Tides & Currents

Tides are caused by the gravitational pull of the Moon and, to a lesser extent, the Sun on the Earth's oceans. The Moon, being much closer, has roughly twice the tidal effect of the Sun.

The Moon pulls the water on the nearest side of the Earth towards it, creating a bulge (high tide). At the same time, centrifugal force creates a second bulge on the opposite side of the Earth. The areas between the bulges experience low tide.

As the Earth rotates, most UK locations experience two high tides and two low tides every ~24 hours 50 minutes (a lunar day). This is called a semi-diurnal tidal pattern.

• High Water (HW) — the highest level reached by the rising tide.
• Low Water (LW) — the lowest level reached by the falling tide.
• Tidal Range — the difference in height between HW and LW.
• Flood — the rising tide (LW to HW).
• Ebb — the falling tide (HW to LW).

The tidal range is not constant — it varies through a roughly fortnightly cycle depending on the alignment of the Sun and Moon.

Spring Tides (nothing to do with the season!):

• Occur when the Sun, Moon, and Earth are aligned (at new moon and full moon).
• The gravitational pulls combine, producing the largest tidal range.
• Higher high tides and lower low tides.
• Stronger tidal streams (currents).

Neap Tides:

• Occur when the Sun and Moon are at right angles to the Earth (first and third quarter moon).
• The gravitational pulls partially cancel, producing the smallest tidal range.
• Moderate high and low tides.
• Weaker tidal streams.

The tide does not rise and fall at a constant rate. It follows a roughly sinusoidal curve — slow at first, fastest in the middle, slow again at the end. The Rule of Twelfths approximates this pattern.

Divide the total time between HW and LW (or LW and HW) into 6 equal hours. The total tidal range is divided into 12 equal parts. The tide rises or falls:

Remember the pattern: 1, 2, 3, 3, 2, 1 — this adds up to 12.

Worked example: If the tidal range is 4.8 metres, each twelfth is 0.4 m. In the 1st hour, the tide rises 0.4 m. In the 2nd hour, 0.8 m. In the 3rd and 4th hours, 1.2 m each. That means half the tidal range (2.4 m) rises in just the middle 2 hours!

While "tides" refer to the vertical rise and fall of water, tidal streams are the horizontal movement of water caused by the tides. Understanding tidal streams is critical for navigation — a 2-3 knot tidal stream can easily exceed your boat speed!

Key terminology:

• Set — the direction the tidal stream flows towards (e.g. a stream setting 090° flows eastward).
• Drift (Rate) — the speed of the stream, measured in knots.
• Flood stream — flows in the direction of the rising tide.
• Ebb stream — flows in the direction of the falling tide.
• Slack water — the brief period at HW or LW when the stream is minimal (changing direction).

Where streams are strongest:

• Narrow channels and headlands (water is squeezed and accelerates).
• Between islands.
• During spring tides (streams can be 50–100% stronger than neaps).
• In mid-channel (weaker near the shore due to friction).

Tidal stream atlases show the direction and rate of tidal streams for each hour relative to HW at a standard port. Arrows show direction; numbers show neap and spring rates. These are available in almanacs, chart plotters, and as separate publications.

Tide tables provide predicted times and heights of high and low water for standard ports. For other locations, you apply time and height differences listed for secondary ports.

Reading a tide table entry:

From this example:

• The morning tidal range is 5.8 - 1.2 = 4.6 m.
• The time from LW to HW is 0928 - 0312 = approximately 6 hours 16 mins.
• Times are usually in UT (Universal Time). Add 1 hour for BST (British Summer Time, late March to late October).

Secondary port corrections: If you need the tide at a secondary port, find its correction in the almanac. For example: "HW Portsmouth = HW Dover +0045, height x 0.85." This means HW is 45 minutes later and 85% of the height.

Calculating depth at a given time:

1. Find the height of tide at the time you need (using Rule of Twelfths or tidal curves from the almanac).
2. Add the charted depth (the number printed on the chart, which is measured from Chart Datum).
3. Subtract your vessel's draught.
4. The result is your clearance under the keel.

Depth at time = Charted depth + Height of tide

Under-keel clearance = Depth at time - Vessel draught