Tides matter more than the wind

On a catamaran, the tide shapes your day more often than the forecast does. A cruising cat will make six to eight knots in easy conditions, and around the UK the stream can run two, three or even four knots through the same water. That is a third of your boat speed. The tide is not a detail you bolt on at the end of planning; it is the frame everything else hangs on. The compensation is that shallow draft gives you options a deep-keeled yacht does not have — you can tuck inside a bank and wait for the stream to turn rather than punching into it for three hours.

What a tidal stream atlas actually tells you

A tidal stream atlas gives the predicted direction and rate of the current across a stretch of coast, hour by hour. The hours are numbered either side of high water at a standard port, so your first job each day is to find that reference port and note the time of high water. Most atlases print two sets of figures — one for springs, one for neaps — and you interpolate between them according to the day's range.

Three habits make the atlas far more useful:

• Use the tidal diamonds on your chart. They give rates and directions at specific positions, and they are the quickest check when a passage crosses a chart boundary.
• Interpolate honestly. If the day sits halfway between springs and neaps, do not use the spring figure just because it feels safer.
• Expect the atlas to be slightly wrong. It is a set of averages; wind, pressure and headlands all bend the stream more than the printed arrows suggest.

Springs, neaps and the rule of twelfths

Spring range around much of the UK is roughly double neap range, so the streams run nearly twice as fast. A passage that feels effortless on a spring tide can feel sluggish a week later. The rule of twelfths estimates how much water moves in each hour of the tide: one twelfth in the first hour, two in the second, three in the third, three in the fourth, two in the fifth, one in the sixth. It works as well for depth under the keel as for the strength of the stream.

Shallow draft tempts catamaran sailors into shortcuts. Take them only with real numbers: your draft plus a sensible under-keel margin, and a depth sounder offset you have checked against a known depth.

Wind against tide: the one to avoid

When wind blows against the stream, the sea stands up quickly. Waves become short, steep and close together, and in shallow water they break. For a catamaran this is the least comfortable and most punishing condition there is: the bridgedeck slams, the rig works hard, and the motion is far worse than the same wind with the tide running underneath you.

The trouble spots are predictable. Overfalls build off headlands — the races off Portland Bill, St Alban's Head and the Needles, the Alderney Race, the Pentland Firth, the north coast of Anglesey. Anywhere the stream is squeezed and accelerated will turn ugly when the wind opposes it.

If the timing is wrong you have three choices, and only one of them is to press on: wait at anchor or in harbour until the stream turns, take an inshore passage where local knowledge says one exists, or give the headland a wide berth.

Planning a passage with a fair current

Good tidal planning is mostly working backwards. Start with your destination and the time you want to arrive, identify the tidal gate — the headland, narrow channel or bar where the stream matters most — then set your departure so you reach it at slack water or with a fair stream.

• Chase the fair stream. Two knots of tide under you for six hours is twelve miles gained.
• Do not fight it. Losing two knots burns fuel and patience. Anchor, eat, and leave on the turn.
• Think in six-hour legs. Most UK coastal streams run about six hours each way, so a thirty to forty mile hop fits one fair tide if you get the start right.
• Watch the wind. A fair stream with a fair wind is a gift; a fair stream with wind against tide means rethinking the plan.

Local knowledge is the last piece

Atlases get you close; local knowledge gets you in. Harbour masters, marina staff and the sailors on the next pontoon know which berth dries first, which bar carries a metre less than charted, and where the stream sets across an entrance. Ask, listen, write it down.

Keep a passage log noting the predicted stream, the actual track over the ground and any surprises. After a season you will have a personal tidal atlas for your own stretch of coast, and that is worth more than any printed page.