Hull Speed
Calculate your boat's hull speed from waterline length using the standard 1.34 sqrt(LWL) formula. Get results in knots, km/h, and mph with interactive speed comparison charts.
About This Calculator
The Hull Speed Calculator computes the theoretical maximum efficient speed of any displacement hull vessel using the standard naval architecture formula. Whether you're a sailor planning passages, a powerboat owner optimizing fuel efficiency, or a boat designer evaluating hull performance, this tool provides instant hull speed calculations in knots, km/h, and mph.
Hull speed is calculated using the formula V = 1.34 × √LWL, where LWL is the waterline length measured in feet. If you enter the length in meters, the calculator automatically converts to feet before applying the formula. The result is then displayed in knots (nautical miles per hour), kilometers per hour, and miles per hour. The concept behind hull speed is rooted in wave-making resistance — when a displacement hull moves through water, it creates a bow wave whose wavelength grows with speed. At hull speed, the bow wave's wavelength equals the waterline length, creating constructive interference that maximizes efficiency.
This calculator also includes an interactive chart showing how hull speed varies with waterline length across a range of vessel sizes. This helps you understand the relationship between boat length and potential speed, which is particularly useful when comparing different boat designs or considering a new vessel purchase.
Regional Notes
United States: Boat lengths are commonly measured in feet, and hull speed is typically discussed in knots. This calculator defaults to feet for US users.
United Kingdom: Both feet and meters are used in UK boating. Narrowboats on canals typically have shorter waterlines (40-70 ft) with hull speeds around 5-8 knots, while seagoing yachts have longer waterlines.
India: Inland waterway vessels and coastal fishing boats use meters for length measurement. The calculator supports meters input for Indian boat owners and operators.
Frequently Asked Questions
What is hull speed?
Hull speed is the theoretical maximum efficient speed of a displacement hull vessel. It occurs when the bow wave's wavelength equals the vessel's waterline length, calculated as 1.34 × sqrt(waterline length in feet). Going faster requires significantly more power as the boat tries to climb its own bow wave.
How do you calculate hull speed?
Hull speed is calculated using the formula V = 1.34 × sqrt(LWL), where LWL is the waterline length in feet. Enter your vessel's waterline length in feet or meters into the calculator to get the hull speed in knots, km/h, and mph instantly.
Can a boat exceed its hull speed?
Yes, boats can exceed hull speed by planing or with specialized hull designs. Planing hulls (like speedboats) rise onto the surface and overcome the bow wave limitation. Long narrow hulls such as catamarans, canoes, and competitive kayaks can also exceed hull speed without planing.
What is the hull speed of a 30 foot boat?
For a displacement hull sailboat with a 30-foot waterline length, the hull speed is approximately 7.3 knots (13.6 km/h or 8.4 mph). This is calculated as 1.34 × sqrt(30) = 7.34 knots.
Does adding weight increase hull speed?
Counterintuitively, adding weight can increase hull speed because it pushes the hull deeper, lengthening the waterline. A longer waterline yields a higher hull speed. However, the extra weight makes the boat harder to accelerate and requires more power to maintain speed.
What is the difference between waterline length and overall length?
Waterline length (LWL) is the length of the boat at the water's surface, while overall length (LOA) includes overhangs at bow and stern. Hull speed depends only on waterline length, not overall length. A boat with a longer waterline relative to its overall length typically has better performance.
Why is hull speed important for sailboats?
Hull speed matters for sailboats because it defines the most efficient cruising speed. Sailing at hull speed synchronizes bow and stern waves, reducing wave-making resistance. Trying to sail faster wastes energy as the boat's bow rises and drag increases dramatically.
What is the Froude number and how does it relate to hull speed?
The Froude number is a dimensionless value used in naval architecture to compare wave-making resistance across different vessel sizes. Hull speed corresponds to a Froude number of approximately 0.4. Modern ship design uses the Froude number instead of hull speed for more precise performance analysis.