Understanding Tides for Small Boat Launching

Why the tide decides whether launching is easy or a struggle

Anyone who has watched a slipway disappear under a rising tide knows the sea keeps its own timetable. Around the UK, tidal ranges vary enormously — from barely a metre in parts of the eastern Irish Sea to well over ten metres in the Bristol Channel and the Severn Estuary. That difference matters hugely when you are reversing a trailer, or pulling a dinghy on a trolley, down a concrete ramp that may be slippery, weed-covered, or simply not there at the right moment.

Good launching is mostly about being in the right place at the right state of tide, with enough water under the hull and enough left over for the return.

Spring and neap tides: the fortnightly rhythm

Tides are driven by the gravitational pull of the moon and the sun. Twice a month — at new moon and full moon, when the sun, moon and Earth line up — those pulls combine and the range between high and low water is at its greatest. These are spring tides, and confusingly they have nothing to do with the season. They happen all year round.

At the first and third quarters of the moon, the sun and moon pull at right angles and partly cancel each other out. The result is neap tides: less water up the slipway at high water, and less water left at low water.

A rough rule of thumb is that the spring range is about twice the neap range in many UK estuaries, though the exact ratio varies by location. In practice:

• Springs give you more water, a longer usable window, and a stronger tidal stream.
• Neaps give you a shorter window and sometimes not enough depth at all for a heavier boat on a shallow ramp.
• The biggest springs of the year tend to arrive near the equinoxes in March and September, when the range can be noticeably larger again.
• Weather matters too: a strong onshore wind and low pressure can add half a metre or more to a high water, while a persistent offshore wind can strip it away.

Chart datum: what your tide table is really measuring

Tide tables list heights above chart datum, which for UK waters is broadly the lowest astronomical tide — the lowest level the tide is predicted to reach under average conditions. It is deliberately chosen so that the depths on a chart, and the readings from an echo sounder, are conservative.

This has two practical consequences. First, if your tide table says the height of water at your chosen hour is 1.2 metres, that is 1.2 metres above datum, not above the seabed. Second, drying heights are measured downwards from the same line, so a slipway marked "dries 1.5m" needs at least 1.5 metres of tide simply to reach the end of the concrete.

You will also see MHWS (mean high water springs) and MHWN (mean high water neaps) quoted for a port. These are averages, useful for comparing one place with another, but for a specific launch use the predicted height for that day.

Local harbour rules and byelaws

Every harbour, estuary and inland navigation has its own authority, and their rules exist for good reason. Before you launch, it is worth checking:

• Whether launching is permitted at all from your chosen slipway, and whether a fee or permit applies.
• Any local notices — dredging, regattas, temporary speed limits, or closed areas around moorings and pontoons.
• Speed limits, which are often 5 or 6 knots inside harbour walls. The wash you make matters to moored boats.
• Where you may and may not leave a trailer, and how long you can occupy the slipway.
• Whether you need to notify the harbourmaster or book a slot at busy times.

Many harbour offices publish a simple leaflet or post notices at the slipway itself. A two-minute read before you go saves a great deal of awkwardness later.

The rule of twelfths and choosing your window

The tide does not rise or fall evenly. In a typical six-hour rise, the flow is slowest at the turn and fastest in the middle. The rule of twelfths splits the range like this:

• First hour: 1/12 of the range
• Second hour: 2/12
• Third hour: 3/12
• Fourth hour: 3/12
• Fifth hour: 2/12
• Sixth hour: 1/12

So with a 4-metre range, the tide rises half a metre in the first hour and a full metre in the third. The two hours either side of low water move slowest of all — often the most forgiving for a shallow slipway, and the safest moment to recover a boat before the water leaves.

A simple pre-launch routine

Before you commit, check the tide table for the nearest standard port, apply the local correction if one is given, and note the predicted heights at the times you intend to launch and recover. Then ask a few practical questions.

• Is the slipway usable at that height, including the part below the waterline?
• Do you have a window that comfortably covers both launch and recovery?
• What is the wind doing, and is it pushing against the tide at the harbour entrance?
• Have you got a painter, a trolley or trailer suited to the gradient, and footwear that grips wet weed?

Do that once or twice and it becomes second nature. The tide stops being an obstacle and starts being a partner — which, on a bright morning with the water just right, is exactly what it should be.