Estimate your boat's top speed based on its specifications
Common planing V-hull
Typical displacement hull
Efficient long-range cruiser
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Download NowEstimating how fast a boat will go is the first sanity check of any design or purchase, and it comes from three things working together: the length that sets the wave barrier, the power available to overcome it, and whether the hull is built to plough through the water or climb on top of it.
A displacement hull is capped near hull speed no matter how much power you bolt on, so its ceiling is geometry. A planing hull trades that ceiling for weight and horsepower, reaching a speed where drag balances thrust, and the relationship is well captured by a simple constant times the power-to-weight ratio.
Enter waterline length, displacement and horsepower, choose the hull type, and the calculator estimates achievable speed and shows which factor, length or power, is currently holding you back.
Hull Speed = 1.34 × √LWL; Planing Speed ≈ C × √(HP ÷ Displacement), C ≈ 200 planing, 170 semi-displacement| Hull Type | Governing Law | Typical Top | Notes |
|---|---|---|---|
| Displacement | 1.34 × √LWL | 6 - 12 kn | Power barely adds speed |
| Semi-displacement | C ≈ 170 | 12 - 20 kn | Partial rise, efficient |
| Planing | C ≈ 200 | 25 - 50 kn | Rides on its own wake |
| High-performance | C > 220 | 50 kn+ | Light, fine entry |
Hull type and the speed law that governs it.
Formula verified against standard naval-architecture references · Updated October 2026
A displacement hull is limited by hull speed, about 1.34 times the square root of waterline length, so its top speed comes from length, not power. A planing hull is set by power-to-weight, estimated as a constant times the square root of horsepower over displacement. Tell the calculator your length, weight, power and hull type and it estimates speed and flags which factor is limiting you.
Only up to a point, and never for a displacement hull once it is at hull speed. On a planer, speed rises with the square root of power, so doubling horsepower adds about forty percent more speed, not double. That steep cost is why chasing the last few knots is expensive in fuel, weight and money, and why choosing a sensible cruise speed beats fitting an oversized engine.
A displacement hull ploughs through the water and is limited by the wave it makes. A planing hull, given enough power, lifts and rides on top of its own wake, escaping hull speed. Semi-displacement sits between, rising partly and trading top speed for efficiency. The type decides which formula governs your expected speed.
Usually load, prop, trim or fouling. The estimate assumes a clean hull, a well-chosen prop and correct weight; a heavy load, wrong pitch, a nose-high transom or growth all subtract knots. Check that the engine reaches rated rpm and that the boat trims level, and the real figure will close on the theoretical one.
Most sailing yachts are displacement hulls, so their motor speed is capped near hull speed and the planing formulas do not apply. A typical 30 foot cruiser makes six to seven knots under power, close to its wave limit. Only outboard-powered or hybrid sailboats with wide transoms begin to plane, in which case the power-to-weight estimate applies.
Both act through the same power-to-weight ratio, but weight is the sneaky one. Because it is easy to add and hard to remove, extra load steadily erodes speed, and it also raises the power needed to plane. When choosing an engine, plan around the heaviest realistic displacement so the boat still performs when loaded.
It is a reliable order-of-magnitude guide rather than a sea-trial result. The constants summarise typical hull quality, and individual boats vary with form, drag, prop and conditions. Use it to compare designs and to set an expected band, then validate against measured performance of the same model, which turns an estimate into a trustworthy number.