Set and Drift: The Invisible River Under Your Keel

Recall from earlier cards that tides make water rise and fall. But that same water also moves sideways — a tidal current that shifts your whole boat like a walkway on a conveyor belt. Two words describe it. Set is the direction the current flows toward (e.g. "the set is 090°" means water moves due east). Drift is its speed in knots. Together they tell you exactly how the sea is pushing you.

Here's why it matters: if you steer for a harbour dead ahead but a current sets across your path, you'll quietly slide sideways and miss it. Suppose you motor at 5 knots toward a mark 5 miles away — about an hour's run. If a 2-knot current sets across your track the whole time, you'll end up roughly 2 miles off to the side unless you correct. The fix is to steer upstream of your target — angle into the current so its push cancels out, holding your intended course over ground.

Timing turns this enemy into an ally. Tidal currents aren't constant: they build to a peak, then slack, then reverse. A rough guide sailors use is the rule of twelfths — a current is strongest around mid-tide and near-zero at slack water (high and low tide). Plan a passage so a 2-knot current runs with you rather than against you and your 5-knot boat effectively makes 7 over the ground; mistime it and you crawl at 3. Over a long channel, catching the fair tide can halve your passage time.

The skipper's habit: before leaving, check the tidal atlas or chart for the set and drift at your planned time, then decide whether to wait for a fair tide or steer a correction — often both.