Determine optimal waterline dimensions and performance characteristics
Traditional cruising sailboat
Performance racing sailboat
Planing motor cruiser
feet
feet
knots
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Download NowWaterline length is the single most influential dimension in a displacement hull, because it sets the wave the boat must travel through and therefore the speed it can hold efficiently. It also shapes the interior layout and the size of the running gear you will build.
The waterline is shorter than the overall length whenever the bow and stern overhang. As a starting point this calculator uses roughly 80 percent of the length overall, a reasonable rule for classic double-ended and transom-stern cruising hulls, then derives the hull speed and length-to-beam ratio from it.
Enter your length overall and beam, and a target design speed, and the tool estimates your waterline length, the classic hull speed, the speed-to-length ratio and the waterline length-to-beam ratio so you can judge the design in one pass.
LWL ≈ 0.8 × LOA; Hull Speed (kn) = 1.34 × √LWL; S/L = Speed ÷ √LWL| LOA (ft) | Est. LWL (ft) | Hull Speed (kn) | L/B @ 8 ft beam |
|---|---|---|---|
| 18 | 14.4 | 5.1 | 1.80 |
| 24 | 19.2 | 5.9 | 2.40 |
| 30 | 24.0 | 6.6 | 3.00 |
| 36 | 28.8 | 7.2 | 3.60 |
| 42 | 33.6 | 7.8 | 4.20 |
Estimated waterline and hull speed at 0.8 × LOA (rounded).
Formula verified against standard naval-architecture references · Updated October 2026
Length overall (LOA) is the boat's full length from the tip of the bow to the end of the stern. Waterline length (LWL) is only the part of the hull that touches the water when the boat floats at its designed load line. Because most hulls overhang at both ends, LWL is always shorter than LOA, and it is LWL, not LOA, that governs hull speed and much of the seakeeping behaviour.
A displacement hull creates a wave train, and the distance between wave crests grows with speed. When that wavelength matches the waterline length, the boat falls into the trough of its own wave and is held back. A longer waterline means a longer wave is needed before that trap closes, so the boat can travel faster before the limit bites. That is why the hull-speed constant is multiplied by the square root of LWL.
For sailing cruisers a length-to-beam ratio around 3.0 to 3.5 gives a good mix of speed and form stability, while racers push past 4.0 for less drag. Powerboats and runabouts often sit nearer 2.5 to 3.5 for a dry, planing ride. Lower numbers feel stiffer and more spacious but carry more wetted drag, so choose based on your priorities.
It is an early-design shortcut, not a measurement. Real LWL depends on how the lines are drawn and how the boat sits when loaded. For laying out a hull or estimating performance to ten percent it is fine, but as soon as you have offsets or a lofted mold, measure the actual load waterline and use that number instead.
Yes, if the hull is shaped to do it. Planing and semi-displacement forms lift and skim over the wave, and modern fin-keel sailboats can surf when the wave outruns them. A true heavy displacement hull can only exceed the figure with a steep, impractical jump in power, so for most DIY cruisers the hull speed is a realistic working ceiling.
Beam does not change the theoretical hull-speed formula, which uses only waterline length. But a wide, flat sections creates more wave drag and makes the boat slam, so a beamy hull often reaches its efficient limit slightly below the classic number. Beam mainly shows up through the length-to-beam ratio and the resulting comfort and stiffness.
Because the wave a displacement hull makes travels with the hull, and its length is tied to the speed at which the hull can ride its own crest. It is the waterline, the length actually in the water fore and aft, that governs that wave system, not the overhanging bow and stern that only enter the picture as the boat heels, trims or pitches. Two boats of equal length overall can sail at quite different hull speeds if their load waterlines differ, so overhang effectively buys you extra waterline only when you heel or drive the boat onto it. That is why a modern wide-transomed hull often planes or rides its wave well past the figure the classic waterline formula predicts.