Hull speed is the ceiling a displacement hull runs into, and it comes from one measurement: waterline length. This works it out, then does the thing people actually need, which is turning it into an arrival time at a speed a cruising boat really averages.
- Realistic cruising average
- Passage time at that average
- Nights at sea
| If you average | Speed | Time | Arrive |
|---|
Where the Formula Comes From
Hull speed in knots is 1.34 multiplied by the square root of waterline length in feet. A 28-foot waterline gives just over 7 knots.
The constant is not arbitrary. A deep-water wave travels at about 1.34 times the square root of its own length, and a moving hull makes waves: one at the bow, one at the stern. As the boat speeds up those waves stretch out, until the wave is as long as the boat and the hull is sitting in a trough of its own making.
Past that point the stern drops into the trough, the bow lifts, and the boat starts trying to climb its own bow wave. Resistance rises steeply. You can push harder, with more sail or more throttle, and get very little back for it.
This is why waterline length matters more than length overall for speed, and why long-ended classic boats are slower than their marketing suggests. It is also the single clearest reason bigger boats are faster: the relationship is a square root, so gains come slowly, but they come reliably.
Why the Passage Table Uses a Lower Speed
Almost nobody averages hull speed over a passage, and planning as though you will is how people arrive off an unfamiliar coast in the dark.
The losses are ordinary rather than dramatic. Windward work means sailing more distance than the straight line. Wind drops overnight. You reef before dark because it is sensible, and carry that reef longer than you strictly need to. Somebody has to cook. A sail change takes twenty minutes during which nobody is trimming.
Two thirds of hull speed is a reasonable planning figure for a cruising boat on a mixed passage, so that is the default. The table shows what a slow, typical and fast passage each look like, because the spread is the useful information. A passage that could take 18 hours or 28 depending on the wind is a passage where the departure time matters.
Plan the arrival rather than the departure. Decide when you want to make landfall, in daylight, with the tide going your way, and work backwards. For anything tidal that is not optional: when to sail covers the seasonal side, and on a longer route the same logic applies to the whole passage, as in how long it takes to cross the English Channel.
When the Formula Does Not Apply
Boats that plane. A dinghy, a sport boat or a light modern hull with a flat run aft can climb over its bow wave instead of pushing through it. Once planing, the speed limit is a different problem entirely.
Multihulls. Long, narrow, light hulls make small waves, so the wave-making ceiling is far less binding. This is why a cruising catamaran routinely exceeds what this formula would predict from its waterline.
Surfing down a wave face. Any boat can exceed hull speed briefly going downhill in a big sea. That is borrowed energy from the wave, not a sustained speed, and chasing it in the wrong conditions is how boats broach. The fastest sailboats covers what happens when designers stop accepting the limit altogether.
Heavy displacement. A very heavy boat may never reach its theoretical hull speed in normal conditions, because it simply cannot carry enough sail to get there. The ratio that predicts this is sail area against displacement, which the sail area calculator works out.
Frequently Asked Questions
Common questions about hull speed and passage planning:
Where Do I Find My Waterline Length?
It is published in the boat’s specifications, usually as LWL. If you only have length overall, waterline is typically 10 to 20% shorter on a cruising boat, and the gap is larger on older designs with long overhangs.
Does Loading the Boat Change Hull Speed?
Slightly, and in an unexpected direction. Loading a boat down sinks it deeper, which can lengthen the waterline a little and nudge theoretical hull speed up, while making the boat slower in practice because it has more weight to drag.
Is 1.34 an Exact Constant?
No. It is the traditional figure, and real hulls vary either side of it depending on how fine they are aft and how they carry their displacement. Treat the answer as a close approximation, not a measurement.
Why Is My GPS Speed Higher Than This?
GPS shows speed over ground, which includes the current carrying you along. Hull speed is about speed through the water. A favourable tide can put you well over hull speed on the display while the hull is doing nothing unusual.
Should I Plan a Passage at Two Thirds of Hull Speed?
It is a sensible default for a cruising boat on a mixed passage. If you have a log of your own passages, use your own average instead, because your boat and how you sail it beat any general rule.