What this calculator does
This boat speed calculator gives a rough top-speed estimate for a planing boat using the Crouch formula, a long-standing rule of thumb from naval architecture that relates speed to horsepower and total weight: speed (mph) equals a hull-type constant multiplied by the square root of horsepower divided by weight.
Boat speed depends heavily on hull design, propeller choice, trim and sea conditions, so treat the result as a ballpark figure for comparing options rather than a guaranteed number. It is aimed at planing hulls (runabouts, bowriders, small cruisers); for the different question of a displacement hull's maximum speed from its waterline length, see the hull speed calculator.
The formula
The Crouch formula is speed (mph) = C x square root of (horsepower / total weight in pounds), where C is a constant chosen for the general type of hull, ranging from around 100 for a slow, heavy cruiser or houseboat up to around 190 for a light racing runabout. Total weight should include the boat, engine, fuel and everyone and everything on board.
| Term | Meaning |
|---|---|
| C (Crouch constant) | A hull-type factor: higher for light, fast-hull racing boats, lower for heavy, slow-hull cruisers. |
| Horsepower | The engine's rated horsepower driving the boat. |
| Total weight | The fully loaded weight of the boat: hull, engine, fuel, gear and everyone on board. |
The inputs explained
| Field | What to enter |
|---|---|
| Engine horsepower | The engine's rated horsepower. |
| Total weight (boat, engine, fuel, crew) (lb) | Total loaded weight of the boat in pounds, including fuel and crew. |
| Hull / boat type | The general hull type, which sets the Crouch constant used in the formula. |
When to use it
Comparing engine options
Before repowering a boat, running a couple of candidate horsepower figures through the same weight shows roughly how much top speed a bigger engine might add.
Estimating the effect of extra load
Loading a boat with fuel, gear and passengers adds weight without adding power; comparing the estimate at light load versus fully loaded shows how much that costs in top speed.
Sanity-checking a horsepower-to-weight ratio
Two boats with very different horsepower and weight can still land on a similar hp-to-weight ratio, and therefore a similar estimated top speed, which is a useful gut check when shopping.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How estimated top speed changes with horsepower at a fixed weight
The same 2,500 lb boat with a range of engine horsepower.
How estimated top speed changes with total weight at fixed horsepower
A fixed 200 hp engine, across a range of total loaded weights.
| Total weight | Estimated top speed |
|---|---|
| 1,500 lb | 54.8 mph |
| 2,000 lb | 47.4 mph |
| 2,500 lb | 42.4 mph |
| 3,500 lb | 35.9 mph |
| 5,000 lb | 30.0 mph |
Questions
How accurate is the Crouch formula?
It is a rough, long-used rule of thumb, not a precise prediction. Real top speed also depends on hull shape, propeller pitch and diameter, trim, hull fouling and sea state, so treat the result as a ballpark figure for comparison rather than a guarantee.
Which hull-type constant should I pick?
Pick the option that best matches the boat: a light, purpose-built racing hull sits at the high end of the constant, an average recreational runabout sits in the middle, and a heavy cruiser or houseboat sits at the low end. When unsure, the average runabout setting is a reasonable default.
What weight should I use?
Use the fully loaded weight the boat will actually run at: the bare hull and engine plus a full tank of fuel, typical gear, and the number of people usually on board, not just the dry weight from a spec sheet.
How is this different from the hull speed calculator?
Hull speed applies to displacement hulls (boats that push through the water rather than rise on top of it) and depends only on waterline length, capping the maximum speed regardless of extra power. This calculator instead estimates the speed a planing hull can reach once it has enough power to get up and ride on top of the water.
For a displacement hull's maximum speed based on waterline length instead of power, see the hull speed calculator.