These days, secondary port tidal heights don’t hold the terrors they used to – not if you’ve a plotter, that is. Here’s a readout from a PC unit. Most hardware plotters offer the same service, if you know where to find it, and it means an end to struggling with interpolation and graphs.
There’s only one problem. Your plotter’s figure may well be different to the one being delivered by another manufacturer’s kit on the yacht fifty yards away. Most of the time, these readouts are fairly useful, but when you’ve cut things fine and need to know the tide to decimals of a metre, perhaps it’s time to lick your pencil, get out the tables and hit the graph after all. Even then, your best-calculated result remains a prediction. Only the fishes know how deep it really is.