BeCuriouzLearn something new every day
Tidal Heights & Under-Keel Clearance: The Rule of Twelfths
💡 Tidal height rises by 1-2-3-3-2-1 twelfths of the range across the six hours from low to high water, letting you predict depth — and your under-keel clearance — at any moment.
Recall that tides rise and fall on a roughly semi-diurnal cycle — about two highs and two lows each day near many coasts. As a skipper, the number you actually care about is under-keel clearance (UKC): the water beneath your keel at a given moment. Get it wrong over a sandbar or drying harbour entrance and you run aground. The good news is that tide tables plus one simple rule let you predict the depth for any time in between.
Tide tables list the height of high water (HW) and low water (LW) and the times they occur. Subtract the two to get the tidal range for that cycle. But the tide doesn't rise at a steady rate — it moves slowly near the turns and fastest in the middle. The Rule of Twelfths approximates this: over the roughly 6 hours between low and high water, the tide moves 1/12 of the range in hour 1, 2/12 in hour 2, 3/12 in hour 3, 3/12 in hour 4, 2/12 in hour 5, and 1/12 in hour 6.
Work a quick example. Suppose LW is 2.0 m at 08:00 and HW is 8.0 m at 14:00 — a range of 6 m, so each twelfth is 0.5 m. By 10:00 (two hours after LW) the tide has risen 1/12 + 2/12 = 3/12 = 1.5 m, giving a height of 3.5 m. Add that predicted height to the charted depth at your spot, then subtract your boat's draft and a safety margin to get UKC. Because the middle hours move fastest, that's when clearance changes most quickly — plan crossings of shallow ground with a generous buffer rather than cutting it fine on a falling tide.