In short
- Tide is the rise and fall of the water. Tidal current is its horizontal flow: flood towards the land, ebb away from it.
- Slack water is the short pause when the current turns. In inlets, bays and estuaries it often comes an hour or more after high or low water.
- Read slack times from a tidal current prediction, never from a tide table.
- Between slacks the stream builds fast: about half its maximum after the first hour and nine tenths after the second.
Tide and tidal current are different things
The tide is vertical. It is the rise and fall of the sea surface, measured as a height above a chart datum, and a tide table gives you the times and heights of high and low water. The tidal current is horizontal. It is the water flowing in and out to make that rise and fall happen, measured as a speed in knots and a direction.
The two are linked, but they don't peak together, and the link changes from place to place. On an open coast the current may run hardest near high water. In the mouth of a bay it may run hardest halfway between high and low. That is why a tide table, however accurate, can't tell you when the current will turn.
Flood, ebb and slack
The flood is the current that comes in with the rising tide, towards the coast or up an estuary. The ebb is the current that goes out, away from the coast or down the estuary. Slack water is the period around the turn when the current is weak or nil. A prediction usually names it by what follows: slack before ebb, after the flood has finished, and slack before flood, after the ebb.
Slack is a moment, not a period. In a strong tidal channel the current may be under half a knot for only twenty or thirty minutes before the new stream starts to build. In some places the current never stops at all: it weakens, swings round and picks up again, and NOAA prediction tables show a minimum current instead of a slack.
Why slack is not high water
Think about a bay connected to the sea by a narrow inlet. Outside, the tide reaches high water and starts to fall. Inside, the bay is still lower than the sea, because the inlet can only let water through so fast. So water keeps flowing in after high water outside, until the levels match. Only then does the current turn. Slack before ebb at the inlet comes after high water on the coast, and the delay can be an hour, two hours or more.
The same thing happens in reverse at low water, and it happens in estuaries, sounds and long channels everywhere. Along open coasts the tide behaves more like a wave travelling along the shore, and the current can turn closer to mid-tide. Neither case is unusual. Both are reasons to read the current prediction for the exact place you will be.
- 1High water outside the inlet, 03:00.
- 2Slack before ebb, an hour after high water. The flood ran in the whole time the tide was already falling.
- 3Maximum ebb, about three hours after slack.
- 4Low water outside, about 09:15.
- 5Slack before flood, an hour after low water.
- 6Maximum flood.
How to read a current prediction
In US waters, NOAA publishes tidal current predictions for thousands of stations. Each day lists the times of slack water and of maximum flood and maximum ebb, with the speed at each maximum. The header gives the flood direction and the ebb direction in degrees true, so you know which way the water is going. Flood speeds are positive and ebb speeds negative, so a line that reads minus 2.8 is an ebb of 2.8 knots.
| Time | Event | Speed |
|---|---|---|
| 02:18 | Slack, before ebb | |
| 05:31 | Maximum ebb | -2.8 kn |
| 08:47 | Slack, before flood | |
| 11:52 | Maximum flood | +2.4 kn |
| 14:40 | Slack, before ebb | |
| 17:58 | Maximum ebb | -3.1 kn |
NOAA stations come in two kinds. Harmonic stations have their own long record of measurements and are predicted directly. Subordinate stations are predicted from a nearby harmonic station, with corrections to the times and speeds. Both are fine to plan with; the subordinate ones are a little coarser, so leave a margin around their slack times.
In the UK and much of Europe the tidal stream is published differently. A tidal stream atlas has one chartlet for each hour from six hours before to six hours after high water at a standard port, such as Dover, with arrows and rates for springs and neaps. Tidal diamonds on the chart do the same for single points: a table under each diamond gives the direction in degrees true and the rate at springs and neaps for every hour relative to high water at the standard port. Work out how far today's range sits between neaps and springs and interpolate the rate.
Rules of thumb for the hours between
Predictions give you the turns and the peaks. For the hours in between, two old rules fill the gap. They describe different things, so keep them apart.
The 50/90 rule is for current. Starting from slack, the current reaches about half its maximum speed after one hour, about nine tenths after two hours, and its maximum at around three hours. It then falls away in the same steps to the next slack. With a 6 hour gap between slacks and a maximum of 3 knots, that is 1.5 knots after the first hour, 2.7 after the second, and 3 at the peak.
- 1Slack: the stream turns.
- 2After one hour the stream is already at half strength.
- 3Maximum, near the middle of the cycle.
- 4The next slack, about six hours after the last.
The rule of twelfths is for height. Over the six hours from low to high water, the tide rises by one twelfth of its range in the first hour, two twelfths in the second, three in the third and fourth, two in the fifth and one in the sixth. It counts from high or low water, while the 50/90 rule counts from slack, and the two do not line up at an inlet where slack lags the tide.
| Hour | Height change in that hour, counted from high or low water | Current at the end of that hour, counted from slack |
|---|---|---|
| 1 | 1/12 of the range | 50% of maximum |
| 2 | 2/12 | 90% |
| 3 | 3/12 | 100% |
| 4 | 3/12 | 90% |
| 5 | 2/12 | 50% |
| 6 | 1/12 | Slack |
Springs and neaps change the size of everything. Spring tides come a day or two after new and full moon, with the biggest range and the strongest streams. Neaps come near the quarter moons, with the smallest range and the weakest streams. A headland that runs at 4 knots on springs may run at 2 on neaps, which can turn a hard tidal gate into an easy one. Current predictions already include this; with atlases and diamonds you interpolate between the spring and neap rates.
Using slack, and working out set and drift
Slack is the easiest time to cross a bar, enter an inlet, transit a narrow cut or round a headland with a race. The sea lies down, the steering is easy and the boat goes where it is pointed. Aim to arrive a little before slack rather than after it: if you are early, the stream is weakening, and if you are late, it is building against or across you.
When you can't make slack, work out what the stream will do to you. The set is the direction the current pushes you, and the drift is its speed. Over a known time, the distance you are carried sideways is the drift multiplied by the time.
Crossing a channel on the flood
You need to cross a 2 nautical mile channel at 5 knots, straight across. The current station shows a flood of 1.5 knots running along the channel at the time you will cross. The crossing takes 24 minutes, 0.4 of an hour, so the flood carries you 1.5 times 0.4, or 0.6 of a mile up the channel.
To land opposite your starting point, aim upstream. Turning about 17 degrees into the current cancels the drift, at the cost of a slightly slower crossing. Cross an hour earlier, near slack, and the drift is almost nothing. One hour after slack, by the 50/90 rule, the flood would be about 0.75 knots and the drift about 0.3 of a mile.
Wind matters as much as the stream. A current running against the wind steepens the sea, so the hours around maximum stream in a breeze from the opposite direction are the ones to avoid. Our guide to wind against tide explains how much current it takes.
