What is the issue?Whether you are navigating by instrument or eyeball, what lies beneath the water is invisible in most places. And sometimes, the main concern of navigation is to avoid an unmarked isolated danger. A good example of this is when hugging the coast and rounding the eastern side of the Isle of Wight. The Cole Rocks as illustrated above, lie 600 metres off Bembridge Ledge and very much in the way of this. There is a safe leisure boat passage between the ledge and the two drying heads of the rock, known as 'The Deep', with soundings of 2-3 metres at CD, and of course passing outside. The latter outside route although longer is the safer path because of the tidal streams, and the Cole Rocks are scarcely ever awash and almost always covered.
Why address this?Operating a boat in the vicinity of hidden isolated dangers makes for a high degree of anxiety, and for the potential of a collision. A good method is to know in real-time exactly how far the hidden danger is from the vessel, and avoiding it will keep the anxiety levels down, the vessel safer, and make the odd shortcut more useful.
How to address this?Place a waypoint on the hidden danger on your chart plotter and you have a real-time feed for its position. This makes it easy to steer clear of it.
When sailing local waters we tend not to set a waypoint but use a plotter as a back-up to eye-ball navigation. Likewise, when we do set a destination waypoint we tend to use it for bearing to the destination, calculating how long you have to go ('Estimated Time of Arrival' or ETA) and speed made good towards it ('Velocity Made Good' or VMG). None of these will suffer from being deactivated for a brief period whilst being in the vicinity of a hidden hazard.
Once you place a waypoint on the danger you can use your GPS to maintain a steady watch on the hazard. Maintain a safe distance off and make certain that you are not closing in on it by watching the bearing altering. You can stay in the cockpit and slide past hazards without making numerous anxious position plots and enjoy the sailing.
With thanks to:Michael Harpur, Yacht Obsession.
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