Calculate displacement hull speed, auxiliary engine horsepower matching, shaft RPM, and recommended prop diameter and pitch for cruising sailboats.
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A sailboat propeller calculator sizes auxiliary marine propellers for displacement hull sailboats. Because cruising sailboats operate within a strict displacement speed ceiling dictated by their waterline length (LWL), sizing requires balancing engine horsepower, shaft reduction gearbox RPM, and cruising drag under sail.
V_{\text{hull}} = 1.34 \times \sqrt{\text{LWL}} \quad ; \quad V_{\text{cruise}} \approx 0.85 \times V_{\text{hull}}Where LWL is waterline length in feet and 1.34 is the Froude speed-to-length ratio constant for standard displacement monohulls. Diameter and pitch are derived from Crouch empirical sizing formulas.
Scenario: A 32-foot cruising sailboat with a 28-foot waterline (LWL), 25 HP diesel engine, 2.5:1 transmission (1,000 shaft RPM), and a 3-blade prop.
Theoretical Hull Speed: 1.34 × √28 = 1.34 × 5.2915 = 7.09 knots.
Target Cruising Speed: 7.09 × 0.85 = 6.03 knots (6.94 mph).
Recommended Prop Dimensions: ~15-inch diameter × 10-inch pitch (3-blade).
As a displacement hull moves forward, it generates a bow wave and stern wave. As speed approaches 1.34 × √LWL, the wavelength matches the waterline length, trapping the boat in a wave trough that requires massive exponential power to climb out of.
A standard rule of thumb for cruising sailboats is 2 to 2.5 horsepower per 1,000 pounds (or 4 to 5 HP per displacement ton) to motor safely against 25-knot headwinds and opposing tidal currents.
Under sail, water flow presses a folding propeller blades together into a low-profile streamlined cone, reducing hydrodynamic drag by up to 90% and adding 0.5 to 1.2 knots of sailing speed.
Prop walk (asymmetric blade thrust) occurs because the lower propeller blade operates in denser, less aerated water than the upper blade. In reverse, a right-hand rotating propeller tends to pull the sailboat stern to port.
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