Hull Speed Calculator

About the Hull Speed Calculator

Hull speed is the theoretical maximum speed a displacement vessel can efficiently travel, fundamentally limited by the wave it creates in the water. As a boat moves through water, it generates a bow wave and stern wave. At hull speed, the wavelength of this wave equals the waterline length of the boat, creating a "wave wall" that traps the vessel.

The formula is elegantly simple: Hull Speed (knots) = 1.34 × √(LWL in feet). This constant (1.34) corresponds to a Froude number of 0.40, the point where wave-making resistance increases exponentially. A 25-foot sailboat has a hull speed of about 6.7 knots, while a 40-foot vessel reaches 8.5 knots—explaining why longer boats are faster.

This calculator computes hull speed from waterline length, shows the power curve as speed approaches hull speed, compares speeds across boat sizes, and explains when boats can exceed hull speed (planing hulls, ultra-light displacement boats). Use the example to confirm the waterline-length unit before comparing boats.

When This Page Helps

Estimate a displacement boat's realistic cruising ceiling, compare hull lengths, and understand why extra waterline length translates into more speed.

How to Use the Inputs

1. Enter the waterline length (LWL) of your boat
2. Select measurement units (feet or meters)
3. Review hull speed in knots and mph
4. Examine the Froude number and speed-length ratio
5. Check the resistance curve showing exponential power at hull speed
6. Compare your boat's hull speed to other vessel sizes

Example Calculation

Tips & Best Practices

• Cruising at 80-90% of hull speed is the most fuel-efficient for displacement boats
• A heeled sailboat has an effectively longer waterline, slightly increasing hull speed
• Catamaran hulls are long and narrow, giving high hull speed despite modest overall length
• Adding waterline length (longer boat) is the only way to increase hull speed for displacement hulls
• Light-displacement boats may surf waves and momentarily exceed theoretical hull speed
• In shallow water, hull speed decreases—the bottom interaction changes wave dynamics

The Physics of Hull Speed

As a displacement boat moves, it creates two wave systems: one at the bow and one at the stern. At low speeds, multiple wave crests fit along the hull. As speed increases, the wave length grows until at hull speed, exactly one wave crest is at the bow and one at the stern. The boat is essentially trapped in a wave trough of its own making. Pushing beyond this requires climbing the bow wave—an exponentially increasing energy demand.

Planing vs Displacement: Breaking the Hull Speed Barrier

Planing hulls are designed to rise up and skim over the water surface rather than pushing through it. This requires a flat or V-shaped bottom, sufficient power, and light displacement. Once a planing hull exceeds hull speed and lifts onto the plane, resistance actually decreases. This is why a 20-foot speedboat can easily reach 40+ knots while a 20-foot sailboat is limited to about 6 knots.

Practical Implications for Sailors and Power Boaters

For cruising sailboats, hull speed determines passage times. A 40-foot waterline cruiser (hull speed 8.5 knots) covers about 170 nautical miles per day. Increasing to 50 feet (hull speed 9.5 knots) adds 20 miles per day. For trawlers and displacement powerboats, running at 80% hull speed saves 30-40% fuel compared to pushing to hull speed.

Frequently Asked Questions

• Can a boat exceed hull speed?

  Yes! Planing hulls (powerboats, light dinghies) lift onto their own bow wave and plane at much higher speeds. Ultra-light displacement boats (ULDBs) can also exceed hull speed. Heavy displacement vessels generally cannot economically exceed hull speed.

• Why is waterline length used, not overall length?

  Waterline length (LWL) determines the wave characteristics. Overhangs (bow and stern extending beyond the waterline) don't contribute to wave making at rest, though they can increase effective LWL when heeled or moving.

• What is the Froude number?

  The Froude number (Fn) is a dimensionless ratio of boat speed to the square root of gravitational acceleration times waterline length. Fn = 0.40 corresponds to hull speed. Above Fn 0.50, resistance increases dramatically for displacement hulls.

• Does hull shape affect hull speed?

  The theoretical hull speed is the same for all displacement vessels of equal waterline length. However, hull shape dramatically affects how much power is needed to approach hull speed and whether the vessel can exceed it.

• How much power increase is needed near hull speed?

  Resistance increases roughly with the cube of speed below hull speed, but near hull speed it increases much faster—approximately with the 6th power. Reaching 90% of hull speed might require 60% of the power needed for 100%.

• What is speed-length ratio?

  SLR = Speed(knots) / √LWL(ft). An SLR of 1.0 requires moderate power. SLR 1.34 is hull speed. SLR above 1.5 indicates planing or semi-planing. Efficient cruising is typically SLR 0.9-1.1.