weight low down in a deep keel. Fast boats typically will have hulls made of light materials, like glass fibre, Kevlar carbon sandwich and fin keels.
A multihull carries no ballast, her exceptional static stability comes from her very wide beam. Faster monohulls will be wider for more stability, and lighter, for less resistance. Multihulls have the advantage that, even without any ballast, they can carry as much sail as a monohull of similar length. They are able to achieve the same stability with a considerably lighter displacement, which means they can sail faster.
Boat speed potential can be compared with the SA/D ratio.
Length waterline 43 ft
Sail Area 1152 sq ft
Displacement 2420 lbs
Ballast 8000 lbs
SA/D ratio 22
Length waterline 43 ft
Beam 23 ft
Sail Area 1141 sq ft
Displacement 16128 ft
Ballast ZERO
SA/D ratio 28.6
The monohull on the left has a very good SA/D of 22.
This puts it into the ‘High Performance Racer’
category. The catamaran on the right, however, carries
almost the same basic sail plan with 8000 lbs less
weight. Instead of 8000 lbs of ballast the cat has
almost 9 ft more beam to achieve the necessary static
stability.
Naturally, if you were to load up the catamaran with 8000 lbs of provisions, she would have considerably more hull resistance and sail slower. One calculation method puts this penalty at 1.5 to 2 kts in average conditions.
