NAVIGATION Tidal range


It’s important to understand that spring and neap refer to variations in tidal range. At high and low water, a spring tide will be both higher and lower than a neap tide – i.e. it has the greater range. By comparison, neap tides won’t dip as low nor rise as high – i.e. have a smaller range. Tides are measured from a ‘datum’, an agreed level which in Britain and the rest of Europe is the Lowest Astronomical Tide (LAT). This is the lowest the tide can be expected to fall if we ignore temporary influences such as wind and barometric pressure. As well as LAT, other abbreviations are common. For example MHWS means Mean High Water Springs and stands for the average height of spring tides over a year. From piece of information, it shouldn’t be hard to figure out the others on the drawing Fig 9:4.


9


Heights are measured above MHWS, not chart datum


MHWS MHWN


Spring range Neap range Height of tide about chart datum Drying height


MLWN MLWS


Chart datum


Fig 9:4 Allowing for tides


As we covered in Chapter 4, a tidal stream means we’re operating on a platform of moving water. A simple example would be someone motoring at 6 knots with a stream of 1 knot from directly abeam. One hour later our boat will have moved 6nm (nautical miles) forward and been carried 1nm sideways by the stream. A boat doing 12 knots would motor twice the distance in the same time, but still would only be carried 1nm downstream. It follows that slower boats are more affected by tidal streams than faster ones.


There are ways of plotting stream effects and computing the course adjustments needed to overcome them but these are fiddly and unnecessary, except perhaps for very slow boats. However, there are some useful tricks which will serve instead.


RYA Seamanship for Sea Anglers 65


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