9 NAVIGATION


Fig 9:6 Bearings and range


Virtually all radars come with an Electronic Bearing Line (EBL) facility. The EBL is a line – usually dotted – that radiates out from your position on the screen and can be rotated to lie over a target to obtain a bearing. When in head-up or course-up modes the bearing will be relative to your heading and must be corrected, as shown below, to produce a compass bearing. The Variable Range Marker (VRM) does a similar job for range. The VRM is a circle – again centred on your boat – that can be expanded and contracted to measure distances away. Unlike bearings, which suffer the distorting effects of beam width, you can consider the range to be accurate whatever the size of the scanner. However, you do need to know what you’re looking at. A boat running parallel to the coast in thick fog ran aground because the echoes he thought were the shoreline turned out to be blocks of flats set many metres inland.


Position by range and bearing


A good way to plot your position on a chart is to use EBL and VRM together.


■ Select a target that can be positively identified (such as a buoy or distinct headland) and lay the EBL over it. Make an allowance for beam width if you think it necessary.


■ Convert the relative bearing into a true one. ■ Obtain the range with the VRM. ■ Reproduce the two on your chart.


Fig 9:7


Range and bearing is a simple way of finding your position


Echo sounders and fishfinders Few sea anglers will need an introduction to fishfinders – those marvellous developments of depth sounders that both trace the undulations of the sea bed and also reveal what potential catches might be lurking in the depths.


Echo sounders – often called depth sounders – and fishfinders work on the same principle. A sound pulse is emitted by a ‘transducer’. The sound echo bounces back from the bottom (or fish) and the time delay is used to compute the depth. This is similar to the way radar works but, of course, with sound waves rather than microwaves. Echo sounders use either rotating LEDs or have digital displays. Fishfinders plot successive soundings as a continuous, rolling representation – a sort of brief history of what went on below you. A fishfinder display might be in black and white or colour – the coloured one being by far the easiest to interpret.


72


RYA Seamanship for Sea Anglers


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