Enter any two of the three values (horsepower, speed, weight) and choose a hull factor (C) to estimate the missing variable for a planing boat.

Boat HP to Speed Calculator

Enter any 2 of the 3 fields (HP, speed, weight). Hull factor (C) is required (a default is provided).

C is an empirical constant that depends on hull/prop setup and conditions; for many planing boats it is often in the ~150–230 range (mph-based). Leave 170 if unsure.

Related Calculators

Boat HP to Speed Formula

This calculator uses Crouch’s planing-hull speed estimate. It is an approximation and depends on an empirical hull factor (C), so it is not a universal “all boats” law (and it is not intended for displacement-hull speed limits).

S_{mph} = C \cdot \sqrt{\frac{HP}{W}}

Variables:

  • Smph is the speed in miles per hour (mph)
  • HP is the engine power in horsepower (mechanical HP)
  • W is the boat weight in pounds (lb)
  • C is the hull factor (Crouch constant, mph-based), chosen empirically for the boat/hull/prop setup

To estimate speed, compute the ratio HP/W, take the square root, and multiply by C. (If you prefer knots: 1 mph ≈ 0.868976 knots.)

Horsepower (Mechanical) to Kilowatts Conversion Table
Horsepower (HP) Kilowatts (kW)
2.51.864
53.729
9.97.382
1511.186
2014.914
2518.643
3022.371
4029.828
5037.285
6044.742
7555.928
9067.113
10074.570
11585.756
12593.213
150111.855
175130.498
200149.140
225167.783
250186.425
*1 HP ≈ 0.7457 kW. Values rounded to 3 decimals.

What is Boat Horsepower?

Boat horsepower (HP) refers to the power output of the boat's engine. It is a measure of the engine's ability to perform work over time. Higher horsepower typically means the boat can achieve higher speeds and handle more weight (all else being equal, such as hull design and propeller efficiency).

How to Calculate Boat Speed?

The following steps outline how to estimate the speed of a planing boat from its horsepower and weight using Crouch’s formula.

  1. Determine the horsepower of the boat (HP).
  2. Determine the boat weight (W).
  3. Choose a hull factor (C) appropriate for the boat (or use a default as a rough estimate).
  4. Calculate the speed using Smph = C · √(HP/W), and convert units if desired (1 mph ≈ 0.868976 knots).
  5. After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Horsepower (HP) = 200

Weight (W) = 3000 lbs

Hull factor (C) = 170

Estimated speed: Smph = 170 · √(200/3000) ≈ 43.92 mph (≈ 38.17 knots).