This hull speed calculator finds the practical top speed of a displacement hull from its waterline length: hull speed (knots) = 1.34 × √LWL (feet) — the wave-making limit every sailor plans passages around.

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The Speed Limit Written by a Wave

A displacement hull doesn’t push through water so much as negotiate with the wave system it creates: the bow throws a wave, the wave’s length grows with the square of speed, and when that wavelength reaches the boat’s waterline length the stern drops into the trough behind the bow crest. From that point the boat is climbing its own bow wave, and resistance rises so steeply that for practical purposes speed stops — at 1.34 × √LWL. The defaults, a 25 ft waterline, give the classic cruising-sailboat number 6.7 knots. The formula explains the used-boat market’s oldest observation (waterline length is speed: a 36-footer’s 8.0 knots vs a 25-footer’s 6.7 is a passage-planning difference of hours per day), and it applies to displacement powerboats, kayaks, rowing shells and ships alike — the constant shifts slightly with hull form (fine hulls cheat toward 1.4+, full hulls sag toward 1.3), but the square root is non-negotiable physics.

The limit’s boundaries are as instructive as the limit. Planing and semi-planing hulls escape it by climbing ONTO the surface (dynamic lift replaces buoyancy), which is why a 25 ft center-console runs at 40 knots — it left the displacement regime the formula describes. Multihulls cheat differently: hulls so slender that their wave-making is feeble can sustain speed-length ratios of 2.0+. And even honest monohulls can surf briefly past hull speed downwind on a wave face — gravity’s loophole, not the formula’s error. For passage planning the sane use of the number is conservative: plan at 75–85% of hull speed for a cruising average, because wind, fouling, sea state and crew patience all tax the theoretical figure. In Froude-number language (for the engineers): hull speed ≈ Froude 0.40, the point where wave-making resistance begins its steep climb — naval architects have designed around this same wall since William Froude’s tank tests in the 1870s.

Worked Examples (Verified)

  • Defaults: LWL 25 ft → 1.34 × 5 = 6.7 kn.
  • 36 ft cruiser: 1.34 × 6 = 8.0 kn.
  • 17 ft sea kayak: 1.34 × 4.12 = 5.5 kn — sprint territory for a strong paddler, which is why touring pace sits at 3–4.
  • 100 ft ship: 1.34 × 10 = 13.4 kn — the square root’s mercy: 4× the length, 2× the speed.
  • Metric check: LWL 7.62 m (= 25 ft) → 2.43 × 2.76 = 6.7 kn ✓ same hull, same wall.

Hull Speed Discipline

  • Use WATERLINE length (LWL), not length overall — overhangs don’t float the wave system; classic long-overhang yachts differ by feet between the two.
  • The constant breathes with hull form: 1.30–1.45 spans most real displacement hulls; 1.34 is the honest standard, not a guarantee.
  • Displacement mode only: planing hulls, foils and slender multihulls live under different physics — don’t apply the wall to them.
  • Loading matters: a cruising boat loaded for a month sits deeper, its effective waterline and trim change, and realized speed sags below the brochure’s light-ship number.
  • Passage planning at 75–85% of hull speed converts this maximum into distance-per-day you can actually promise a crew.

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Frequently Asked Questions

How do you calculate hull speed?

Multiply 1.34 by the square root of the waterline length in feet: hull speed (knots) = 1.34 × √LWL. A 25 ft waterline gives 6.7 knots; a 36 ft waterline gives 8.0. In metric, hull speed ≈ 2.43 × √LWL(metres) knots. This calculator applies the standard constant and converts the result to mph and km/h as well.

What is hull speed in simple terms?

It’s the speed at which a displacement boat’s bow wave grows as long as the boat itself. Below it, the hull rides fairly level between its own waves; at it, the stern squats into the trough and the boat is perpetually sailing uphill — adding power beyond that point mostly makes a bigger wave, not a faster boat. It’s a practical wall, not a physical absolute: light, fine hulls can bully slightly past it, and surfing down a wave face can carry any hull beyond it briefly.

Why does a longer boat go faster?

Because wave physics prices speed by wavelength, and wavelength is capped by waterline length. A longer hull’s wave trough sits further behind the bow, so the “uphill” point arrives at a higher speed — by the square root of the length ratio: quadruple the waterline (25 → 100 ft) and hull speed only doubles (6.7 → 13.4 kn). This is why racing rules obsess over waterline, why overhangs that immerse at heel effectively lengthen the boat under sail, and why “she’s fast for her size” always means fast for her waterline.

Can a boat exceed its hull speed?

Yes, in three legitimate ways: planing (dynamic lift lifts the hull out of displacement mode — powerboats, dinghies, boards), extreme slenderness (multihulls and rowing shells sustain speed-length ratios near 2 because their waves are tiny), and surfing (gravity down an ocean wave face adds the energy the wave system won’t). What can’t be done is brute-forcing a heavy displacement hull far past it under power — fuel burn roughly doubles for the last knot before the wall and nearly stops buying speed after it, which is why displacement trawlers cruise at 70–80% of hull speed on purpose.

Is hull speed the same as top speed?

For a displacement hull under power in flat water, close — most reach 90–100% of calculated hull speed at full throttle, and engine sizing assumes it. Under sail, realized speeds depend on wind and point of sail; a boat may never touch hull speed upwind in a chop yet surf past it downwind. Hull speed is best treated as the design reference: passage plans at 75–85% of it are realistic, and a boat that can’t reach ~90% of it in calm conditions is telling you about fouling, prop mismatch or an optimistic waterline measurement.

What waterline length should I enter?

The length of the hull AT the waterline when loaded as you sail it (LWL or DWL), not the deck length (LOA). Manufacturers publish it; classic designs with long overhangs can have LOA exceeding LWL by 20%+ — using LOA inflates the answer fictionally. Loaded cruising weight immerses more hull and can slightly extend the effective waterline versus the light-ship spec. When in doubt, measure between the waterline’s bow and stern intersection points at rest, loaded.

What is the Froude number and how does it relate?

The Froude number (Fr = speed ÷ √(g × LWL)) is naval architecture’s dimensionless speed. Hull speed corresponds to Fr ≈ 0.40 — the neighborhood where wave-making resistance steepens for displacement forms. Below Fr 0.3, friction dominates and speed is cheap; above ~0.5 you’re paying planing prices without planing. The 1.34 constant in the sailor’s formula is simply Fr 0.40 wearing imperial units — same physics Froude tank-tested in the 1870s, still how ships are scaled from models today.

Is this hull speed calculator free?

Yes — free, instant, no sign-up. Enter your waterline length in feet for hull speed in knots, mph and km/h.