Boat Hull Speed Calculator

The speed limit written into your hull

Every displacement hull has a practical speed limit set by physics: as the boat moves faster, the bow wave it creates gets longer, and when that wave stretches to the full waterline length, the boat ends up trying to climb uphill over its own wave. That speed is the hull speed, and this calculator finds it from your waterline length.

The result explains a lot of boating frustration. Pouring on more throttle past hull speed burns dramatically more fuel for almost no extra speed, because the resistance curve goes nearly vertical. Knowing the number tells you the efficient cruising speed to target instead.

How to use the hull speed calculator

Enter your boat's waterline length in feet, which is the length of the hull actually in the water, not the overall length including bow pulpits and swim platforms. Brochures list both; use the LWL figure. The tool returns hull speed in knots with mph and km/h conversions.

If you do not know your LWL, estimate it at about 90 percent of overall length for a typical sailboat, or check the manufacturer's specs. A foot or two of error moves the answer only slightly, since the formula uses a square root.

The formula and a worked example

Hull speed in knots equals 1.34 times the square root of the waterline length in feet. The 1.34 constant comes from wave physics: it is the speed length ratio at which the bow wave's wavelength matches the waterline.

Worked example: a 36 foot waterline. The square root of 36 is 6, times 1.34 gives 8.04 knots, which converts to 9.25 mph or 14.89 km/h. A smaller 25 foot waterline gives 1.34 times 5, which is 6.70 knots. Notice the pattern: doubling the speed requires quadrupling the waterline, which is why long hulls are fast and short ones are not.

When boats beat hull speed

Hull speed is a soft wall, not a law. Planing hulls, like most powerboats, climb on top of the water and leave the formula behind entirely. Racing sailboats with very light, easily driven hulls can surf past it downwind, and multihulls with their skinny hulls routinely exceed it. Foiling boats ignore it completely.

But for a heavy displacement monohull, the wall is real. Cruising sailors treat about 80 to 90 percent of hull speed as the sweet spot: nearly full speed at a fraction of the fuel or effort. Push to 100 percent only when the schedule demands it.

Using hull speed for trip planning

Passage planning starts here. Divide your route distance by a realistic cruising speed, usually 75 to 85 percent of hull speed for a sailboat under power, to get travel time. A 100 nautical mile passage on a 36 foot boat at 6.5 knots takes about 15.5 hours, an overnight trip, while assuming an optimistic 8 knots would promise 12.5 hours and leave you arriving after dark.

Fuel planning follows the same logic. Most displacement boats burn roughly with the cube of speed, so backing off from 8 to 6.5 knots can cut fuel burn nearly in half. The calculator's number is the anchor for both the schedule and the fuel budget.

Displacement versus planing: picking your boat

Hull speed only governs displacement hulls, the heavy boats that push water aside rather than skimming over it. Traditional sailboats, trawlers, and cargo ships live in this world, where the 1.34 rule sets a firm practical limit. If your boating dreams involve efficient long passages and sea kindly motion, displacement is the right physics and hull speed is your planning number.

Planing hulls play a different game. Most powerboats, from center consoles to sport cruisers, climb onto the surface once they have enough power, leaving wave making resistance behind. They burn far more fuel doing it, but speed is limited by horsepower and nerve rather than waterline. The tradeoff is comfort: planing hulls pound in chop that a displacement hull would shrug through.

Multihulls cheat the formula with geometry. A catamaran's narrow hulls have such a favorable length to beam ratio that they exceed displacement speeds routinely, which is why cruising cats sail rings around monohulls of the same length. Foiling boats go further still, lifting the hulls out entirely. When choosing a boat, decide which physics you want to live under, then use this calculator to understand its limits.

Multihull sailors should treat the calculator's number as trivia rather than a target, since their boats live beyond it by design. But even catamaran owners benefit from knowing the displacement reference: it marks the speed where a monohull of the same length would hit the wall, which explains the effortless way a cat walks away from heavier boats in moderate breeze.

Keep the number in your logbook. It settles more dock debates than any other single figure in sailing, and it keeps every fuel budget honest.

Worked Example: 36-Foot Waterline

A cruiser with 36 feet of waterline: hull speed = 1.34 x sqrt(36) = 1.34 x 6 = 8.04 knots. Push past 8 knots and the stern wave grows faster than the speed does; fuel burn per mile roughly doubles between 7 and 9 knots on a typical displacement trawler. The economical cruise is usually 80 to 90 percent of hull speed, about 6.5 to 7.2 knots here, where the bow wave stays small and the engine sips. Enter 36 above and the calculator returns 8.0 knots with the efficient cruise band.

A 22-foot waterline daysailer: 1.34 x sqrt(22) = 1.34 x 4.69 = 6.29 knots. That number explains why small boats feel slow on long passages: 6 knots for 10 hours is 60 nautical miles, a full day to cover what a 40-footer does before dinner. Displacement length is destiny, and no amount of sail area changes the wave physics, only how quickly you reach the wall.

When Boats Beat Hull Speed

Planing hulls ignore the limit entirely. A planing powerboat climbs on top of its bow wave and leaves displacement physics behind, which is why a 25-foot center console does 40 knots while a 25-foot sailboat does 6.7. Surfing down waves lets displacement boats briefly exceed it too; ocean racers surf at 15-plus knots on hulls rated for 9. And multihulls cheat with narrow hulls: a catamaran's fine hulls make small waves, so the practical limit sits far above the 1.34 formula.

Ultralight displacement boats stretch the constant itself. Racing designs with very light, long hulls use effective constants of 1.5 to 1.7 instead of 1.34, because their waves are smaller relative to length. The formula is a rule of thumb calibrated to traditional cruising shapes; the physics underneath, wave speed matching waterline length, never changes, but the constant moves with the design.

The Fuel Cost of Fighting the Wave

Horsepower demand climbs with the cube of speed near hull speed. Going from 6 to 7 knots on our 36-footer might take 25 HP; going from 7 to 8 takes 50; pushing to 8.5 takes 90 and gains half a knot. That cubic curve is why displacement cruisers have small engines and big fuel tanks: the engine is sized for the efficient cruise, not the maximum. Owners who repower bigger expecting proportional speed gains learn the wave math the expensive way.

Designing Around the Limit

Naval architects design cruising sailboats to sit just under hull speed at typical engine RPM, because horsepower past that point buys almost nothing. A 40-foot waterline cruiser with an 8.5-knot hull speed carries a 50 HP diesel that pushes 7.5 knots all day; doubling to 100 HP would gain half a knot and halve the range. Buyers comparing boats should compare waterline length first and engine size second. The longer hull is faster, quieter, and more efficient at every speed below the limit, which is every speed that matters.