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1.3 times square root of the lwl
Hull speed is about 1.3 times the square root of your waterline length. For the Hunter 22, that is 5.7 knots (I can't find your lwl.) Above the hull speed, your need a lot more energy to lift the boat onto it's bow wave and drive it faster. So auxiliaries are generally not sized to drive the boat beyond the hull speed in flat water because that leads to lots of wasted energy, and oversized auxiliary. The other consideration is having reserve power to drive the boat into the waves. Motoring into a 3 foot chop can cut your speed in half of the same power in flat water. So your expected conditions do play a role. David Lady Lillie
Hull speed is about 1.3 times the square root of your waterline length. For the Hunter 22, that is 5.7 knots (I can't find your lwl.) Above the hull speed, your need a lot more energy to lift the boat onto it's bow wave and drive it faster. So auxiliaries are generally not sized to drive the boat beyond the hull speed in flat water because that leads to lots of wasted energy, and oversized auxiliary. The other consideration is having reserve power to drive the boat into the waves. Motoring into a 3 foot chop can cut your speed in half of the same power in flat water. So your expected conditions do play a role. David Lady Lillie
HP per ton
Generally - the recomendation is 3-5 HP per ton. For example, the J22 I sail does fine with 4-5 HP, a Catalina 25 generally needs 8-10HP, a 27 will want 12-20. Remember that displacement as given by many (most?) mfrs is (unfortunatly) light boat weight - so don't forget to add crew, fuel, fluids and stuff. A boat that is big for its weight will have windage issues, also if you need to fight big wind and current, you will want want power on the high side of the above range. The usual formula for hull speed is 1.34 * square root of waterline length (DWL). It is not actually a hard limit - just an area of a knee in a curve of power vs speed that represents greatly increasing difficulty to get more speed.
Generally - the recomendation is 3-5 HP per ton. For example, the J22 I sail does fine with 4-5 HP, a Catalina 25 generally needs 8-10HP, a 27 will want 12-20. Remember that displacement as given by many (most?) mfrs is (unfortunatly) light boat weight - so don't forget to add crew, fuel, fluids and stuff. A boat that is big for its weight will have windage issues, also if you need to fight big wind and current, you will want want power on the high side of the above range. The usual formula for hull speed is 1.34 * square root of waterline length (DWL). It is not actually a hard limit - just an area of a knee in a curve of power vs speed that represents greatly increasing difficulty to get more speed.
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