Boat Speed Calculator
Estimate a planing boat's top speed from horsepower and weight using Crouch's formula, in knots, mph, and km/h.
🚤 What is the Boat Speed Calculator?
The boat speed calculator estimates the top speed of a planing powerboat from two numbers you already know: the engine's horsepower and the boat's weight. It uses Crouch's formula, a classic rule of thumb in naval architecture that links speed to the power-to-weight ratio through a single hull-efficiency constant. The result appears in knots, miles per hour, and kilometres per hour at once.
Boaters use it to answer everyday questions: how fast will my rig go if I repower with a bigger outboard, how much speed will I lose if I load four passengers and a full fuel tank, and is this used boat's advertised top speed realistic for its power and displacement? Dealers, restorers, and anyone planning an engine swap can sanity-check a claim before spending money, and race-boat builders can compare hull types by changing only the Crouch constant.
The most important detail is what the formula does and does not cover. Crouch's formula applies to planing hulls that ride on top of the water at speed, so it does not describe displacement hulls such as sailboats and trawlers, which are limited by hull speed instead. It also treats propeller efficiency, trim, and sea state as fixed, so it gives a top-speed estimate rather than a guaranteed number. Real results usually fall within about 10 to 15 percent of the figure shown.
This tool is useful because it turns a horsepower rating and a weight into a clear speed estimate in one step, handles the pound and kilogram conversion for you, and shows the working so you can see exactly how power, weight, and hull type combine to set a boat's top speed.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 - Bass boat, 250 hp on 2,000 lb
Example 2 - Family cruiser, 300 hp on 4,500 lb
Example 3 - Racing catamaran, 400 hp on 1,200 kg
❓ Frequently Asked Questions
🔗 Related Calculators
How do you calculate boat speed from horsepower?
Use Crouch's formula: speed in knots equals a constant C times the square root of horsepower divided by weight in pounds. For a 2,000 lb boat with 250 hp and C = 150, speed = 150 × sqrt(250 / 2000) = 53 knots, about 61 mph. The constant C reflects the hull's efficiency.
What is Crouch's formula?
Crouch's formula is a classic planing-speed estimate: V = C × sqrt(hp / displacement), where V is in knots, hp is shaft horsepower, displacement is the boat's weight in pounds, and C is a hull-type constant. It was popularised by naval architect Dave Gerr and gives a quick top-speed estimate for planing powerboats.
What Crouch constant should I use for my boat?
Use 150 for average runabouts and cruisers, 190 for high-speed runabouts and light race boats, 210 for race boats, 220 for stepped hydroplanes, and 230 for racing power catamarans and vee-bottoms. A higher constant means a lighter, more efficient, more slippery hull design.
How accurate is the boat speed calculator?
Crouch's formula is an estimate, typically within about 10 to 15 percent of real top speed for a well-matched planing hull. It ignores propeller efficiency, trim, water and air conditions, and drag details, so treat it as a planning figure, not a guarantee.
Does the boat speed formula work for sailboats?
No. Crouch's formula is for planing powerboats that ride on top of the water. Displacement hulls such as sailboats and trawlers are limited by hull speed, which depends on waterline length: about 1.34 × sqrt(waterline length in feet) in knots.
Why does doubling horsepower not double boat speed?
Because speed rises with the square root of power. In Crouch's formula, speed is proportional to sqrt(hp), so doubling horsepower multiplies speed by sqrt(2), about 1.41. That is why the last few mph of top speed are so expensive in engine power.
Does adding weight slow a boat down?
Yes. Speed is inversely proportional to the square root of weight, so a heavier boat is slower for the same power. Cutting weight is often the cheapest way to gain speed. Doubling the weight cuts top speed by about 29 percent.
What units does this calculator use?
You enter engine power in horsepower and weight in pounds or kilograms. The calculator converts kilograms to pounds internally, then reports the estimated top speed in knots, mph, and km/h at once, along with the weight-to-power ratio in pounds per horsepower.