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.

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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, official charts, and current weather sources, and follow all applicable FAA, ICAO, USCG, and other regulatory guidance. Do not rely on this tool for real-world flight, marine, or trailering decisions. Verify all results independently before operating any aircraft or vessel.

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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. At hull speed the speed-to-length ratio S/L = v ÷ √LWL equals 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.

Practical Tips

The “hump speed” is the drag peak a boat must push through between displacement speed (below hull speed) and full planing (well above): the resistance curve climbs steeply through this region, so it is the most fuel-hungry place to operate. The hump sits roughly between hull speed (S/L ≈ 1.34) and full planing (S/L ≈ 2.5–3), not at a single S/L figure. Downwind in a seaway a displacement boat can surf past its hull speed on the face of a wave, gravity supplying the extra thrust the engine or sails cannot. The 1.34 coefficient itself is simply what the deep-water wave equation reduces to once LWL is measured in feet and speed in knots - it is not a tuning factor. Treat every figure here as a planning study only; real top speed depends on hull form, displacement, propeller, fouling, and sea state.

Hull Speed Calculator

Frequently Asked Questions

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