Boat Speed Calculator

How fast is 80 hp? On a planing boat that weighs 2,000 pounds loaded, 80 horsepower gives an estimated top speed of about 39 mph (34 knots) for an average hull, by Crouch’s planing speed formula. With the constant for heavy runabouts and cruisers the estimate drops to about 34 mph; with the one for high-speed runabouts it rises to about 43 mph. The calculator above estimates a planing boat’s top speed from its horsepower and loaded weight, or the horsepower needed for a target speed, in mph, knots or km/h.

How Fast Is 80 HP on a Boat?

Weight matters as much as power. These are Crouch’s formula estimates for 80 rated horsepower (76.8 shaft hp) at different loaded weights, for heavy runabouts and cruisers (C 150), average boats (C 175) and high-speed runabouts (C 190), in mph:

Loaded weight	C 150 (mph)	C 175 (mph)	C 190 (mph)
1,500 lb (680 kg)	39	46	49
2,000 lb (907 kg)	34	39	43
2,500 lb (1,134 kg)	30	35	38
3,000 lb (1,361 kg)	28	32	35
4,000 lb (1,814 kg)	24	28	30

The numbers are estimates for planing hulls with a well-matched propeller, not test results. A real boat can be faster or slower depending on its hull, propeller, trim and load. To convert mph to knots, divide by 1.151.

Boat Speed Formula (Crouch’s Formula)

The formula is named after George F. Crouch, the American naval architect who designed Baby Bootlegger, the Gold Cup winner of 1924 and 1925, and boat designers still use it for a quick first estimate. Naval architect Dave Gerr gives it with speed in knots, the loaded weight (displacement) in pounds and shaft horsepower, the power that reaches the propeller shaft. Gerr takes shaft horsepower as 95% to 96% of the engine’s rated (brake) horsepower; the calculator uses 96%. With two or more engines, add their horsepower together.

Worked example: 80 hp × 0.96 = 76.8 shaft hp. 2,000 ÷ 76.8 = 26.04, and √26.04 = 5.10. With C = 175, the speed is 175 ÷ 5.10 = 34.3 knots, or 39.5 mph and 63.5 km/h (1 knot = 1.151 mph = 1.852 km/h).

Because speed follows the square root of power divided by weight, doubling the horsepower raises the estimate by about 41%, and adding 20% more weight lowers it by about 9%.

Choosing the Boat Type Constant

C is an experience-based constant for the type of hull. Gerr suggests these values:

C	Type of boat
150	Heavy runabouts, cruisers, passenger vessels
175	Normal average for most ordinary boats
190	High-speed runabouts, very light high-speed cruisers
210	Race boat types
220	Three-point hydroplanes, stepped hydroplanes
230	Racing power catamarans and sea sleds

Values over 190 are uncommon for normal boats, and Gerr recommends 175 when in doubt, which is the calculator’s default. If you know the speed, weight and power of a similar boat, naval architect Eric Sponberg suggests working out its own constant instead: C = knots × √(pounds ÷ shaft hp). For example, Gerr reports sea trials of the Gerr 34 sportfisherman, designed by his office: 21,130 pounds with two 340 hp diesels (652.8 shaft hp) and a top speed of 29.5 knots. That works out to C = 29.5 × √(21,130 ÷ 652.8) ≈ 168, while the average value of 175 estimates 30.8 knots, about 4% high.

Speed by Weight per Horsepower

In Crouch’s formula only the ratio of weight to power matters, so one table covers any engine size. Divide the loaded weight by the total rated horsepower and read across; the columns are the same three boat types, in mph:

Pounds per hp	C 150 (mph)	C 175 (mph)	C 190 (mph)
10	53	62	68
15	44	51	55
20	38	44	48
25	34	39	43
30	31	36	39
40	27	31	34
50	24	28	30

For example, a 3,000-pound boat with 150 hp carries 20 pounds per horsepower, for an estimated 44 mph with an average hull.

Horsepower Needed for a Target Speed

Solved for power, the formula gives the shaft horsepower for a target speed: shaft hp = pounds × (knots ÷ C)². Divide by 0.96 for the engine’s rated horsepower. To reach 35 knots (40.3 mph), a 6,000-pound boat with C = 175 needs 6,000 × (35 ÷ 175)² = 240 shaft hp, or 250 rated hp. Choose Horsepower in the calculator to run this for your boat.

The result is a formula estimate, not a horsepower rating. Before you add power, check the maximum horsepower on your boat’s capacity plate and never exceed it. Federal rules (33 CFR 183) require a plate with that maximum on outboard-powered monohull boats less than 20 feet long, except sailboats, canoes, kayaks and inflatable boats, and many states prohibit installing a motor above it.

What Weight to Use

Use the loaded weight, also called displacement: the boat with its engines, fuel, water, people and gear, as it actually runs. Fuel and water add up quickly; 100 gallons of gasoline weighs about 618 pounds. The Fuel and Water Weight Calculator gives the weight of any amount of fuel or water.

When the Estimate Does Not Apply

• Displacement hulls. Crouch’s formula is for planing boats only. Displacement hulls, such as cruising sailboats and trawlers, don’t plane, and their practical top speed depends mainly on waterline length; use the Hull Speed Calculator for them.
• Boats that are not fully planing. Gerr treats a boat as planing when its speed in knots is at least 3 times the square root of its waterline length in feet. For a 20-foot waterline that is 3 × √20 = 13.4 knots (15.4 mph). Below that, a boat is in the semi-planing range, which the formula is not meant for.
• Propeller and setup. The estimate assumes a propeller and gearing well matched to the boat, and Mercury warns that the wrong propeller can significantly affect a boat’s performance. The Propeller Pitch Calculator works out speed from engine RPM, pitch, gear ratio and slip.

To turn a speed into trip time, use the Speed, Distance and Time Calculator; to convert between knots and mph, use the Knots to MPH Converter.

Frequently Asked Questions