CalcHull Speed Calculator
Last updated: 2026-05-07
Enter your email and download a PDF report with your results.
| Length (m) | |
|---|---|
| City | 5 m |
| Suburban | 8 m |
| Highway | 10 m |
| Long haul | 15 m |
| International | 20 m |
Hull Speed Calculator: Find Your Vessel's Maximum Efficient Speed
The Hull Speed calculator computes the theoretical maximum speed of a displacement vessel using its waterline length. Whether you sail a 30-foot sloop or operate a 50-foot trawler, hull speed defines the point where your boat transitions from efficient displacement motion into wave-climbing territory where fuel consumption skyrockets. Every mariner should know their vessel's hull speed for efficient cruising, passage planning, and understanding their boat's fundamental hydrodynamic limits.
You may also find the Fuel Burn Calculator, True Airspeed Calculator, and Crosswind Component Calculator useful.
Hull Speed Formula
Hull Speed (knots) = 1.34 × √LWL
Where LWL is the length of the vessel at the waterline in feet. The constant 1.34 is derived from the Froude number, which relates wave-making resistance to vessel speed and length. The square root relationship means that doubling waterline length only increases hull speed by about 41%, not 100%.
The physics behind hull speed lies in the wave system created by the hull moving through water. As a displacement boat moves forward, it generates a bow wave and a stern wave. When the boat reaches a speed where the wavelength equals the waterline length, the bow and stern waves interact constructively, and the boat sits in a wave trough between its own bow and stern waves. Adding more power at this point mostly creates bigger waves rather than increasing speed, making hull speed a practical efficiency ceiling for displacement vessels.
Worked Examples
Example 1: Cruising Sailboat
A 35-foot cruising sailboat has a waterline length of 30 feet. The skipper wants to know the boat's hull speed for passage planning across a 200-nautical-mile offshore passage.
Hull speed: 1.34 × √30 = 1.34 × 5.48 = 7.34 knots
In ideal conditions, this sailboat can maintain about 7 knots without excessive fuel consumption from the auxiliary engine. A 200-nautical-mile passage at 6 knots (85% of hull speed, a realistic cruising average under sail) would take approximately 33 hours. The skipper should plan for at least two overnight watches and carry sufficient provisions.
Example 2: Coastal Trawler
A 45-foot power trawler has a waterline length of 41 feet. The owner wants to determine the most fuel-efficient cruising speed for a trip from Miami to Key West, a distance of 160 nautical miles.
Hull speed: 1.34 × √41 = 1.34 × 6.40 = 8.58 knots
At hull speed, this trawler burns approximately 8 gallons per hour, completing the trip in about 18.6 hours and using 149 gallons. However, by reducing speed to 7 knots (approximately 82% of hull speed), fuel consumption may drop to 4-5 GPH, cutting fuel usage to about 91 gallons — a 38% fuel savings for only a 19% increase in trip time. The owner might choose either option depending on whether time or budget is the priority.
Common Uses
- Determining the most fuel-efficient cruising speed for displacement power vessels and sailboats under power
- Passage planning and estimating time of arrival for offshore sailing and power cruising trips
- Comparing vessel performance during sea trials and boat buying decisions based on hull design
- Setting realistic speed expectations for new boat owners transitioning from planing boats to displacement hulls
- Calculating speed-to-length ratios for performance evaluation against similar vessels in the same class
- Understanding the relationship between waterline length and speed for designing or modifying vessels
Common Mistakes
- Using overall length (LOA) instead of waterline length (LWL) — hull speed depends on the waterline length, which is typically shorter than the overall length, especially on sailboats with long overhangs
- Assuming hull speed is the absolute maximum possible speed — many boats can exceed hull speed by 10-30% when surfing down waves or running before strong winds, though efficiency drops dramatically
- Ignoring the effect of hull shape and displacement — the 1.34 constant assumes an average displacement hull; lightweight modern designs can achieve speed-length ratios above 1.4, while heavy traditional hulls may top out at 1.2
- Forgetting that hull speed applies only to displacement mode — planing hulls, multihulls, and powerboats with sufficient horsepower operate in a completely different regime where hull speed is irrelevant
Pro Tip
For maximum fuel efficiency, operate your displacement vessel at 80-85% of hull speed rather than at hull speed itself. The power required increases with the cube of speed, meaning running at 85% of hull speed typically uses 40-50% less fuel than running at 100% of hull speed, while only adding 15-20% to your passage time. This "sweet spot" varies slightly by hull design but is universally more economical. On long passages, this speed reduction transforms into significant cost savings and extended range, making it the preferred choice for experienced cruisers and commercial vessel operators who prioritize efficiency over speed.
Frequently Asked Questions
Hull speed is the theoretical maximum speed of a displacement hull before it must start climbing its own bow wave. Attempting to exceed hull speed requires a disproportionate increase in power for minimal speed gain. Understanding hull speed helps sailors and power boat operators choose efficient cruising speeds and manage fuel consumption.
Yes, but with significant penalties. To exceed hull speed, the boat must climb its bow wave and transition into semi-planing or planing mode, requiring dramatically more power. Some lightweight catamarans can exceed hull speed with moderate power. Planing hulls are designed to operate above hull speed.
Displacement hulls push through the water and are limited by hull speed. Planing hulls rise onto the water's surface at speed, reducing drag. Most sailboats use displacement hulls, while powerboats and ski boats use planing hulls designed for higher speeds.
Waterline length is the most important factor for a displacement hull's maximum speed. Longer waterline length means higher hull speed. This is why racing sailboats have long overhangs that increase waterline length when the boat heels, effectively raising the hull speed.