How this instrument works
As a displacement-hull boat — a sailboat, trawler, canal boat, or most any non-planing hull — moves through the water, it pushes up a bow wave whose length grows with speed. Once that wave's length matches the boat's own waterline length, the hull is effectively trying to climb its own bow wave, and pushing harder buys very little extra speed for a lot more fuel. The naval-architecture rule of thumb for where that happens is hull speed (knots) = 1.34 × √(waterline length in feet).
This is genuinely useful for anyone shopping for or owning a displacement-hull boat: a longer waterline buys real speed with no extra horsepower, which is why a 36-foot trawler comfortably outruns a 16-foot boat of the same hull type even at modest power, and why bolting on a bigger engine past a certain point mostly burns more fuel rather than adding knots.
The formula is a rule of thumb, not a hard physical limit — with enough power, a hull can be pushed past it (semi-displacement and planing hulls are built to do exactly that), and some sources use a slightly higher constant like 1.4 for easily-driven hulls. But 1.34 is the standard, widely cited figure for a conventional displacement hull, and it's accurate enough to plan a cruise or sanity-check a boat's specs.
- Enter Waterline length (ft) — the length of the hull at the waterline, not overall boat length (a longer bow overhang doesn't count).
- Read Hull speed (knots) — the practical displacement-hull speed limit for that waterline length.
- Use this for displacement hulls — sailboats, trawlers, canal boats. Planing powerboats routinely exceed this figure; the formula doesn't apply to them.
- If you only know overall boat length, waterline length is typically somewhat shorter — check the boat's specifications for the LWL figure specifically.
Worked example — a 16 ft waterline length
Enter 16 into Waterline length (ft) — a small daysailer or trailerable cruiser. Hull speed reads 1.34 × √16 = 1.34 × 4 = 5.36 knots.
That 5.36-knot figure is roughly what this boat can sustain in calm water without disproportionate extra fuel burn — pushing to 6 or 7 knots is possible with much more power, but each extra knot past hull speed costs far more than the last one did.
Questions
Why can't I just add more horsepower to go faster?
Below hull speed, extra power converts fairly efficiently into extra speed. Near and past it, more and more of that power goes into pushing a taller bow wave rather than moving the boat forward, so the fuel-per-knot cost rises sharply. A displacement hull can be pushed past this figure, but the return on horsepower drops off fast — which is exactly why displacement-hull boats are usually built and operated to cruise near, not past, it.
Does hull speed apply to powerboats and planing hulls?
Not once they're up on plane. A planing hull is shaped to rise up and skim across the water's surface rather than push through it, which lets it exceed the displacement-hull speed limit substantially — that's the whole point of the hull form. This formula describes displacement hulls: sailboats, trawlers, canal boats, and powerboats still operating in displacement mode below planing speed.
Why is the constant 1.34 specifically?
It comes from the physics of surface waves: a boat's bow wave travels at a speed set by its wavelength, and 1.34 is the point (in knots and feet) where that wave's length matches the boat's own waterline length. Some sources round it to 1.4 or use higher figures for unusually easily-driven hull shapes, but 1.34 is the standard, most widely cited value for a typical displacement hull.
How is this different from this site's hull-speed calculator in the Sports section?
Same formula, same physics — 1.34 × √(waterline length) either way. This page is written for general boat ownership and cruising: sizing up a boat, understanding fuel burn, comparing hull lengths. The Sports section's hull-speed instrument covers the identical calculation from a sailboat-racing angle, where hull speed matters for tactics and boat selection rather than cruising economy.
What waterline length do I use if I only know my boat's overall length?
Waterline length (LWL) is measured where the hull actually meets the water, and it's typically shorter than the boat's overall length once you exclude bow and stern overhangs — check the manufacturer's specification sheet for LWL specifically rather than estimating it from overall length, since the difference varies a lot by hull design.