Tidal calculations can be tricky. Sometimes back-to-basics is best – certainly don’t believe everything you find on the web. For pilotage in shoal waters a sailor needs to know the depth, which depends upon the height of tide. To calculate height of tide for times between high and low water, the conventional method involves a pencil-and-paper exercise, working from tidal curves published in almanacs, or often these days, the number is plucked straight from a phone app, often based on generic tidal curves. However, in shallow estuaries the banks impede the flow of water, distorting those tidal curves. Inaccurate or overly simplified tidal curves can lead to dangerous inaccuracies in your expected depth. On countless rivers around our coast, you wouldn’t want to arrive at the rivermouth bar only to be driven aground because you’re an hour too early, or too late. The same is true for marinas and harbours protected by locks and sills.
A full analysis requires intricate maths, usually on a computer, but some computerised output on websites contains gross errors. In those cases, using a pencil to sketch adjustments to the almanac’s tidal curve can actually yield more accurate results. For this it is vital that the skipper should have a good mental picture of what happens when the sea flows into and out of the estuary.
TIDAL SILLS
One obvious way in which a shallow estuary can interfere with the flow is by a natural sill, such as a drying bar. To illustrate general principles, I’ll start with a simple example of a man-made tidal
