NAVIGATION Belt and braces navigation
Navigators should be prepared to use all the tools at their disposal. The arrival of GPS and radar has had the unfortunate effect of making some of us neglect more conventional methods. For example, running along a well defined depth contour – or between two depth contours – is a good way of working your way along a coast in poor visibility. Of course, since the contours marked on the chart are effectively for the lowest possible tide (apart from in exceptional circumstances), you have to allow for the tidal height at that time. Handily, many chart plotters will display tidal information. Or you may choose to use The Rule of Twelfths (see page 69). In the days before there were any fancy instruments, navigators employed all manner of odd tricks – one of them interestingly predicting echo sounders and radars. When closing a cliff-lined shore in fog they used a loud noise (a shout, a whistle or even a musket shot) and counted the seconds before the echo returned. Sound travels at about 377yds (340m) per second and you have to allow for it to go there and back. Another useful guide is that, with your eyes just 5ft (1,52m) above the waterline, if you can just see the breakers at the foot of a cliff, then that cliff is about 2.5nm away. Not exactly rocket science but – who knows? – it might come in useful one day…
Unmarked rivers
It’s not unusual for creeks and rivers either to have very few navigation marks or none whatsoever. Even marks that might be found are often simple withies – sticks stuck in the mud, here today gone tomorrow. Luckily for us, rivers are fairly predictable. Along the straighter reaches the faster, deeper water tends to be in the middle. On bends the centrifugal force throws the stream outwards, carving out the bottom where it runs. So the general rules are:
■ Stick to the centre where the river runs straight. Keep to the outside where it bends. The inside of bends can be very shallow indeed (Fig 9:8).
Fig 9:8 ■ Be watchful for rocks on the banks – they could extend out into the river.
■ Rivers often have bars at their entrance. Be very careful entering, especially with an onshore wind.
■ If you’re a small vessel, don’t forget your obligations under Colreg rule 9(b). You must not impede another vessel which can only navigate in the deeper channel.
NOTE: Whether navigating conventionally or by electronics, there’s much more to navigation than could ever be covered here. Anyone wanting to learn more is urged to read the RYA Navigation Handbook which covers the subject in far greater depth.
9
RYA Seamanship for Sea Anglers
73
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