A single wave is fully described by three traits: amplitude, frequency, and phase. For sailors, amplitude is noted as significant wave height, from crest to trough, in feet. This is actually twice the true amplitude. It is defined as the average crest to trough height of the tallest one-third of the waves. Some waves will be higher than that. Frequency (peaks per unit of time) is denoted by its inverse, wave period (time between peaks), in seconds. Phase is unimportant.
Wave speed
and periodIn deep water, wave speed (velocity of a particular peak in ft per second) is based on a complex formula involving the gravitational constant. After all the math is done, wave speed is directly proportional to period: Longer waves move faster. This is not the speed of the water, it is the speed of the wave. In deep water, as a close approximation, wave speed in knots is a bit more than three times the wave period in seconds.
For a cruising sailor, wave period is usually more useful than wave speed. Period is what marine forecasts normally give you, and from it you can infer whether you’re dealing with short, steep chop or long, fast-moving swell.
A marine weather forecast might report waves by citing the direction, height and period in a form like SW seas 5 ft at 3 seconds where, as for wind, direction is where the wave is coming from. Short period waves every 3–6 seconds are steep, choppy, and uncomfortable. Medium period waves every 7–10 seconds are moderate and normal. Longer period waves, called swell, are smooth and rolling.
Wave speed and period help determine how a boat will interact with the sea. They become particularly important in heavy weather, when sailing downwind, and when entering shallow water or inlets. For sailors, this has several practical consequences:
Following seas. When a boat is moving in roughly the same direction and at nearly the same speed as the waves, a crest can remain near the boat for a long time. The boat may accelerate down the wave face, surf, broach, or become difficult to steer.
Head seas. When sailing into waves, boat speed plus wave speed determines how often crests arrive. Even fairly long-period waves can produce rapid, uncomfortable pitching when heading directly into them. Changing course 20–40° can greatly alter the encounter rate and motion.
Wave period tells you a great deal about the sea. In deep water, longer-period waves travel faster than shorter-period waves. For example, deep-water swell with a 10-second period travels at about 30 knots, while 15-second swell travels at roughly 45 knots. This is why distant storm swell can arrive well before the storm itself.
Shallow water changes everything. As waves enter water shallower than roughly half their wavelength, they slow down, shorten, steepen, and eventually may break. This is particularly important when approaching bars, shoals, reefs, and inlets. A relatively benign offshore swell can become dangerous over a shallow entrance.
Wind against current can create steep seas. If waves travel into an opposing current, their speed relative to the bottom decreases and their wavelength shortens. The waves then become higher and steeper. This is why an inlet with an ebb current opposing an onshore swell can be dramatically worse than the same inlet at slack water. It is why the Gulf Stream gets choppy in a northerly wind.
Sailing
in wavesWhen sailing in waves, work with the water—don't fight it. Sail into waves or away from them. Avoid sailing across them and taking them broadside. The most important factor is angle relative to the waves. Taking waves on the bow at 30° to 45° (quartering into them) is usually smoother than hitting them head-on, which causes violent pitching. Running directly downwind in large waves risks broaching (sideways capsize) or pitchpoling (flipping end-over-end).
As waves build, depower the sails by flattening them and easing the traveler to reduce heeling. Waves make a heeled boat harder to control. Reef early as seas build. It is always better to have too little sail than too much. In confused waves or steep chop, a bit more power can help the bow punch through rather than hobby horsing.
Boat speed matters. Too slow and waves push you around. Too fast downwind and you risk surfing out of control. Go fast enough to have steerage way but not so fast as to be overpowered.
Going downwind, head up slightly before a wave lifts the stern to avoid broaching. Bear off slightly as the wave passes to keep momentum. Watch the waves coming and steer proactively.
In steep or breaking waves, steer to avoid taking a wave broadside. Keep crew weight centered and low, away from the bow and stern. Heavy weight in the bow makes pitching worse.
Running before big waves is the most dangerous point of sail. To stay in control, sail by the lee (with care), or gybe, while possibly towing a drogue to slow the boat and stabilize steering. Heaving to is an option if the boat can do it and you have practiced it.
Slow down before conditions deteriorate, not after. Keep hatches closed and crew clipped in with harnesses. Lastly, trust the boat: Modern sailboats are remarkably seaworthy in waves if sailed well. The core idea is to be in harmony with the sea. Set your speed, angle to the waves, and sail plan so the waves become something to work with rather than obstacles to fight. Then steer accordingly.