liminfoHull Speed Calculator
Free web tool: Hull Speed Calculator
Hull Speed
7.3 knots
Hull Speed
13.6 km/h
About Hull Speed Calculator
The Hull Speed Calculator helps sailors, naval architects, and boat enthusiasts determine the theoretical maximum displacement hull speed using the classic 1.34 × √LWL formula, where LWL is the waterline length in feet. This speed—measured in knots—represents the point at which a displacement hull begins to climb its own bow wave, requiring exponentially more power to go faster. Input your waterline length in feet or meters and get instant results in both knots and km/h.
Beyond hull speed, this tool includes a marine fuel consumption estimator. Enter your engine horsepower, fuel type (gasoline or diesel), and tank capacity in gallons to compute burn rate (gal/hr), estimated run time, approximate hull speed based on power, and projected nautical mile range. Diesel engines use the standard 0.04 gal/hp/hr efficiency factor while gasoline outboards use 0.06 gal/hp/hr, consistent with ABYC and NMMA guidelines.
Whether you are planning a long offshore passage, comparing hull designs, or simply curious about your boat's theoretical performance limits, this calculator provides the essential numbers instantly in your browser. All calculations run locally with no data transmitted to any server, making it safe for sharing sensitive vessel specifications.
Key Features
- Hull speed calculation using the industry-standard 1.34 × √LWL formula
- Supports both feet and meters input with automatic unit conversion
- Dual output in knots and km/h for international users
- Marine fuel burn rate estimation for gasoline (0.06 gal/hp/hr) and diesel (0.04 gal/hp/hr) engines
- Tank size and horsepower inputs to calculate estimated run time in hours
- Nautical mile range estimate based on engine power and fuel capacity
- Separate tabs for hull speed and fuel consumption workflows
- 100% client-side — no data uploaded, works offline once loaded
Frequently Asked Questions
What is hull speed?
Hull speed is the theoretical maximum speed of a displacement hull, defined as the speed at which the waterline length equals the wavelength of the bow wave. The standard formula is 1.34 × √(LWL in feet), resulting in speed in knots. Beyond this speed a displacement hull must climb its own wave, requiring dramatically more power.
How accurate is the 1.34 × √LWL formula?
The formula is a good rule of thumb for traditional displacement hulls with beam-to-length ratios typical of sailboats and trawlers. Planing hulls, catamarans, and semi-displacement hulls can exceed hull speed with sufficient power. The constant 1.34 applies to sea water; some sources use 1.34–1.51 for beamier or lighter hulls.
How do I convert my boat length to waterline length?
The waterline length (LWL) is shorter than the overall length (LOA). For most monohull sailboats, LWL is roughly 80–90% of LOA. Check your boat's spec sheet or measure from where the bow enters the water to where the stern exits. Use the unit selector in this calculator to input meters or feet.
What fuel burn rate should I use for my outboard motor?
As a rule of thumb, gasoline outboards consume approximately 0.06 gallons per horsepower per hour at wide-open throttle, while diesel inboards use about 0.04 gal/hp/hr. Actual consumption varies with throttle position, hull resistance, sea conditions, and engine age. These estimates are best used for conservative passage planning.
How is estimated range calculated?
Range is estimated by multiplying run time (tank size ÷ burn rate) by the estimated speed derived from √HP × 1.5. This is a rough approximation; real-world range depends on hull efficiency, current, wind, and sea state. Always carry a fuel reserve of at least 30% for marine safety.
Can planing boats use this hull speed calculator?
The hull speed formula applies to displacement mode only. Planing hulls—typically powerboats with planing speeds above ~1.4 × √LWL—transition out of displacement mode and are not constrained by hull speed. The fuel consumption tab is still useful for planing boats to estimate range at cruise RPM.
What is the difference between knots and km/h for boat speed?
1 knot equals 1.852 km/h. Knots are the international standard for marine and aviation navigation because they relate directly to nautical miles (1 nm = 1 minute of latitude). The calculator shows results in both units so you can cross-reference with speed logs or compare with international routing tools.
Is this calculator suitable for professional vessel design?
This tool provides quick estimates based on widely-used rule-of-thumb formulas. For professional naval architecture, resistance predictions should be validated with Froude number analysis, tank testing, or CFD software such as Maxsurf or Rhino/Orca3D. Use these results for preliminary planning and preliminary comparisons only.