All about tides

If you sail in tidal waters you will need to understand the effects of tides. Your navigator or skipper will work out the tidal calculations needed for your journey but as a member of crew it is useful to understand the basics and be able to read a tide table.

The science bit

So how exactly do tides work? Well, tides are caused by the gravitational pull of the sun and the moon on the oceans. Twice a month when the sun, earth and moon are in line the effect is the greatest causing a SPRING TIDE, with the difference between high water and low water is at its most extreme. At the half moon phase, when the sun and the moon are at right angles and the pull has weakened, this is called a NEAP TIDE.

Spring tides follow seven days after Neap tides, then seven days later it is Neaps again and so on.

How to read Tide Tables...

The tide goes in and out twice a day, so there are usually two high waters (HW) and two low waters (LW) each day, usually about six hours apart. The falling tide (when the tide goes out) is called the EBB and the rising tide (when the tide comes in) is called the FLOOD.

Tide Tables tell you the times of high and low water each day as well as the height of tide above chart datum (depth of water shown a chart, also known as the Lowest Astronomical Tide or LAT, and is the lowest the water is ever likely to be), for each tide…..

This tells us:

• The first high tide of the day is 43 minutes past midnight with 4.5 metres above chart datum
• The tide drops to 2 meters above chart datum at low tide at 7.15 in the morning
• The second high tide on this day takes place at 1.16 in the afternoon
• With the final low tide happening at just before 8pm in the evening with 1.8 meters above chart datum

You also need to be aware that:

• Tide times are different around the coast so make sure you use the right tide table for the harbour you are in!
• Times may need to be adjusted for Summer Time – if they do there will be a note on the page, so make sure you check the information properly.

It might all sound a little tricky but sailing is about brains as well as brawn!

Which way?

As well as the rise and fall of tides you also need to know about tidal stream – this is the direction that the tide is flowing and affects both the speed of the boat and its course. The skipper of sailing boat will plan to travel with the tide (in the same direction as the tide) rather than against it, so you may have a leave in the middle of the night or early hours of the morning for your trip to make the most of the tide. Spring tides that we talked about earlier will run faster than neap tides as there is more water to shift!

To find out the tidal stream your skipper or navigator will use a chart or a Tidal Atlas. This will tell him what the tidal stream is doing and how it changes hour by hour.

When you’re out on the water you won’t be able to see how the tide is moving until it flows past a fixed object like a buoy.

It’s a slightly different story if you are out on a motor cruiser. In a motor cruiser you can go at a faster speed which means you don’t have to wait for a fair tide; in fact, you may choose to travel against the tide. This is because the sea is smoother when wind.