Boat Speed Calculator
Find your boat top speed with Crouch formula using shaft horsepower, displacement in pounds, and hull type. Free calculator shows mph, knots, and km/h results.
About This Calculator
The Boat Speed Calculator uses Crouch formula to estimate a boat's top speed based on its engine power, weight, and hull type. This is the standard formula used by naval architects and boat designers for preliminary performance analysis.
Crouch formula is expressed as S = √(P / D) × C, where S is the speed in miles per hour, P is the shaft horsepower delivered by the engine, D is the displacement of the boat in pounds, and C is the Crouch constant that varies by boat type. The formula gives a theoretical top speed that helps in comparing design alternatives before building.
Displacement is the weight of water displaced by the hull, which equals the total weight of the boat including passengers, fuel, and cargo. It is based on Archimedes principle: a floating object displaces its own weight in water. Heavier boats require more power to achieve the same speed as lighter ones.
The Crouch constant varies from 150 for heavy cruisers and passenger vessels up to 230 for lightweight racing catamarans and sea sleds. This constant accounts for differences in hull efficiency and design. Lighter, more efficient hull shapes have higher constants and therefore higher speeds for the same power-to-weight ratio.
Regional Notes
Worldwide: Crouch formula uses imperial units (horsepower, pounds, miles per hour). The calculator also provides speed in knots (used globally in maritime navigation) and km/h (metric system).
US: Boating is widespread with horsepower ratings typically between 150-400 hp for recreational boats. Displacement commonly ranges from 3,000-10,000 lbs for speedboats.
UK & Europe: While metric units are standard, Crouch formula is still applied in imperial units. Boat speeds are often discussed in knots for navigation and km/h on inland waterways.
India: Recreational boating uses horsepower ratings similar to international standards. Displacement and speed conversions to knots are commonly used for coastal navigation.
Key Features
- Calculate top speed from horsepower and displacement
- Select from five boat types with appropriate Crouch constants
- Results in mph, knots, and km/h
- Visual bar chart comparing speed across boat types
- Doughnut chart showing power vs displacement ratio
- Shareable URL with your inputs and results
Frequently Asked Questions
What is Crouch formula for boat speed?
Crouch formula calculates boat top speed as S = √(P / D) × C, where S is speed in mph, P is shaft horsepower, D is displacement in pounds, and C is the Crouch constant based on boat type.
How do I calculate my boat speed?
Enter your boat's shaft horsepower and displacement in pounds, then select your boat type. The calculator applies Crouch formula to compute top speed in mph, knots, and km/h.
What is the Crouch constant for different boat types?
Cruisers and average runabouts have a Crouch constant of 150. Light high-speed cruisers and high-speed runabouts use 190. Racing boats use 210. Hydroplanes use 220. Racing catamarans and sea sleds use 230.
How do I increase my boat top speed?
Increasing shaft horsepower or reducing displacement increases top speed. A lighter hull or more powerful engine both improve the power-to-weight ratio that drives Crouch formula.
What is the difference between mph, knots, and km/h?
Miles per hour (mph) is the standard land speed unit. Knots are nautical miles per hour used in maritime and aviation: 1 knot equals 1.151 mph. Kilometers per hour (km/h) is the metric speed unit: 1 mph equals 1.609 km/h.
What factors affect boat speed besides power and displacement?
Hull design, length, hull material, propeller efficiency, water conditions, and load distribution all affect real-world speed. Crouch formula provides a theoretical estimate for preliminary design analysis.
How accurate is Crouch formula?
Crouch formula gives a good preliminary estimate of top speed for planning and design. Actual speed varies due to hull efficiency, propeller slip, sea conditions, and other real-world factors not accounted for by the simplified formula.
What is displacement in boating?
Displacement is the weight of water a boat displaces when floating, equal to the boat's total weight. It is measured in pounds or tons and is a key factor in determining speed via the power-to-weight ratio.