The tide moves that many twelfths of its range in six equal slices of the fall, and the same again on the rise.
Your boat needs a height of tide. The fall takes it away and the rise brings it back, so there are two times to know, not one.
Around the same mean level, neaps bring high water down and low water up. Dover goes from 6.8 and 0.8 m to 5.4 and 2.2 m here.
Assume six hours where the fall takes five, and three hours in the rule says 50% has gone when 65% has.
At 2, 3 and 4 hours it is exact: 25%, 50% and 75%. The worst error is inside the first and last hours.
The rule says 8.3% where the curve says 6.7%. On a 2 m neap that same gap is 3 cm.
Half a tidal cycle is 6 h 12 m, not six hours, and the whole pattern slides about 50 minutes later every day.
Springs come when the Sun and Moon pull in line, at new and full moon. Neaps come at the quarters.
Roughly 1 cm per millibar away from 1013. A 980 mb depression adds about 33 cm, and that is not in your tide table.
Depths are measured from about the lowest astronomical tide, so the height of tide is almost always something you add.
The ones whose curve is not a sine wave at all. Southampton and the Solent have a double high water and a long stand, Poole is similar, and parts of the Bristol Channel have a much faster flood than ebb. There a published tidal curve for the port is the only thing that works.
Because the tide follows the Moon, not the Sun. The Moon returns to the same meridian every 24 hours 50 minutes, so a semidiurnal cycle is 12h 25m and each half is 6h 12m. That is why tides run about 50 minutes later each day.
Because they often differ. In many estuaries the shape of the seabed distorts the wave as it runs in, so the flood is shorter than the ebb. Set each to what your almanac gives.
It depends where the height you need sits. Neaps raise low water, so a spot that shuts for an hour or two at springs can stay open all tide. They also lower high water, so a bar you can only cross near the top of a spring tide can stay shut.
Chart datum, approximately the lowest astronomical tide. That is why charted depths look alarmingly shallow: they are the worst case, and the height of tide is what you add. A negative height of tide is possible and rare.
That the tide is semidiurnal: one smooth fall and one smooth rise, with the next high water the same height as this one. Both curves are approximations to a real tide, which is the sum of dozens of harmonic constituents.
That the numbers you type are right. This page does no astronomy and has no tide table in it, so garbage in is garbage out, and at sea that has consequences.
That the weather is doing nothing. Pressure and wind move real tides by more than the rule’s own error, and surge is not in this page or in your tide table.
Port ranges are nominal typical figures for scale, and the neap heights come from holding the mean level still, not from a tidal prediction. Verified in tide_model.py.
Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.