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When a planing hull is moving slowly (compared to its maximum potential when planing), it is (in effect) a displacement hull. Since the Columbia 30 has a planing hull, there are 2 different sailing modes of the Columbia 30's speed potential: (1) displacement mode and (2) planing mode. Much of the time (say in wind velocity under 10 knots), the boat sails in the displacement mode.
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| A Columbia 30 approaching planing speed off San Diego |
The speed of conventional displacement sailing yachts is limited by their waterline length. The generally accepted principle of the maximum theoretical hull speed of a displacement vessel is determined by the square root of the waterline length multiplied by 1.34. Displacement sailboats cannot go faster than their theoretical maximum hull speed because they create a wave at the bow and a wave near the stern as they begin to make good headway. And, there is a physical law that correlates the distance between the waves with the maximum speed that these waves can travel: the greater the distance between these waves, the faster they can go. Thus, as long as the conventional displacement hull is moving through the water, it cannot travel much faster than the speed allowed by the distance between the waves it creates. This is why the maximum theoretical speed of a displacement hull is a function of its waterline length. In other words, the longer the waterline, the faster the potential speed of a displacement vessel.
Because light-air sailing necessitates displacement sailing, we designed the Columbia 30 with the greatest waterline length that we could in a 30-foot sailing yacht, in order to achieve the fastest displacement speed possible. At rest, the waterline is 27 feet, which means that while in displacement mode the boat has a potential top speed of around 7 knots. This theoretical speed increases slightly as the 30 moves and the stern wave moves up. The upward movement of the stern wave increases the effective waterline length of the boat, which becomes close to 30 feet, delivering a theoretical top displacement speed of around 7 ½ knots.
As power and speed increase, however, a planing hull lifts itself up on top of its own bow wave (see the photo above). This causes the boat to displace much less water. As a result, there is much less wetted surface on the hull bottom, meaning much less drag as well. The speed of the boat dramatically increases when planing.
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| The Planing Hull of the Columbia 30 |
Therefore, the advantage of the Columbia 30's planing hull is that it is capable of much greater speed than a displacement hull. Another set of physical laws apply to the speed potential while planing: namely, the horsepower to weight ratio, or Sail Area to Displacement (SA/D) ratio. It's the same principle as a Formula-1 race car - just keep adding horsepower and removing drag and away you go…the Columbia 30 is capable of speeds of around 20 knots, or about 3 times its theoretical maximum displacement hull speed.
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