Find your boat's theoretical maximum speed based on waterline length. Essential for understanding your displacement hull's performance limits.
Calculate the theoretical maximum speed for your displacement hull
Formula: Speed (knots) = 1.34 × √(LWL in feet)
Hull speed is the theoretical maximum speed a displacement hull can efficiently travel through the water. Beyond this point, the boat creates a bow wave that's nearly impossible to climb over without significantly more power.
The physics: As a displacement hull moves, it creates waves. At hull speed, the wavelength of the bow wave equals the boat's waterline length. To go faster, the boat would need to climb its own bow wave — requiring exponentially more power.
For planing boats, hull speed is just the transition point to a planing state — they're designed to exceed it easily.
The classic hull speed formula is:
The constant 1.34 comes from the physics of wave propagation. Some sources use 1.4 or 1.5 for semi-displacement hulls that can exceed theoretical limits with more power.
Notice that doubling the waterline length doesn't double the speed — you only gain about 41% more speed. This is why longer boats feel faster even at the same hull speed percentage.
Understanding hull speed helps you:
Set realistic expectations: A 25-foot sailboat won't cruise at 15 knots, no matter how much canvas you raise. Knowing your hull speed lets you plan passages accurately.
Save fuel: Displacement powerboats are most efficient at 70-80% of hull speed. Pushing beyond this wastes fuel fighting physics.
Recognize vessel limits: If you need higher speeds, you need a different hull type, not a bigger engine.
Measure performance: Comparing your actual speed to theoretical hull speed tells you how efficiently you're sailing or motoring.
Note: Some modern hull designs (like those with fine entries and wide sterns) can slightly exceed the 1.34 constant. However, for practical planning, the traditional formula remains reliable.
Find your boat's waterline length (LWL) — the horizontal distance from where the bow enters the water to where the stern exits. This is different from overall length (LOA), which includes overhangs. Check your boat's specifications or measure at rest in the water.
Our calculator uses the standard formula: Speed (knots) = 1.34 × √(LWL in feet). This constant comes from wave physics — specifically the relationship between wavelength and wave speed in open water.
The result is your theoretical maximum efficient speed for a displacement hull. You'll also see conversions to mph and km/h. Remember: planing hulls can exceed this limit, but displacement hulls cannot do so efficiently.
When a boat moves through water, it creates a bow wave. As speed increases, this wave gets longer. At hull speed, the wavelength of the bow wave equals the boat's waterline length — the boat sits in a "trough" between its own waves. Going faster would require climbing over the bow wave, which takes exponentially more power. This physical barrier is why displacement hulls have a practical speed limit.
Most efficient cruising happens at 70-80% of hull speed. At this speed, you're moving well without fighting the steep resistance curve that appears near hull speed. For a boat with an 8-knot hull speed, efficient cruising would be 5.6 to 6.4 knots. Pushing to hull speed burns significantly more fuel for only marginally more speed.
Displacement hulls push through the water and are limited by hull speed. Semi-displacement hulls can partially lift at higher speeds, exceeding theoretical limits. Planing hulls are designed to rise up and skim across the water surface, breaking free from hull speed entirely. Our calculator applies to displacement and semi-displacement hulls at lower speeds.
Hull speed increases with the square root of waterline length. A 36-foot waterline has a hull speed of 8.0 knots, while a 16-foot waterline maxes out at 5.4 knots — almost 50% slower despite being less than half the length. This is why ocean cruising yachts are long and narrow, and why commercial ships are so massive.
The standard hull speed formula is: Speed (knots) = 1.34 × √(Waterline Length in feet). For example, a boat with a 25-foot waterline has a hull speed of 1.34 × √25 = 1.34 × 5 = 6.7 knots. This formula is derived from wave physics and applies to displacement hulls.
Displacement hulls cannot efficiently exceed hull speed — the power required increases exponentially. However, semi-displacement hulls can partially overcome this limit, and planing hulls are specifically designed to break free by lifting out of the water. Light displacement sailboats can sometimes "surf" down waves above hull speed.
Semi-displacement hulls can exceed the 1.34 constant because they partially lift at higher speeds. For these hulls, use a constant of 1.4 to 1.5 instead. A semi-displacement hull with a 30-foot waterline might achieve 8.2 knots (using 1.5) versus 7.3 knots for a pure displacement hull.
Yes — our calculator works for sailboats. Sailboat hull speeds use the same 1.34 formula based on waterline length. Most cruising sailboats are displacement hulls and follow this rule closely. Racing sailboats with ultra-light displacement can exceed calculated hull speed by surfing or planing.
No. Overall length (LOA) includes bow and stern overhangs that extend beyond the waterline. Waterline length (LWL) is the actual length touching the water at rest. A 35-foot LOA boat might have only a 28-foot waterline. Always use LWL for hull speed calculations.
A hull speed chart lists pre-calculated speeds for various waterline lengths. Common reference points: 15 ft = 5.2 knots, 20 ft = 6.0 knots, 25 ft = 6.7 knots, 30 ft = 7.3 knots, 40 ft = 8.5 knots, 50 ft = 9.5 knots. Our calculator gives you precise results for any length.
Traditional hull speed formulas don't directly apply to catamarans. Due to their narrow hulls and reduced wave-making resistance, catamarans can exceed speeds predicted by the standard formula. Performance catamarans routinely sail at speed-length ratios above 1.34.
Kayaks follow the same formula despite their small size. A typical 16-foot touring kayak has a hull speed of about 5.4 knots (6.2 mph). Racing kayaks at 18 feet achieve 5.7 knots. This explains why longer kayaks feel noticeably faster despite similar paddling effort.
Hull speed increases with the square root of length. Doubling the waterline length increases hull speed by about 41% — a 50-foot boat (9.5 knots) is significantly faster than a 25-foot boat (6.7 knots). Longer boats also feel more comfortable because their longer wavelength creates smoother motion.
The most fuel-efficient cruising speed is typically 70-80% of hull speed. This is the "speed-length ratio sweet spot" where you achieve good progress without fighting the exponential resistance increase near hull speed. For an 8-knot hull speed boat, cruise at 5.5-6.5 knots.
The 1.34 constant is accurate for traditional displacement hulls with moderate beam and conventional hull shapes. Modern hull designs with fine entries and wide sterns may slightly exceed this (up to 1.4). Very heavy displacement boats may fall short. For practical purposes, 1.34 remains the standard reference.
Weight doesn't directly change hull speed (which depends only on waterline length), but it affects how much power is needed to reach hull speed. Heavier boats require more power and sink deeper, which can slightly increase waterline length. The formula assumes the boat is floating at designed waterline.
Explore our guides on hull types, propellers, and engine efficiency.
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