Sailboat Propeller Calculator
Calculate displacement hull speed, auxiliary engine horsepower matching, shaft RPM, and recommended prop diameter and pitch for cruising sailboats.
Sailboat Propeller Calculator
Performance Results
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What Is a Sailboat Propeller Calculator?
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.
How to Use This Calculator
- Enter your sailboat waterline length (LWL in feet, not LOA).
- Enter your auxiliary diesel or electric inboard engine horsepower (HP).
- Input your propeller shaft RPM at cruising speed (Engine RPM divided by transmission reduction ratio, typically 2:1 or 2.5:1).
- Choose 2-blade (low sailing drag), 3-blade (all-around motoring power), or 4-blade configuration.
Displacement Hull Speed & Prop Sizing
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.
Worked Example
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).
Tips & Key Notes
- A 2-blade fixed prop offers less drag while sailing, but a 3-blade prop provides significantly better stopping power in marinas and punch through head seas.
- Folding or feathering propellers (such as Max-Prop or Gori) reduce sailing drag by up to 1 knot while delivering full 3-blade reverse thrust.
- Ensure you have at least 10% to 15% of the propeller diameter in tip clearance between the blade tips and the bottom of the hull.
Frequently Asked Questions
Why is sailboat speed limited by waterline length (LWL)?
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.
How much horsepower per ton does a sailboat need?
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.
What is the advantage of a folding propeller on a sailboat?
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.
What causes prop walk on a sailboat?
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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