Wind against tide: why the sea stands up, and how to time it

What wind against tide means

Wind against tide, or wind over tide as British sailors say, is any stretch of water where the wind blows in roughly the opposite direction to the tidal stream. A northeasterly breeze over an ebb that runs to the northeast is fine. The same breeze over a flood running southwest is not.

The trap is in the conventions. Wind direction is where the wind comes from. A southwesterly blows from the southwest towards the northeast. Current direction is where the water goes to, which is why tables call it the set. So a southwesterly wind and a current setting southwest are opposed, even though both carry the word southwest. When you read a forecast next to a tidal current table, turn one of them round before you compare.

The same thing happens with ocean currents. The Gulf Stream runs north at two to four knots, which is why boaters crossing to the Bahamas wait for the wind to come out of anything but the north.

What the current does to the waves

Waves keep their period when they ride into a current: the same number of crests passes a fixed buoy each minute. When the water under them runs the other way, each wave makes less progress over the ground, so the crests bunch up. The wavelength shortens, and because the energy in the wave train has to go somewhere, the height grows.

Steepness is height divided by wavelength. When height rises and wavelength falls together, steepness climbs twice as fast as either one. Open-water waves break once they reach a steepness of about one in seven, which is why a modest sea can turn into breaking crests in a strong opposing stream without the wind changing at all.

1. 1The same wind in both panels, blowing left to right.
2. 2Panel A: the current runs with the wind. Waves stretch out and lie down.
3. 3Panel B: the current runs against the wind. The crests bunch up and grow.
4. 4Where the steepness passes about one in seven, the crests break.

A current running with the waves does the opposite. It stretches them out and lowers them, which is why a fair tide on a windy day can feel kinder than the forecast suggests.

How much current it takes

There is a hard limit. In deep water, a wave stops making headway when the opposing current reaches a quarter of the speed that wave would travel in still water. A wave's still-water speed in knots is roughly three times its period in seconds, so the stopping current is roughly three quarters of the period in knots.

Two things follow. First, the short wind waves you get on a breezy afternoon are exactly the ones an ordinary tidal stream can stall. A 3 knot ebb is common in inlets and sounds, and it is enough to stop 4 second waves completely. Second, long swell rides through most tidal streams, but it still steepens over a shallow bar on the ebb, where the shoaling and the current work together.

Long before a wave is stopped, it is already taller and steeper than it was. You do not need to reach the limit in the table to have an uncomfortable hour. Treat it as the point where a sea starts to break in earnest, not the point where it starts to get worse.

Where it bites hardest

Tidal streams are strongest where the water is squeezed, and that is where wind against tide does its work.

• Inlets and harbour bars. The ebb pours out against an onshore wind and the incoming swell, over water that is getting shallower. US East Coast inlets on the ebb in an easterly, and West Coast river bars on the ebb against the swell, are the classic cases.
• Headlands and races. Streams accelerate round a point. Portland Bill and the Alderney Race in the English Channel, or The Race at the eastern end of Long Island Sound, can be smooth at slack and confused a couple of hours later.
• Narrow sounds and canals. A channel that funnels both wind and stream lines them up head to head, so a moderate breeze can raise a short, sharp chop.
• Ocean currents. The Gulf Stream against a northerly is the best known example in North America. The current never stops, so the only fix is the wind.

Timing a passage around it

1. Find the pinch points. Mark the inlets, bars, headlands and narrows on your route. Open water in between rarely matters as much.
2. Get the stream times for each one. In US waters, NOAA tidal current predictions give the times of slack, maximum flood and maximum ebb, with the speed. In the UK and Europe, use a tidal stream atlas or the tidal diamonds on the chart, referenced to high water at the standard port.
3. Lay the wind forecast over them. For the hours you will be at each pinch point, note the wind direction and strength. Turn one direction round so you compare like with like.
4. Work back to a departure time. From your speed and the distance to each pinch point, find the departure that puts you there near slack or with a fair stream. If two pinch points want different times, the one with the stronger stream wins.
5. Check the period. A short period wind sea is the most sensitive. If the forecast shows 3 to 5 second waves and a stream of 2 knots or more against them, give that spot your full attention.

Slack water is not the same moment as high or low water in most places, and at some stations it comes an hour or more later. Use current predictions for slack, not tide tables. Our guide to tidal currents and slack water explains why.

1. 1Flood: the stream runs with the onshore wind. Waves lengthen and lie down.
2. 2Slack: the stream turns. The calmest time to cross a bar or round a headland.
3. 3Ebb: the stream runs against the wind. Worst near maximum ebb, in the middle of the red.
4. 4The wind stays the same all day. Only the stream changes.

Worked example: an inlet on the ebb

If you cannot make slack, the next best is the first hour of the flood, when the stream runs with the wind and the sea at the mouth lies down. The hours either side of maximum ebb are the ones to avoid.

Sources and further reading

• NOAA Tides and Currents: tide and tidal current predictions
• NOAA Ocean Service: tutorial on ocean and tidal currents
• National Weather Service marine forecasts
• Met Office guide to coast and sea forecasts