Sailboat Hull Speed Calculator
Sailboat hull speed in knots, mph, m/s.
What it does
Displacement hulls hit a wave wall set by waterline length. Bow and stern waves sync at 1.34 times root length in knots, so a 25-footer tops near 6.7 knots while a 64-footer nears 10.7. Longer hulls climb longer waves.
The tool above runs that root rule with unit conversions attached. Enter waterline feet and it prints knots, mph, and m/s. Defaults give 6.7 knots; 16 ft gives 5.36 while 36 gives 8.04.
Boating references agree. BoatUS Foundation states the 1.34 root rule with a 64-ft worked check, and Brown sailing notes place cruisers at 1.2-1.4 with the physics tracing wave speed at Froude 0.40. US Sailing governs racing around the same water.
Planing and multihulls break the wall by design. Power and surf lift hulls over bow waves where displacement boats climb them, so this page bounds cruisers, not racers. Fouling, load, and seas cut real speeds below theory.
How to use
- Enter the waterline length in feet.
- Read knots first.
- Read mph beside it.
- Read m/s for metric crews.
- Plan passages under hull speed.
- Size engines past it for planing.
Formula
- LWL
- Waterline length in feet, 0 to 1,000.
BoatUS states the 1.34 root rule with a 64-ft worked check; Brown sailing FAQ gives the 1.2-1.4 cruiser band.
Worked example
Example: 25 ft waterline
Defaults: 25 ft waterline.
- Input: 25 ft waterline.
- Root: sqrt(25) = 5.
- Knots: 1.34 x 5 = 6.7.
- MPH: 6.7 x 1.15078 = 7.71.
- m/s: 6.7 x 0.51444 = 3.45. Tool shows 6.7 knots.
Reference table
| LWL ft | Knots | MPH |
|---|---|---|
| 16 | 5.36 | 6.17 |
| 20 | 5.99 | 6.90 |
| 25 | 6.70 | 7.71 |
| 30 | 7.34 | 8.45 |
| 36 | 8.04 | 9.25 |
| 64 | 10.72 | 12.34 |
Limits
- Displacement hulls only.
- Calm water theory; seas cut speed.
- Loaded and fouled hulls slower.
- Engine sizing separate.
Common mistakes
- Using overall length, not waterline.
- Expecting planing speeds from math.
- Ignoring fouling and load.
- Mixing knots with mph.
- Sizing engines to theory alone.
Try it yourself
Quick checks. Answers sit tucked under each question.
16 ft waterline?
Root 4. Knots 1.34 x 4 = 5.36. MPH 6.17. m/s 2.76.
36 ft waterline?
Root 6. Knots 1.34 x 6 = 8.04. MPH 9.25. m/s 4.14.
Questions
How is hull speed figured?
1.34 times root of waterline feet. Here 1.34 x 5 = 6.7 knots.
How do knots convert?
Knots times 1.15078: 7.71 mph here. Times 0.51444 for m/s: 3.45.
Is 1.34 exact?
About 1.2 to 1.4 for cruisers; 1.34 is the standard planning figure.
Do all boats obey it?
Yes. Planing hulls, multihulls, and surf exceed displacement math.
Do longer boats go faster?
Yes. Longer waterlines raise the wave speed the hull must climb.
What slows real boats?
Yes. Fouling, load, and sea state cut real speeds below theory.