Hull Speed Calculator
Calculate the theoretical maximum hull speed of a displacement vessel based on waterline length to determine efficient cruising speed for sailboats, kayaks, and canoes.
This free online hull speed calculator provides instant results with no signup required. All calculations run directly in your browser — your data is never sent to a server. Enter your values below and see results update in real time as you type. Perfect for everyday calculations, homework, or professional use.
Length of the hull at the waterline, not the overall length.
Results
Hull Speed
6.7 knots
Hull Speed
7.7 mph
Hull Speed
12.4 km/h
How to Use This Calculator
Enter your input values
Fill in all required input fields for the Hull Speed Calculator. Most fields include unit selectors so you can work in your preferred unit system — metric or imperial, whichever matches your problem.
Review your inputs
Double-check that all values are correct and that you have selected the right units for each field. Incorrect units are the most common source of calculation errors and can produce results that are off by factors of 2, 10, or more.
Read the results
The Hull Speed Calculator instantly computes the output and displays results with units clearly labeled. All calculations happen in your browser — no loading time and no data sent to a server.
Explore parameter sensitivity
Try adjusting individual input values to see how the output changes. This is a quick and effective way to develop intuition about how different parameters influence the result and to identify which inputs have the largest effect.
Formula Reference
Hull Speed Calculator Formula
See calculator inputs for the governing equation
Variables: All variables and their units are labeled in the calculator interface above. Input fields accept values in multiple unit systems — select your preferred unit from the dropdown next to each field.
When to Use This Calculator
- •Use the Hull Speed Calculator when you need accurate results quickly without the risk of manual computation errors or unit conversion mistakes.
- •Use it to verify calculations made by hand or in spreadsheets — an independent check can catch errors before they lead to costly decisions.
- •Use it to explore how changing input parameters affects the output — a quick way to develop intuition and identify the most influential variables.
- •Use it when collaborating with others to ensure everyone is working from the same numbers and applying the same assumptions.
About This Calculator
The Hull Speed Calculator is a free, browser-based calculation tool for engineers, students, and technical professionals. Calculate the theoretical maximum hull speed of a displacement vessel based on waterline length to determine efficient cruising speed for sailboats, kayaks, and canoes. It implements standard formulas and supports both metric (SI) and imperial unit systems with automatic unit conversion. All calculations are performed instantly in your browser with no data sent to a server. Use this calculator as a quick reference and sanity-check tool during design, analysis, and learning. Always verify results against primary engineering references and applicable standards for any safety-critical application.
About Hull Speed Calculator
Hull speed represents the practical maximum speed at which a displacement vessel can travel efficiently before wave-making resistance becomes prohibitive. As a boat moves through water, it creates a bow wave and stern wave. At hull speed, the wavelength of the bow wave equals the waterline length of the vessel, and increasing speed beyond this point requires disproportionately more energy as the boat effectively tries to climb over its own bow wave. This concept is fundamental to naval architecture, sailing strategy, and human-powered boating. The formula is remarkably simple: hull speed in knots equals 1.34 times the square root of the waterline length in feet. This means that longer boats have inherently higher top efficient speeds, which is why racing sailboats and long-distance kayaks are built with maximum waterline length for their class. Understanding hull speed helps boaters choose appropriate vessels for their intended use and plan realistic transit times.
The Math Behind It
Formula Reference
Hull Speed Formula
Hull Speed (knots) = 1.34 x sqrt(Waterline Length in feet)
Variables: Waterline Length = length of hull at waterline in feet; 1.34 = Froude speed-length ratio coefficient.
Worked Examples
Example 1: 25-foot sailboat
A sailboat with a 25-foot waterline length.
The hull speed is 6.7 knots (7.7 mph / 12.4 km/h).
Example 2: 16-foot touring kayak
A kayak with a 15-foot waterline length (slightly less than overall length).
The kayak's hull speed is approximately 5.2 knots (6.0 mph).
Common Mistakes & Tips
- !Using overall length instead of waterline length, which overestimates hull speed by 5-15 percent for boats with significant bow and stern overhang.
- !Assuming hull speed is an absolute maximum that cannot be exceeded, when in reality it represents the point of diminishing returns for displacement hulls.
- !Applying the hull speed formula to planing boats, which can exceed hull speed once they rise onto the plane.
Related Concepts
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Frequently Asked Questions
Can a boat exceed its hull speed?
Displacement hulls can technically exceed hull speed, but the power required increases dramatically. At hull speed, doubling the speed might require 8-10 times the power. Planing hulls overcome this limitation by rising on top of the water, reducing displacement and wave-making resistance. Surfing a wave can also temporarily push a displacement vessel beyond hull speed because the wave provides the energy. Modern racing sailboats with foils can exceed hull speed by lifting completely out of the water.
Why are longer boats faster?
Hull speed is proportional to the square root of waterline length, so longer boats have a higher theoretical maximum efficient speed. A 36-foot waterline produces a hull speed of 8.0 knots, while a 16-foot waterline yields only 5.4 knots. This is why ocean-crossing vessels and racing boats maximize waterline length within their design constraints. However, longer boats are also heavier and harder to accelerate, so the advantage applies primarily to sustained cruising rather than short sprints.
What is a good cruising speed relative to hull speed?
Most displacement vessels cruise most efficiently at 60-80 percent of hull speed, which is called the speed-length ratio sweet spot. At this range, wave-making resistance is moderate and fuel (or physical effort for human-powered boats) is used efficiently. Pushing to 90-100 percent of hull speed consumes significantly more energy per distance traveled. For a sailboat with a 6.7-knot hull speed, an efficient cruising speed would be 4.0-5.4 knots.