Boat Speed Calculator

Work out your boat's actual speed in knots, mph, and km/h from distance and time, then compare it to the theoretical hull speed limit for a displacement hull.

Quick Facts

Hull speed formula
V (knots) = 1.34 x sqrt(LWL in feet)
Approximates the top efficient speed of a displacement-hull boat based on waterline length.
Unit conversion
1 knot = 1.15078 mph = 1.852 km/h
A knot is one nautical mile per hour.

Your Results

Calculated
Boat speed
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Distance ÷ time, in knots
Speed (mph)
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Statute miles per hour
Hull speed limit
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1.34 x sqrt(LWL), theoretical max
% of hull speed
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Your speed vs. theoretical max

Ready

Enter distance, time, and waterline length, then press Calculate.

Understanding Boat Speed and Hull Speed

This calculator answers two related questions: how fast is your boat actually moving, and how fast can a displacement-hull boat efficiently go given its length. The first is simple arithmetic — distance divided by time. The second uses a well-established rule of naval architecture called hull speed, which estimates the practical speed limit of a boat that pushes through the water rather than planing across it.

Calculating actual speed

Boat speed over water is measured the same way any speed is measured: speed = distance ÷ time. Because boats are traditionally measured in nautical miles and knots, entering distance in nautical miles and time in minutes lets the calculator convert your time to hours and divide directly. One knot is one nautical mile per hour, equal to about 1.15078 mph or 1.852 km/h.

The hull speed formula

For a displacement hull — a boat that moves through the water rather than skimming over it — the traditional hull speed formula is:

V (knots) = 1.34 × √LWL, where LWL is the waterline length in feet.

This comes from the physics of the bow and stern waves a hull generates as it moves. As a displacement boat speeds up, the wavelength of its bow wave grows until, at hull speed, the wave crests reach roughly the length of the boat itself. Pushing beyond that point means climbing up and over your own bow wave, which requires a sharp, disproportionate increase in power for each additional knot. A 16-foot waterline gives a hull speed of about 5.4 knots; a 25-foot waterline gives about 6.7 knots; a 40-foot waterline gives about 8.5 knots.

Reading the comparison

The calculator reports your measured speed alongside the hull speed limit and the ratio between them. A ratio well under 100% means you are cruising efficiently in displacement mode. A ratio near 100% means you are pushing close to the wall that displacement hulls face. A ratio over 100% means the boat is exceeding its theoretical displacement hull speed — normal and expected for planing hulls and semi-displacement hulls, which are designed to rise up and skim rather than plow through the water, but a sign of heavy fuel consumption for a pure displacement hull.

Frequently Asked Questions

What is hull speed and how is it calculated?
Hull speed is the traditional estimate of the fastest speed a displacement-hull boat can reach efficiently, before it would need to climb over its own bow wave. It is calculated as V = 1.34 x the square root of the waterline length in feet, with the result in knots. A 25-foot waterline boat has a hull speed of about 6.7 knots.
How do I calculate my boat's actual speed?
Actual boat speed is distance traveled divided by time elapsed. If you cover 5 nautical miles in 20 minutes (1/3 hour), your speed is 5 / (20/60) = 15 knots. This calculator converts that speed to knots, mph, and km/h automatically.
What is a knot and how does it convert to mph?
A knot is one nautical mile per hour. Since a nautical mile is about 1.15078 statute miles, 1 knot equals about 1.15078 mph and 1.852 km/h. A boat cruising at 20 knots is traveling roughly 23 mph.
Can a boat go faster than its hull speed?
Yes, but not efficiently as a pure displacement hull. Planing hulls and semi-displacement hulls can exceed the 1.34 x sqrt(LWL) hull speed limit by rising up and skimming across the water rather than pushing through it, which requires substantially more power per added knot.

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