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Farr 11.6 (Farr 38)
10,525 Posts
Actually, most Catamarrans do not plane. Most Cats and certainly cruising cats are really displacement vessels than operate in that grey zone of ''semi-displacement'' speeds. The typically quoted formula for Hull Speed equal to 1.34 times the square root of the waterline length is based on the wave making of the hull.

The faster a boat goes the bigger its stern and bow waves become. Also the faster a boat goes the further aft that the bow wave migrates. At some speed around 1.34 times the square root of the waterline the bow wave and stern waved meet and combine creating this big wave that the boat must climb onto to go any faster. Boats that are on a plane have climbed up on to the top of this wave and have ceased to be in displacement mode (where the weight of the boat equals the weight of water displaced). Boats that are surfing are on the downward inclined plane of a wave and the force of that downward incline offsets the force needed to climb up on a plane.

Climbing up on a wave is only one way to assault the limits of hull speed. The other way is to make smaller waves. The shallow and very narrow hulls of a multihull, (or high speed monohulls for that matter) produce smaller waves and so it takes less force to push the boat beyond the theortical hullspeed. Many modern race boats will routinely travel at 1.5 times thier hull speeds and Cats can reach speeds of 2 to 2.5 times their hulls speed without actually planing.

The other issue in terms of speed is the amount of time spent at or near hullspeed. The easily driven hulls of lighter and narrow boats spend a lot more of their sailing time at or near thier theoretic hullspeeds that heavier, deeper and wider boats with the same theoretical hullspeed which is why lighter boats tend to be substantially faster than heavier boats.

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