Hull Speed Calculator (Metric)
Hull speed from waterline metres: 2.43 × √LWL(m) — the same wave wall in the units most of the world's boats are documented in.
2.43 is just 1.34 with the foot-to-metre conversion absorbed. Naval architects prefer the dimensionless form: hull speed is Froude number ≈ 0.40, the same physics in any unit system.
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.
Hull speed from waterline metres: 2.43 × √LWL(m) — the same wave wall in the units most of the world's boats are documented in.
About Hull Speed Calculator (Metric)
European boat papers, Asian shipyard specs and most of the world's sailing literature quote waterlines in metres — and the 1.34 constant silently assumes feet. This metric version uses the correctly converted 2.43 × √LWL(m), shows the km/h figure ferry schedules think in, and introduces the dimensionless truth underneath: hull speed is simply Froude number 0.40, the formulation naval architects use because it survives any unit system.
How to use Hull Speed Calculator (Metric)
- 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) = 2.43 × √LWL(m); equivalently Froude number Fn ≈ 0.40 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 (Metric)?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula hull speed (kt) = 2.43 × √LWL(m); equivalently Froude number Fn ≈ 0.40 with sources cited on the page
- ✓2.43 is just 1.34 with the foot-to-metre conversion absorbed. Naval architects prefer the dimensionless form: hull speed is Froude number ≈ 0.40, the same physics in any unit system.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Why is the metric constant 2.43?+
Pure unit conversion: 1.34 × √(ft per m) = 1.34 × √3.281 = 2.43. The physics is identical; only the bundled conversion changes. Quoting 1.34 with metres in hand under-predicts hull speed by 45% — a classic spreadsheet error in boat-buying comparisons.
What is the Froude number this tool reports?+
Speed normalized by wave physics: Fn = V/√(g·L). It's the ratio that makes a 9-metre yacht and a 300-metre ship comparable — both meet their bow-wave wall near Fn 0.40. Model-basin testing, resistance curves and design literature all speak Froude; the 'hull speed' of sailors is one point on that axis.
Do canal and inland-water boats obey the same limit?+
More strictly: shallow water modifies wave behavior (waves slow as depth drops), creating an earlier, harsher resistance hump — the reason canal barges plod and why displacement boats squat noticeably in shallow channels. Depth-limited 'hull speed' is governed by √(g×depth); below roughly a waterline of depth, the shallow limit takes over from the length limit.
How fast can I actually cruise relative to hull speed?+
Economical cruising for displacement hulls sits around 75–85% of hull speed (Fn ≈ 0.30–0.34), where the resistance curve is still gentle: a 9.2-m waterline cruiser with a 7.4-kt hull speed cruises sweetly at 6–6.3 kt. The last knot to hull speed can double fuel burn — our boat-fuel tools price exactly that curve.
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