# Boat Speed Calculator

URL: https://speedycalculator.com/calculators/everyday-life/boat-speed/
Last updated: 2026-10-25

> The boat speed calculator estimates the top speed of a planing powerboat from its shaft horsepower and displacement using Crouch's formula, and works backwards to the power needed for a target speed.

## Inputs

- Shaft horsepower (hp)
- Displacement (loaded weight) (lb, kg, g, oz)
- Boat type (Crouch constant)

## Results

- Estimated top speed
- Weight per horsepower

## Formula

```
Speed (mph) = C × √(shaft hp ÷ displacement in lb)
Required hp = (speed ÷ C)² × displacement
```

## Explanation

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

FieldWhat to enterShaft horsepowerIn hpDisplacement (loaded weight)Units: lb, kg, g, ozBoat 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)7523.7112.52915033.5187.537.522541.130047.4

## Worked 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.

## FAQ

**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).


## Sources

- [Crouch, G. – Planing formula (Motor Boating, 1930s)](https://en.wikipedia.org/wiki/Hull_speed)