Forecast wave height is the number most boaters use to decide whether to go, and it is the number most likely to mislead them. Two seas with identical wave heights can be a pleasant afternoon and an ordeal. What separates them is the period, and almost nobody reads it.
What "wave height" actually means
Forecasts give significant wave height: the average height of the highest third of the waves. It is a statistical value, not a maximum. The practical consequences:
- Roughly one wave in ten will be noticeably higher than the forecast figure.
- The largest wave you meet in a few hours may approach twice that height.
So a "3 foot" forecast is not a promise that nothing bigger than three feet will arrive. It is a promise that something bigger will.
The number that decides comfort: period
Period is the time in seconds between crests. Together with height it defines steepness, and steepness is what beats up a small boat.
- 3 ft at 9 seconds — long, rolling swell. A small boat rides over it.
- 3 ft at 3 seconds — short, stacked wind chop. Pounding, spray, and a crawl to windward.
A useful rule of thumb: when the period in seconds is less than roughly the height in feet multiplied by two, the sea is steep and uncomfortable. Below four seconds, treat any height with respect.
Small Craft Advisory, and what triggers it
In the United States a Small Craft Advisory is issued for sustained winds or frequent gusts in a range that typically starts around 20-25 knots, or for seas considered hazardous to small vessels — the exact thresholds are set locally and vary by region and by season.
Two things are worth understanding about it. First, "small craft" is not formally defined: the advisory leaves the judgement to you. Second, it is issued by forecasters who know that specific piece of water, which makes it more valuable than any generic table — including this one.
Direction changes everything
Head seas are the most tiring but the most controllable: slow down and pick your way through. Beam seas roll the boat and make for a wet, uncomfortable passage. Following seas are the ones that catch small boats out — a wave overtaking from astern can slew the bow and, in the worst case, broach the boat.
Watch also for a confused sea, where wind waves and swell arrive from different directions. The combination is unpredictable and considerably worse than either component suggests.
Where waves get worse than forecast
- Inlets and river mouths, where the outgoing current opposes incoming swell. This is the single most dangerous place for a small boat on an otherwise fine day.
- Shoaling water. As depth decreases, waves slow, shorten and steepen.
- Around headlands, where wind accelerates and currents converge.
Practical limits
For a boat under about 20 feet, in the hands of someone who is competent rather than expert: up to 1 foot is easy, 1-2 feet with a long period is comfortable, 2-3 feet is working conditions, and above 3 feet you want a good reason and a short trip. Every one of those numbers drops when the period is short or the sea is on the beam.
And as always, the trend matters more than the snapshot: heading out into a falling sea is a different day from heading out into a building one.
Domande frequenti
What wave height is safe for a small boat?
For a boat under about 20 feet with a competent skipper: up to 1 foot is easy, 1 to 2 feet with a long period is comfortable, 2 to 3 feet is working conditions and above 3 feet you want a good reason and a short trip.
Where do waves get worse than forecast?
At inlets and river mouths where outgoing current opposes incoming swell, in shoaling water where waves slow and steepen, and around headlands where wind accelerates and currents converge.