Hull Speed Calculator
The displacement-hull speed limit: 1.34 × √LWL — why your boat stops accelerating, with the wave physics that builds the wall.
At hull speed the bow wave's length matches the hull: the boat sits in a trough of its own making, and more power buys mostly a deeper hole. It's a soft wall — semi-displacement and planing hulls cheat it with dynamic lift.
Formula
⚠️ For planning and education only — verify with your vessel's documentation, naval-architecture data and official sources. Not for navigation or stability decisions on real voyages without professional data.
The displacement-hull speed limit: 1.34 × √LWL — why your boat stops accelerating, with the wave physics that builds the wall.
About Hull Speed Calculator
Every displacement boat carries an invisible speed limit shaped like a wave: at 1.34 times the square root of waterline length, the bow wave it generates is exactly one hull-length long, the stern settles into the trough, and further power digs the hole deeper instead of going faster. This calculator computes that limit in knots, mph and km/h — and explains the Froude-number physics that planing hulls spend their horsepower escaping.
How to use Hull Speed Calculator
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula hull speed (kt) = 1.34 × √LWL(ft) — where wavelength of the bow wave equals the waterline substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Hull Speed Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula hull speed (kt) = 1.34 × √LWL(ft) — where wavelength of the bow wave equals the waterline with sources cited on the page
- ✓At hull speed the bow wave's length matches the hull: the boat sits in a trough of its own making, and more power buys mostly a deeper hole. It's a soft wall — semi-displacement and planing hulls cheat it with dynamic lift.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Why 1.34 — where does the constant come from?+
Deep-water wave physics: a wave's speed in knots is 1.34 × √(wavelength in feet). When the boat's speed makes its bow-wave wavelength equal the waterline length, bow and stern each sit on a crest with a trough amidships — the hull is trapped in its own wave system. The constant is the wave-speed relation, not a property of boats.
Is hull speed a hard limit?+
It's a steep hill, not a wall: drag rises sharply approaching it, and pushing past demands disproportionate power — a heavy displacement cruiser may need triple the horsepower for one extra knot. Light, flat-sterned hulls (semi-displacement) climb partway up their bow wave with enough power; true planing hulls climb fully on top and leave the limit behind.
Why does waterline length, not overall length, govern?+
The wave system forms along the immersed body: overhangs above the water contribute nothing at rest. The classic sailing-yacht trick exploits it — long overhangs immerse as the boat heels, stretching the effective LWL and raising hull speed precisely when racing hard. Modern plumb-bow designs simply buy the waterline outright.
What hull speeds do typical boats have?+
√LWL arithmetic: a 25-ft waterline gives 6.7 kt, 36 ft gives 8.0, a 50-ft cruiser 9.5, a 100-ft classic 13.4. The square root is why doubling the boat adds only 40% to the speed — and why container ships (1,000+ ft waterlines, hull speeds past 40 kt) cruise at 20+ knots while staying in displacement mode.
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