⛵ Boat Speed Calculator

How to Use This Boat Speed Calculator

Hull speed is the natural speed limit of a displacement sailboat, set by its waterline length. Enter the waterline length in feet and get hull speed in knots, plus mph and km/h conversions and a realistic cruising speed for passage planning.

The Hull Speed Formula

The 1.34 constant comes from wave physics: it is the speed in knots where the bow wave\'s wavelength matches the waterline length. At that speed the stern sits in the wave trough and the boat must climb uphill to go faster, which takes disproportionate power. The formula assumes a typical displacement hull; light, flat-bottomed boats plane and multihulls cheat it entirely.

Worked Example

A cruiser with a 30-foot waterline: square root of 30 is 5.477, times 1.34 gives 7.34 knots hull speed. In mph: 7.34 x 1.15078 = 8.4 mph. In km/h: 7.34 x 1.852 = 13.6 km/h. For passage planning at 80 percent: 5.87 knots, so a 60-nautical-mile crossing takes about 10 hours 13 minutes. Budgeting the full 7.34 knots would promise an 8-hour trip you will almost never achieve against real wind and current.

What Hull Speed Means in Practice

Hull speed is a soft wall, not a hard one. Approaching it, the stern wave grows, the bow rises, and fuel burn or sail effort climbs steeply for tiny gains. That is why experienced sailors cruise below it: the boat is quieter, the motion is kinder, and the fuel lasts. Racing sailors flirt with it constantly, surfing down waves to spike past it for seconds at a time. If your boat regularly exceeds its calculated hull speed with ease, it is probably a planing or semi-displacement design, and the formula was never meant for it.

Waterline Length vs Overall Length

Marketing loves overall length; physics only cares about waterline. A 35-footer with long classic overhangs might float on 27 feet of waterline at rest, giving 6.96 knots, then stretch toward 30 feet when heeled, nudging past 7.3. Modern wide-stern designs float nearly their full length all the time, so their at-rest number is honest. When comparing boats, always compare waterline lengths. Two boats both called 32-footers can differ by a full knot of hull speed, which is an hour on every ten miles of passage.

A Second Worked Example: The 36-Foot Cruiser

A cruiser with a 36-foot waterline: the square root of 36 is 6, times 1.34 gives 8.04 knots hull speed. In mph: 8.04 times 1.15078 is 9.25 mph. In km/h: 8.04 times 1.852 is 14.9 km/h. For passage planning at 80 percent: 6.43 knots. A 120-nautical-mile passage at that pace takes 120 divided by 6.43, which is 18.7 hours, about 18 hours 40 minutes. Plan the full 8.04 knots and you promise yourself a 15-hour trip that reality will not deliver.

Compare with the 30-footer from the first example at 7.34 knots. Six extra feet of waterline buys 0.7 knots, which sounds small until you multiply it across an ocean crossing: on a 2,000-mile passage it is the difference of roughly two days. Waterline length is the cheapest speed money can buy, which is why cruisers keep getting longer.

Displacement vs. Planing vs. Multihull

Hull speed only limits displacement hulls, boats that push through the water rather than skimming over it. A planing powerboat climbs up and over its own bow wave once it has enough power, leaving hull speed behind entirely. That is why a 25-foot center console does 40 knots while a 40-foot sailboat does 8. Different physics, different limits.

Multihulls cheat differently. A catamaran's long, narrow hulls generate a much smaller bow wave, so the wave-making resistance that creates the hull-speed wall stays low. A 40-foot cruising cat routinely sails at 10-12 knots, well past the 8.5-knot hull speed its waterline would suggest for a monohull. The trade is motion and cost: cats are pricier per foot and move differently in a seaway. But on pure speed per foot of waterline, narrow hulls win.

Passage Planning Mistakes

First: planning at hull speed instead of cruising speed. Hull speed is a ceiling measured in flat water with a clean bottom. Real passages have fouling, waves, and wind angles that never cooperate for 18 hours straight. The 80 percent figure exists because decades of cruisers learned the hard way. Second: ignoring current. One knot of adverse current over a 24-hour run steals 24 nautical miles, which can be a third of a day's progress. Check current charts, not just wind forecasts.

Third: forgetting that wind against current steepens waves and slows the boat beyond what either would do alone. Fourth: planning a dawn arrival by cutting it fine. Arriving at an unfamiliar harbor at night because the math was optimistic is how groundings happen. Build a buffer, plan to arrive with daylight to spare, and heave to or slow down rather than rush the last miles in the dark.

Why Hull Speed Is a Soft Wall, Not a Hard One

The classic formula, 1.34 times the square root of waterline length in feet, gives hull speed in knots, and it describes the speed where the bow wave's wavelength equals the boat's waterline length. Past that point the boat starts trying to climb its own bow wave, and the power needed rises brutally. But 1.34 is not a law of physics; it is a rule of thumb. Light, narrow hulls can exceed it with modest extra power, which is why racing sailboats routinely sail above their "hull speed," while heavy, full-keeled cruisers hit the wall exactly where the formula says. Treat the number as the start of the expensive zone, not a speed limit sign.

Worked Example: Fuel Burn Past the Hump

A 30-foot waterline trawler has a hull speed of 1.34 times the square root of 30, about 7.3 knots. Run it at 6.5 knots and a small diesel sips maybe 1.5 gallons an hour. Push to 8.5 knots, just past hull speed, and consumption can triple to 4 or 5 gallons an hour while speed rises barely 15 percent. Over a 100-nautical-mile passage, the slow trip takes 15.4 hours and burns about 23 gallons; the fast trip takes 11.8 hours and burns around 50 gallons. You save 3.6 hours and spend 27 extra gallons to do it. At marina fuel prices that is the most expensive three hours of the trip, and the wake you throw the whole way annoys everyone anchored near shore.