Tidal Streams – Theory tips – RYA Day Skipper & Yachtmaster

Under international regulations (Safety of Lives at Sea - SOLAS) all boats have to do a passage plan (sometimes called a voyage plan) before leaving harbour. In tidal waters you need to be able to take account of the effect of the tide on your boat.

What are tidal streams?

Tides are the flow of water around the earth caused by a combination of the earths rotation and the effect on the oceans caused by the gravitational forces formed between the sun, moon and earth. This horizontal movement is called the tidal current, or tidal stream.

The largest movement (and hence the strongest tidal streams) usually is at the times of high and low water.

The shape of the land also affects the speed of the tidal stream (it's faster in tidal estuaries and narrow straits). The highest tide in the world is the Bay of Fundy in Canada - and its here you get the fastest tidal streams.

The weakest tidal streams are known as slack tides.

Where can I find the tidal stream data

Information on the tides that are flowing can be found in a Tidal Stream Atlas such as the English Channel Tidal Stream Atlas which is available at the RYA shop.

The tidal atlas gives you a pictorial representation of the tidal flow. You select the correct page for the tidal hour at the port listed at the top of the page, then select the arrow closest to where you will be. The direction of the arrow shows the direction of tidal flow, and the two numbers represent the speed of the tide (for example 09.14). The smaller number is for neaps and the larger number is for springs. This doesn't mean 14 knots for a spring tide in our example - they do not print the decimal point so its 0.9 knots for neaps and 1.4 knots for springs.

Another source of tidal information is from tidal diamonds. These are shown as magenta diamonds on the chart with a letter inside.

With tidal diamonds you again need to work out the tidal hour in relation to the nominated port. Each tidal diamond has data listed for direction of flow (dir) in degree, and then two columns for springs (Sp) and Neaps (Np).

If its not exactly springs or neaps you can take a figure in between for mid range for both tidal streams or tidal diamonds. If it doesn't line up with springs, neaps or mid range then you have to do "computation of rates". Details of how to do this will be in a future Theory Tip.

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