Hull Speed Calculator — 1.34 Rule for Any Waterline Length

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What is hull speed?

Hull speed is the speed at which a displacement boat’s bow wave grows as long as the boat itself. Push faster and the boat starts climbing its own bow wave, which takes a lot more power for very little extra speed. The classic formula is simple: hull speed in knots equals 1.34 times the square root of the waterline length in feet. A boat with a 25-foot waterline has a hull speed of about 6.7 knots. A 36-foot waterline gives about 8 knots.

The formula comes from wave physics. A boat moving through water creates waves, and the wavelength of those waves grows with the square of the speed. When the wavelength equals the waterline length, the bow wave and the stern wave reinforce each other, and resistance climbs steeply. That is the wall that displacement boats hit, and it is why longer boats are faster: a longer waterline means a longer wave, which means a higher speed before the wall appears.

How the calculator works

Enter the waterline length — the length of the hull actually in the water, not the overall length including bowsprits and swim platforms — and choose feet or metres. The tool returns the hull speed in knots, kilometres per hour, and miles per hour, and it states the formula plainly so you can check the arithmetic.

Note the unit trap: the 1.34 constant only works with feet. If you enter metres, the tool converts to feet first. A 10-metre waterline is about 32.8 feet, giving a hull speed of about 7.7 knots. Enter 10 as if it were feet and you would get 4.2 knots, which is badly wrong.

A worked example

Take a typical 30-foot cruising yacht with a waterline of 27 feet. The square root of 27 is about 5.2, and 1.34 times 5.2 is about 7 knots. Under engine, that boat will cruise happily at 5 to 6 knots and need rapidly more power as it approaches 7. Under sail, it will reach 7 knots in a good breeze on a reach, and going beyond that needs surfing down waves or planing conditions.

Now take a 45-foot yacht with a 40-foot waterline. The square root of 40 is about 6.3, giving a hull speed of about 8.5 knots. That extra length buys roughly a knot and a half of speed, which over a 100-mile passage saves about three hours. This is why passage-making boats tend to be long: waterline length is the cheapest speed money can buy.

Why it is not a hard limit

Hull speed is a guideline, not a wall. Light, flat-bottomed boats can plane and leave the displacement regime entirely, travelling well above their nominal hull speed. Surfing down a wave can push any boat past it briefly. Racing multihulls, with their very long, slim hulls, operate by different rules and routinely exceed the formula’s prediction.

Heavy, full-keeled boats, on the other hand, may struggle to reach their hull speed at all. Their resistance curve climbs earlier, and they need a lot of sail area or engine power to get near the number. The formula also assumes calm water. Into a head sea, a boat’s practical speed falls well below hull speed, and no formula can fix that.

Different writers use slightly different constants — 1.30 and 1.40 also appear. Treat the result as accurate to within about half a knot.

Practical uses

Passage planning is the main one. If you know your hull speed, you can estimate passage times honestly instead of hoping for speeds your boat cannot sustain. Divide the distance by a realistic cruising speed — usually 70 to 80 percent of hull speed for a comfortable passage — and you get a time you can actually plan around.

Engine sizing is another. An engine that pushes a boat to hull speed in flat water is well matched; a bigger engine mostly burns more fuel for little gain. Sailors choosing between two boats can also use hull speed as a quick comparison: all else equal, the longer waterline is the faster boat.

Common mistakes

The classic mistake is using overall length instead of waterline length. Overhangs, bowsprits, and swim steps do not count, because they are not in the water making waves. On a boat with long overhangs, the difference can be several feet, which changes the answer by half a knot or more.

The second mistake is treating hull speed as a target rather than a ceiling. Cruising at hull speed all day is noisy, wet, and fuel-hungry. Most experienced sailors cruise well below it and arrive relaxed, with fuel to spare.

Frequently asked questions

Can my boat go faster than hull speed? Yes, briefly, by surfing or planing, and routinely if it is a light planing design or a multihull. But sustained cruising above hull speed in a displacement boat is expensive in power.

Does the formula work for powerboats? Only for displacement powerboats. Planing powerboats leave the displacement regime behind and the formula does not apply.

Why 1.34? It comes from the physics of waves in deep water, where wave speed in knots relates to wavelength in feet by a factor close to 1.34. Different hull shapes shift it slightly.

What about shallow water? Shallow water changes wave behaviour and can lower the practical speed further. The formula assumes deep water.