Hull Speed Calculator

Frequently Asked Questions

What is hull speed?

For a displacement boat, hull speed (kt) ≈ 1.34 × √(waterline length in ft). A 25 ft LWL sailboat: 1.34 × √25 = 6.7 kt - the practical maximum before bow wave length equals hull length and drag explodes.

Can a displacement boat exceed hull speed?

Only briefly and with enormous power - wave drag rises as roughly the cube of speed near hull speed. A 40 ft LWL trawler (hull speed 8.5 kt) might reach 9 kt at huge fuel cost but not 12.

How does planing change the rule?

Planing hulls climb over their bow wave above ≈ 2 × √(LWL) kt and ride on lift, not displacement. A 20 ft planing skiff with adequate HP can cruise at 25+ kt - hull speed is irrelevant once on plane.

What about catamarans and ultralight displacement?

Long, narrow, light hulls (cats, racing keelboats) defeat the simple 1.34 formula because they make less wake. Speed/length ratios of 1.6–2.0 are routine, and modern foiling boats break the formula entirely.

Provided by AllCalculators.io
Free online calculators for everyday. No registration required.

Educational only. Not for flight or navigation operations.

Aviation & Marine Disclaimer: Educational only. Not for flight or navigation operations.

This calculator is for educational and informational purposes only and is not a substitute for official flight or navigation planning. Always use current performance charts, an approved POH/AFM, certified navigation tools, and follow all applicable FAA, ICAO, USCG, and other regulatory guidance. Verify all results independently before operating any aircraft or vessel.

JavaScript is required to use the interactive calculator above. The questions and answers below remain readable without JavaScript.

How This Calculator Works

Every displacement vessel - sailboats, trawlers, traditional powerboats, kayaks - is constrained by the wave system it creates. As speed rises, the bow wave lengthens; when the wave is exactly as long as the waterline of the boat, the stern sits in the trough and the bow on the crest, and the boat is effectively climbing its own wave. To go faster requires enormous additional power because the boat must literally lift onto the bow wave (plane) or push it apart. That speed-length limit is called hull speed. This calculator implements the classic Froude approximation: hull speed in knots ≈ 1.34 × √LWL (waterline length in feet). Use for planning study; actual top speed varies with hull shape, displacement, prop, and sea state.

Formula / Method

Hull speed (kt) = 1.34 × √LWL (ft). The constant 1.34 derives from the relation v = √(g × L ÷ 2π) for the speed of a deep-water wave of length L: convert to ft and kt and the coefficient becomes 1.34. The hull speed is the speed-to-length ratio S/L = v ÷ √LWL = 1.34. Modern semi-displacement and planing hulls can reach S/L of 1.5–2+ with sufficient power; true planing boats break free from the wave entirely and operate on hydrodynamic lift, but they pay an enormous drag penalty in the “hump” between displacement and planing speeds.

Worked Example

A 32-ft sailboat with a 28-ft waterline (LWL): hull speed = 1.34 × √28 = 1.34 × 5.29 ≈ 7.09 kt - consistent with the “6–7 knot” cruising speeds typical of mid-size cruising sailboats. A 22-ft sport fishing boat with full planing hull and 250 hp can easily hit 30+ kt at S/L > 6 because it is no longer behaving as a displacement vessel. A 100-ft superyacht with 80-ft LWL: hull speed = 1.34 × √80 ≈ 12.0 kt. Notice the law of diminishing returns - doubling LWL only raises hull speed by √2 ≈ 1.4×.

Common Mistakes

Using LOA (length overall) instead of LWL (length at the waterline) - bowsprits, swim platforms, and overhanging sterns do not count; only the wetted waterline forms the wave system. Treating hull speed as an absolute limit - long, fine, light hulls (rowing shells, multihulls) can exceed it; heavy, beamy hulls struggle to reach it. Assuming a planing-capable boat is “planing” whenever it goes fast - the transition through the hump speed is what costs fuel; many semi-planing boats sit just below the hump and burn far more than at displacement speed. Applying the formula to multihulls - each hull's wave system is narrow, so hull speed is much higher (or effectively irrelevant) for catamarans and trimarans.

Tips & FAQ

Can I exceed hull speed? Yes - with enough power. For a heavy cruising sailboat in 25 kt of wind running downwind on a big wave, surfing past hull speed is routine. For a powerboat, exceeding hull speed requires enough thrust to climb the bow wave. What is the “hump speed”? The drag peak between displacement (below hull speed) and planing (well above) - the worst part of the curve, often around S/L = 2–2.5. Why 1.34? It is the coefficient derived from the deep-water wave equation when LWL is in feet and speed in knots. Disclaimer: planning study only; actual top speed depends on hull form, displacement, propeller, fouling, and sea state.

Frequently Asked Questions

Related Aviation & Marine Calculators