Tides
The arithmetic I keep forgetting and having to reconstruct on a cold beach, written down here so that I stop doing that.
The rule of twelfths
Between high and low water there are roughly six hours. The range does not drain evenly across them. Split it into twelve parts and spend them like this:
| Hour | Share of range | Running total |
|---|---|---|
| First | 1/12 | 1/12 |
| Second | 2/12 | 3/12 |
| Third | 3/12 | 6/12 |
| Fourth | 3/12 | 9/12 |
| Fifth | 2/12 | 11/12 |
| Sixth | 1/12 | 12/12 |
It is an approximation that assumes a clean semidiurnal tide and a coastline that does not do anything clever. In an estuary, or anywhere with a sill or a narrows, it can be badly wrong. Treat it as a way to decide, not a way to be certain.
Springs and neaps
- Spring tides
- Sun and moon pulling along the same line, a day or two after new and full moon. Biggest range, strongest currents, shortest slack.
- Neap tides
- Pulling at right angles, around the quarters. Range can be half the spring figure, and slack water lasts long enough to be worth planning around.
- The lag
- Neither arrives on the day of the moon phase. Locally it runs about two days behind, which is long enough to catch you out if you plan by the calendar instead of the table.
How long slack actually lasts
Shorter than the name suggests. On a big spring it can be under ten minutes of genuinely still water, with the turn arriving faster than you expect. On a neap it can stretch past half an hour. The current also does not reverse at high water in most places; there is an offset, sometimes an hour or more, and the only way to know it is local knowledge or a tidal stream atlas.
The practical version: if you need slack water to get somewhere, arrive early and accept that you will stand around. If you plan to arrive exactly on time, you will arrive during the turn.