Tides Explained for Sailors

Tides are the rise and fall of the sea, driven by the gravitational pull of the moon and sun — and for sailors they affect two things that really matter: how much water there is (the depth) and which way it flows (the tidal stream). Understanding both is a core skill, because the tide can be a powerful friend or a serious hazard depending on how well you read it.

What causes tides

Tides are caused mainly by the moon's gravity, with the sun adding to or subtracting from the effect. As the earth turns, the moon's pull raises the sea into bulges that sweep around the globe as the rise and fall we call the tide. Most coasts experience two high tides and two low tides roughly every day — a pattern called semi-diurnal.

Spring and neap tides

The size of the tide changes through the month:

• Spring tides — the largest range (biggest difference between high and low water), around the new and full moon, when the sun and moon pull in line. Springs bring stronger currents and more extreme highs and lows.
• Neap tides — the smallest range, around the half moon, when the sun and moon pull at right angles. Neaps are gentler.

Knowing whether you are on springs or neaps tells you how much water to expect and how strong the currents will run.

Tidal streams — the flow

Just as important as the height is the tidal stream — the horizontal flow of water that goes with the tide. For sailors this is often the bigger factor: a fair (favourable) stream can add knots to your progress over the ground, while a foul (against-you) stream can slow you to a crawl or stop you dead. Measured in knots, the stream also affects how you approach a dock or mooring, and even which side of a racecourse pays. Planning passages to catch a fair tide is one of the classic arts of seamanship.

Why it matters

Put simply, the tide changes depth and flow:

• Depth — enough water to avoid running aground in shallow areas, and to clear the bottom at anchor or in a berth.
• Flow — how fast and which way the water moves, affecting your speed, your course, and how you dock or moor.

Reading the tide

You find out what the tide is doing from tide tables (times and heights of high and low water) and tidal stream information (which way and how fast the water flows at each stage), published in almanacs, apps and on chart plotters. Reading them lets you plan when there will be enough water and when the flow will help rather than hinder. It is a habit every coastal sailor develops, and the sailing terms glossary covers the related terms.