Here are some generic sailing calculations, these calculations are based on theoretical conditions (i.e., no current, no wind resistance, no excessive drag, 100% efficiency, etc.
and should be used for comparison purposes only. These calculations should be used as “Rules of Thumb” and not on how a specific sailboat will perform in all weather and sea conditions. Hull Speed Formula Determines the theoretical boat speed for a displacement hull shape. This formula assumes that there is no current, no wind resistance and the hull is clean and free of objects that could cause excess drag in the water. Hull Speed = (1.34) * (LWL^0.5) = In Knots The calculation uses a notation of LWL to the POWER of 0.5. This is the same as the square root of the quantity. Horsepower Required for Hull Speed Determines the theoretical horsepower required for a displacement hull shape to reach hull speed. This formula assumes that there is no current, no wind resistance and the hull is clean and free of objects that could cause excess drag in the water. Horsepower = Displacement/{(150^2)/(Hull Speed^2)) As can be seen from this equation doubling the speed requires 4 times the horsepower. Maximum Boat Speed for a Propeller As a propeller turns, its pitch determines how far it travels through the water. No propeller is 100% efficient so that in reality it "slips" by moving water aside rather than straight backward. For a planing hull shape, this propeller slip can be as low as 10%. For a displacement hull shape with a high speed propeller, this slip is usually around 45%. This means that the maximum vessel speed is a function of the shaft speed in revolutions per minute, the pitch of the propeller, and the amount of propeller slip. Assuming no propeller slip, the maximum boat speed will be: Boat Speed = (RPM * 60 * Pitch)/72913.2 = In Knots If the propeller slip is 45% then the propeller's forward motion is 55% utilized so the maximum speed for this combination would be: Boat Speed = {(RPM * 60 * Pitch)/72913.2}* 0.55 = In Knots However, since the maximum boat speed is the theoretical hull speed for a displacement hull shape, the propeller will function with an inefficient slip if calculated propller speed is greater than the hull speed, since you can't go faster than the theorectical hull speed. As a general rule, if you purchase a typical outboard motor for use on a sailboat, you should instruct the dealer to provide the lowest pitch and largest diameter propeller possible consistent with the motor's designed operating speed and horsepower. The "standard" propellers provided with these motors will just not be efficient with a sailboat since most "standard" propellers are designed to be used by planing hull shape boats (power boats). Either you will be running at 1/2 throttle and therefore never use all the available horsepower, or you will be running at full throttle and wasting the energy in propeller slip. Sail Area To Displacement Ratio - SA/D Used for sailboat comparisons. High numbers are associated with racing boats. The calculation is Sail Area in square feet divided by displacement in cubic feet to the 2/3 power. The cubic feet of displacement is calculated as the equivalent of sea water. Therefore there is one cubic foot of displacement for every 64 pounds of cruising weight of sailboat. SA/D = Sail Area / (Displacement^0.667) Displacement to Length Ratio - D/L Used for boat comparisons. Low numbers are associated with racing boats. The calculation is Displacement in long tons (2240 pounds) divided by the quantity 0.01 times the length of the waterline cubed. D/L = (Displacement) / [(0.01 * LWL)^3] Capsize Formula Used to determine how well a sailboat is designed for offshore use. This formula is very generic and should be used as a guide only. Sailboats with capsize ratios below two are said to be more suitable for offshore use. The calculation is Beam dimension in feet divided by the cube root of Displacement expressed in cubic feet of sea water (64 pounds per cubic foot). Capsize Ratio = Beam / [(Displacement / 64)^0.333] These calculations can be found in most sailing manuals. Smooth sailing Clyde
and should be used for comparison purposes only. These calculations should be used as “Rules of Thumb” and not on how a specific sailboat will perform in all weather and sea conditions. Hull Speed Formula Determines the theoretical boat speed for a displacement hull shape. This formula assumes that there is no current, no wind resistance and the hull is clean and free of objects that could cause excess drag in the water. Hull Speed = (1.34) * (LWL^0.5) = In Knots The calculation uses a notation of LWL to the POWER of 0.5. This is the same as the square root of the quantity. Horsepower Required for Hull Speed Determines the theoretical horsepower required for a displacement hull shape to reach hull speed. This formula assumes that there is no current, no wind resistance and the hull is clean and free of objects that could cause excess drag in the water. Horsepower = Displacement/{(150^2)/(Hull Speed^2)) As can be seen from this equation doubling the speed requires 4 times the horsepower. Maximum Boat Speed for a Propeller As a propeller turns, its pitch determines how far it travels through the water. No propeller is 100% efficient so that in reality it "slips" by moving water aside rather than straight backward. For a planing hull shape, this propeller slip can be as low as 10%. For a displacement hull shape with a high speed propeller, this slip is usually around 45%. This means that the maximum vessel speed is a function of the shaft speed in revolutions per minute, the pitch of the propeller, and the amount of propeller slip. Assuming no propeller slip, the maximum boat speed will be: Boat Speed = (RPM * 60 * Pitch)/72913.2 = In Knots If the propeller slip is 45% then the propeller's forward motion is 55% utilized so the maximum speed for this combination would be: Boat Speed = {(RPM * 60 * Pitch)/72913.2}* 0.55 = In Knots However, since the maximum boat speed is the theoretical hull speed for a displacement hull shape, the propeller will function with an inefficient slip if calculated propller speed is greater than the hull speed, since you can't go faster than the theorectical hull speed. As a general rule, if you purchase a typical outboard motor for use on a sailboat, you should instruct the dealer to provide the lowest pitch and largest diameter propeller possible consistent with the motor's designed operating speed and horsepower. The "standard" propellers provided with these motors will just not be efficient with a sailboat since most "standard" propellers are designed to be used by planing hull shape boats (power boats). Either you will be running at 1/2 throttle and therefore never use all the available horsepower, or you will be running at full throttle and wasting the energy in propeller slip. Sail Area To Displacement Ratio - SA/D Used for sailboat comparisons. High numbers are associated with racing boats. The calculation is Sail Area in square feet divided by displacement in cubic feet to the 2/3 power. The cubic feet of displacement is calculated as the equivalent of sea water. Therefore there is one cubic foot of displacement for every 64 pounds of cruising weight of sailboat. SA/D = Sail Area / (Displacement^0.667) Displacement to Length Ratio - D/L Used for boat comparisons. Low numbers are associated with racing boats. The calculation is Displacement in long tons (2240 pounds) divided by the quantity 0.01 times the length of the waterline cubed. D/L = (Displacement) / [(0.01 * LWL)^3] Capsize Formula Used to determine how well a sailboat is designed for offshore use. This formula is very generic and should be used as a guide only. Sailboats with capsize ratios below two are said to be more suitable for offshore use. The calculation is Beam dimension in feet divided by the cube root of Displacement expressed in cubic feet of sea water (64 pounds per cubic foot). Capsize Ratio = Beam / [(Displacement / 64)^0.333] These calculations can be found in most sailing manuals. Smooth sailing Clyde