Crouch’s planing formula has been used by boat designers for nearly a century. It links speed to the power-to-weight ratio through a constant C that reflects how efficient the hull is: about 150 for cruisers and average runabouts, 190 for light high-speed boats, 210 for racing boats and up to 230 for racing catamarans and sea sleds.
How to use the Boat Speed Calculator
- Shaft horsepower
- Displacement (loaded weight)
- Boat type (Crouch constant)
- Every result field is editable: type into Estimated top speed and Shaft horsepower plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the shaft horsepower (roughly 90–95% of the engine’s rated power for outboards), the loaded displacement and the type of boat. Type a target speed into the result to find the horsepower needed.
Tips
- Use your own measured figures – fuel economy, speed and prices vary a lot with driving style, traffic, load and season.
- Pick the unit in each field’s menu (miles or kilometres, mpg or L/100 km); the calculator converts everything for you.
- Fill up to the brim twice and divide the distance by the fuel added to measure real fuel economy.
- For vehicle modifications, treat results as a starting point and confirm with the parts maker or a professional mechanic.
Common mistakes to avoid
- Mixing US and imperial (UK) gallons – a UK gallon is about 20% bigger.
- Using the advertised fuel economy instead of real-world figures.
- Forgetting tolls, parking and wear and tear when comparing driving with public transport.
Inputs explained
| Field | What to enter |
|---|---|
| Shaft horsepower | In hp |
| Displacement (loaded weight) | Units: lb, kg, g, oz |
| Boat type (Crouch constant) | Choose one: Cruisers, passenger vessels, average runabouts (150), Light high-speed cruisers, high-speed runabouts (190), Racing boats (210), Hydroplanes (220) … |
What each result means
- Estimated top speed = [Boat type (Crouch constant)]×sqrt([Shaft horsepower]÷([Displacement (loaded weight)]÷0.45359237))×0.44704
- Weight per horsepower = [Displacement (loaded weight)]÷0.45359237÷[Shaft horsepower]
Example values
How the main result changes when Shaft horsepower changes and everything else stays at the example values above:
| Shaft horsepower (hp) | Estimated top speed (mph) |
|---|---|
| 75 | 23.7 |
| 112.5 | 29 |
| 150 | 33.5 |
| 187.5 | 37.5 |
| 225 | 41.1 |
| 300 | 47.4 |
Formula
Speed (mph) = C × √(shaft hp ÷ displacement in lb)Required hp = (speed ÷ C)² × displacementWorked example
Inputs — Shaft horsepower: 150 hp; Displacement (loaded weight): 3,000 lb; Boat type (Crouch constant): Cruisers, passenger vessels, average runabouts (150).
Results — Estimated top speed = 33.5 mph; Weight per horsepower = 20 lb/hp.
Frequently asked questions
How fast will my boat go with a bigger engine?
Speed rises with the square root of power: doubling horsepower raises top speed by about 41%.
What displacement should I use?
The loaded weight – hull, engine, fuel, gear and people.
How accurate is Crouch's formula?
Within about 5–10% for planing hulls when the right constant is used; it does not apply to displacement hulls.
What formula does the Boat Speed Calculator use?
Speed (mph) = C × √(shaft hp ÷ displacement in lb).