How this instrument works
Hull speed is the traditional naval-architecture estimate of the fastest a displacement hull can move before it runs out of usable power against its own bow wave. As a boat speeds up, the wave it pushes ahead of itself grows longer; once that wave's length roughly equals the boat's waterline length, the hull sits trapped in the trough between its own bow and stern waves, and each further knot costs disproportionately more power. The rule of thumb v = 1.34 x sqrt(LWL) — knots, waterline length in feet — marks roughly where that wall appears.
The 1.34 figure comes from the physics of surface gravity waves: the speed of a wave train, in knots, works out to about 1.34 times the square root of its wavelength in feet. At hull speed, the boat's own wavelength has grown to match its waterline length, so the same constant carries over to describe the boat. It's a rule of thumb rather than a hard ceiling — actual hull shape, beam, and displacement shift the real number, and light, flat-bottomed, or planing hulls can exceed it substantially by climbing up and over their bow wave instead of staying trapped behind it.
For racing sailors, hull speed is a quick way to compare boats before a start: a longer waterline buys a genuinely higher speed ceiling, which is why offshore racing yachts are built long and lean rather than short and beamy, and why a bigger boat routinely out-hulls a smaller one on a light-wind reach even with an identical sail plan. It also doubles as a sanity check on GPS speed-over-ground data — sustained readings well above the hull-speed figure usually mean the boat has come up onto a plane or is surfing down a wave face, not cruising along in normal displacement mode.
- Enter the boat's Waterline length (ft) — the hull's length at the waterline when normally loaded, not overall length on deck.
- Read Hull speed (knots) — the classic displacement-hull speed-limit estimate for that waterline length.
- Compare two boats by re-entering each one's waterline length; the longer waterline always wins on hull speed alone.
- Treat the result as a rule-of-thumb ceiling, not a guarantee — hull form, displacement, and sail power all affect how close a real boat gets to it.
Worked example — a 25 ft waterline length
Enter a waterline length of 25 ft. Hull speed = 1.34 x sqrt(25) = 1.34 x 5 = 6.70 knots — a typical displacement-mode speed ceiling for a small-to-mid-size cruising or racing sailboat with that waterline.
Questions
Why does a longer waterline mean a higher hull speed?
Because hull speed scales with the square root of waterline length, so a longer hull can outrun a shorter one even with an identical hull shape and sail area. Doubling the waterline length only raises hull speed by a factor of about 1.41 (sqrt 2), not by double — which is why big gains in speed potential require disproportionately bigger boats, not just modest length increases.
Can a sailboat actually go faster than its calculated hull speed?
Yes, and it happens routinely. Light, flat-bottomed, or planing-capable hulls can climb up and over their own bow wave rather than staying trapped behind it, and any hull can briefly exceed hull speed surfing down a wave face in strong following seas. Heavy, full-keeled displacement cruisers have the hardest time breaking through, which is exactly the hull shape the rule of thumb describes best.
Is this the same formula used for powerboats?
The underlying wave physics is the same for any displacement hull, sail or power, so 1.34 x sqrt(waterline length) applies equally. Most powerboats, though, are designed to plane rather than stay in displacement mode, so their cruising and top speeds sit well above this figure, and the formula matters mainly at idle or trolling speed.
What waterline length should I use if I don't know it exactly?
Waterline length (LWL) is usually listed in a boat's specification sheet or class rules, and it's shorter than the boat's overall length on deck (LOA) because bow and stern overhangs sit above the water. If only LOA is available, treat this calculator's result as an upper-bound estimate — the true hull speed from the shorter, real waterline length will be a little lower.
How is this different from the site's boat-speed calculator?
It isn't a different formula — the everyday-life boat-speed calculator on this site runs the identical 1.34 x sqrt(waterline length) rule for a general-purpose hobbyist boat. This page frames the same number for racing and performance sailing, where hull speed is used to compare boats and judge sail-power ceilings rather than just estimate a cruising day's travel time.