Tide Level Chart

Tide tables give the times and heights of high and low water, but you often need the level in between — to launch a boat, cross a bar, walk a causeway or fish a rising tide. Enter the high and low water from your official tide table and this chart draws the tide curve, lists the height hour by hour, and tells you when the water will reach the depth you need.

Why tides rise and fall

Tides are caused mainly by the Moon’s gravity, with help from the Sun, pulling on the oceans as the Earth rotates. Most coasts have two high and two low waters each day (semi-diurnal tides), about 6 hours 12 minutes apart, so tides happen roughly 50 minutes later each day. Some places have one high and one low a day (diurnal) or two unequal tides (mixed). Around new and full moon, the Sun and Moon line up to produce larger spring tides; around the quarter moons, smaller neap tides.

Reading a tide table

Always use the official tide table for the nearest reference port or secondary station, and apply any time and height corrections it lists. Depth of water = charted depth + tide height.

The rule of twelfths

The tide changes slowly near high and low water and fastest in the middle — half the range in the third and fourth hours. The rule of twelfths is a quick mental approximation that assumes six equal hours between high and low water. The cosine method gives a smooth curve closer to a real semi-diurnal tide; both are estimates.

Worked example

Low water is at 06:10 with 0.8 m and high water at 12:25 with 4.6 m, a range of 3.8 m over 6 h 15 min. A boat needs 2.5 m of tide to cross a sandbar. The cosine curve shows the tide reaching 2.5 m shortly after 09:00 — just under halfway through the rise — and the height at 09:00 is about 2.4 m. By the rule of twelfths the timing is similar. The skipper plans to cross after 09:30 for a safety margin.

Clearance under keels and bridges

• Under-keel clearance: depth of water (charted depth + tide height) − boat draught. Keep a safety margin for waves, squat and chart inaccuracies.
• Bridge clearance: charted clearance is usually given above a high-water datum; at lower tides there is more room. Check the datum used on your chart.
• Drying heights: underlined figures on charts show heights above chart datum of areas that dry at low water — they cover when the tide exceeds that height.

Tide and safety on the shore

Walkers, anglers and beachcombers are regularly cut off by the rising tide on sandbanks, causeways and headlands. Check the time of high water before setting out, allow plenty of margin, and turn back early. Remember the tide rises fastest in the middle hours. Onshore winds and low atmospheric pressure can make tides higher than predicted; offshore winds and high pressure can make them lower.

Fishing and tides

Many anglers find fish feed more actively when water is moving, especially during the middle hours of the flood or ebb, when currents stir up food. Slack water around high and low tide can be quieter. Local knowledge matters: some marks fish best on a rising tide, others at low water.

Tidal streams

Tides move water horizontally as well as vertically. Tidal streams (currents) are usually strongest around the middle of the flood and ebb — the same hours when the height changes fastest — and weakest near slack water. In narrow channels and around headlands, streams can be strong enough to stop small boats or swimmers. Use tidal stream atlases or diamonds on charts to plan passages with the stream rather than against it.

Limitations

This chart interpolates between two tide table values you supply; it does not predict tides by itself. Real tides can be asymmetric — rising faster than they fall — in estuaries and shallow areas, and some ports have double high waters or long stands. For navigation where depth is critical, use official tidal curves for your port, local tidal stream atlases and a margin of safety.

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