Horsepower to Speed Calculator
Calculate theoretical boat speed based on engine horsepower and hull characteristics. Optimize your boat's performance with our marine speed calculator.
How horsepower relates to boat speed
Boat speed isn't a simple multiple of horsepower. It depends on how the hull moves through the water. Displacement hulls push water aside and are capped near their hull speed no matter how much power you add. Planing hullsclimb up and ride on top of the water, so for them speed scales with the square root of the power-to-weight ratio. This calculator picks the right model for the hull type you select.
Crouch's planing speed formula
For planing boats the calculator uses Crouch's formula:
Speed (knots) = C × √(HP ÷ weight in lbs)
The constant C reflects hull efficiency — about 150 for an average runabout, 190 for a light high-performance hull, and lower for heavy cruisers. Displacement and semi-displacement hulls are limited by hull speed (1.34 × √waterline length in feet), which the calculator also reports.
Why doubling power doesn't double speed
Because drag rises steeply with speed, a planing boat needs roughly four times the horsepower to double its speed. A 1,000 kg boat with 100 HP that does ~30 knots would need closer to 400 HP to reach 60 knots. For displacement boats the extra power buys almost nothing past hull speed — it just increases fuel burn. Check the cost of that with the fuel economy calculator.
Choosing the right engine size
Most planing boats want 25–40 HP per tonne of loaded weight to plane comfortably. To size an engine rather than predict speed, use the boat engine size calculator or the power-to-weight ratio calculator.
Note: this is a theoretical estimate. Actual speed varies with hull design, loading, propeller, trim and sea conditions.
Frequently Asked Questions
How much horsepower do I need to reach a given boat speed?
For planing boats, Crouch's formula estimates speed: knots = C × √(HP ÷ weight in lbs), where C is roughly 150 for an average cruiser and 190 for a high-performance hull. Displacement hulls are capped near their hull speed regardless of power. Enter your horsepower, weight and waterline length and this calculator estimates the speed for your hull type.
Does doubling horsepower double boat speed?
No. Because water resistance rises sharply with speed, you typically need roughly four times the power to double the speed of a planing boat. For displacement hulls, extra power beyond hull speed produces almost no speed gain — it just burns more fuel.
What is the difference between hull speed and actual top speed?
Hull speed (1.34 × √waterline length in feet) is the efficient speed limit for a displacement hull. Planing and semi-displacement hulls can exceed it with enough power, climbing onto their bow wave. This calculator shows both the displacement hull-speed limit and the estimated top speed for your power and hull type.