Why is tide so important when passage planning?

Those of us that sail in tidal waters will be aware of the shear weight of water that can run through in opposite directions every 6 hours. There are many areas around the British coast where tidal streams can reach 9 knots and more during spring tides and most small vessels simply cannot make progress against that sort of force.

Tidal streams are at their strongest at pinch points such as narrow channels and headlands, deep narrow channels such as the Needles Channel on the western tip of the Isle of Wight or the area of water between Alderney and the Cherbourg peninsula are renowned for strong tidal flows. These areas are known as ‘tidal gates’ and a prudent skipper will plan his voyage so as to arrive at these gates when tide is not running against them. If it is, you may be well advised to avoid the area and sail the long way around, or stop for a while and wait for the tide.

But tidal gates are not the only thing to consider. After all, it usually makes sense to leave at a time when you get the most benefit from tide flowing in your favour. Balancing this with when you will arrive at tidal gates is a combination of mathematics and experience. There is also something else to consider; sea state.

We all know that the sea state is, to a large extent, governed by wind speed, but not speed alone. The direction of the wind is important as wind blowing offshore from land will have had little chance to build a sea (fetch) and will not usually greatly worry the inshore yachtsman. However, where wind is blowing in the opposite direction to a strong tidal stream, steep waves can quickly be induced making for a rough and uncomfortable sea state. 

The prudent skipper will need to assess his vessel and his crew before setting out on passage in such conditions. A strong spring tide going East around St Catherine’s Point on the Isle of Wight will create a steep, rough sea very quickly when you are beating into 25 knots of Westerly breeze. Are you and your crew prepared for this ?

Finally, when planning your passage remember that tide is relevant not just because of it’s effects on the sea state and your course. It will also need to be considered when piloting into harbour. The amount of water under you may affect your planned approach and time of arrival. Many marinas are only accessible at certain times and knowing how much tide you have on arrival is critical. 

To those of you planning to sail mainly in the warm, non-tidal, waters of the Mediterranean tide might seem like an awful lot of bother, but if you harness its power to your advantage it can have a very positive effect on your speed over ground and estimated time of arrival at your next port. Let’s hope the bars are still open.

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