Guide
How big is three feet, really?
Wave height sounds simple until you're standing in a boat trying to decide whether it's safe to head out. Here's what the numbers actually describe.
When a forecast gives a wave height, it almost always means significant wave height, denoted Hs. This is the average of the highest third of waves during a measurement period. The definition sounds technical but has a practical point: it roughly matches what an experienced sailor would estimate by eye, since we automatically ignore the smallest ripples.
Individual waves are always higher than the forecast
The most important thing to understand is that Hs is not the maximum value. In a normal wave field, the highest wave over an hour is roughly 1.8 times the significant wave height, and over a longer period individual waves can reach double that. A forecast of 1.5 metres therefore means you'll regularly meet a three-metre wall of water. Always factor that in when judging your margins.
Feet and metres
Feet are used a lot in boating contexts since much of the terminology is English. A foot is exactly 0.3048 metres. Three feet is therefore 0.91 metres, five feet 1.52 metres, and ten feet just over three metres. A quick rule of thumb: divide feet by three to get roughly metres.
| Metres | Feet | Sea state |
|---|---|---|
| 0–0.3 m | 0–1 ft | Ripples. Barely noticeable on board. |
| 0.3–0.6 m | 1–2 ft | Small waves. Pleasant going in any boat size. |
| 0.6–1.0 m | 2–3 ft | Light sea. Small boats start slapping into the waves. |
| 1.0–1.5 m | 3–5 ft | Moderate sea. Uncomfortable in a 20-footer, calm in a 50-footer. |
| 1.5–2.5 m | 5–8 ft | Rough sea. Smaller boats need to slow down. |
| 2.5–4.0 m | 8–13 ft | Very rough sea. Only for larger, seaworthy boats. |
| 4.0 m + | 13 ft + | High sea. Individual waves can be almost double the height. |
Wavelength matters as much as height
Two-metre waves with a twelve-second period are long, gentle swells that a boat follows almost unnoticed. Two metres with a four-second period are short, steep walls that slam hard into the hull. It's the steepness – the ratio between height and length – that decides whether the sea feels pleasant or brutal. Enclosed seas like the Baltic are notorious for short, choppy waves, while Atlantic swell can be considerably higher but far gentler to travel in.
The boat's length relative to the wave
A boat reacts completely differently depending on how it relates to the wave's length. A 20-foot open boat is shorter than most waves and therefore follows every crest and trough – the motion becomes quick and jerky already in a one-metre sea. A 32-foot sailboat with a deep keel has enough mass to dampen the motion and can travel comfortably in two metres. A 50-footer bridges smaller waves entirely: it barely notices a metre, but in return takes hard slams when it comes down into the trough in truly rough seas.
Heading relative to the sea also matters. Straight into the waves gives heavy pitching but good control. Running with the sea is more comfortable but risks the boat broaching uncontrollably in a breaking wave. Beam-on seas are usually the most uncomfortable – that's when rolling is worst and seasickness sets in.
Try it yourself
Set the wave height in the visualiser and see the sea both from the side and from the shore – or add a boat and see how the hull length changes the whole experience.
Visualise waves