Fog Blog – Navigating in Restricted Visibilty

Posted: 26 October 2012 by Jim Barden in Navigation
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Of all the different types of weather we may encounter at sea,  fog must be one of the most daunting and one which carries the most risk ; especially in crowded European waters which can be alive with commercial shipping, and where the risk of collision is high.

The best advice, offered by Des Sleightholme,for when fog is forecast might be :  ‘Stay home. Keep Hens’

However  if we’re already underway when the fog creeps in,  then some strategy to get safe and keep safe must employed.  As confidence with the various techiniques grows you should be able to navigate blind, if the ground permits, to most destinations.

Get prepared early.  If visibility is noticeably  deteriorating, get ready for it.  Get a reliable fix on the chart by conventional means – visible bearings, GPS etc.  If necessary work up an EP from you last known position (this shouldn’t be any more than an hour old on your chart).  If you happen to be near a recognisable mark get to it and ask the helm to hold station while you figure what to do next.

We are going to consider later the navigational aspects of dealing with fog,  but here are some basic precautions you should consider in immediately :

  • Fog Horn Ready – sound appropriate signal ( one long – under power, or one long and two short – sailing : every two minutes.
  • Lights On – make the vessel more visible with the appropriate lights for your category
  • All Crew in Lifejacket and on-deck – the collision you fear could happen very suddenly resulting potentially in all hands in the water.  On deck they also provide extra eyes and ears.
  • Radar Reflector Up – if it’s not already, hoist your reflector high.  If you can’t find it, string your pots and pans together on a halyard !
  • Monitor VHF – and contact relevant Port Traffic Control in necessary
  • Liferaft / Tender Ready
  • Engines on or ready
  • Navigate away from potential danger of collision.

If fog, shallow is safe, away from commercial shipping and also the most likely place to find a reliable contour of depth to follow (or a safe place to anchor)

With reference to your position and any shipping channels indicated on the chart ,measure up a course to steer to the nearest strong contour of depth and away from shipping channels.  This should be one which is has a fairly consistant direction, if possible close and parallel to the shore. ( the odd mark on it or near it along the way wouldn’t go amiss. )

Calculate the height of tide.  Add this to the value of the contour (and subtract draught if your echo sounder reads depth below keel) —  this is the displayed  echo-sounder depth we’re looking for. For example if we decide to follow the 5m contour, and calculate the height of tide to be 2.3m – we are actually looking for a depth of 7.3m to follow.  With a draught of 1.8m and an echo sounder reading below the keel, this would be 5.5m displayed.

If there is no strong contour to follow nearby then just head shallow and anchor.

Otherwise measure the rough direction of the contour you intend to follow, this will be the heading we turn onto when the required displayed depth is reached.  No adjustment to the heading is usually required for tidal stream here as in general the tides follow the contours. (but make note of exceptions to this where the contour might veer away from the tidal stream)

This information should be passed to the helm and crew as a clear and concise set of instructions :

“Head 030°(M) , maintain speed of 4kts. Depth should fall.  When we get to 5.5m dispayed (10 mins or so)  turn starboard onto a mean course of  095(M)  ± 20°  and  maintain depth between 5 and 6m.  Feed me depth readings at every 0.5m of change and let me know when you get to 5.5m”

When the boat hits the desired depth and turns to follow the contour make a note of the time / or start your stopwatch and mark this on the chart of this where you think you have joined the contour.

Now every six minutes  ( 1/10 hr) you can plot your estimated position along this line.  This should be fairly straight forward as you will cover one tenth of your boat speed in knots (± any estimated tidal stream) every six minutes.   Our arrival to or near to any marks on or near to the contour can now be predicted and the crew told when , where and what to keep a look out for.  On sighting and identifying each mark we can fix our postion again and make a note of time.  This also allows us to recalculate our actual ground speed rather using an estimate of tide along with a possibly inaccurate log/speedo.

Changes or trends in the contour’s direction should also be noted and this information passed onto the crew before it happens  eg :

“ in the next few minutes the contour will start curving to the north onto a rough heading of 005°(M) ±10°.  Alter course as nessessary to follow it and maintain depth 5.0 -6.0 “

Sometimes it might be useful to sketch the shape of the contour and mark it up with rough courses for the helm to look at in the cockpit.  Also don’t forget that every half hour (or at least every hour) the tidal height may have changed significantly , so the indicated depth you wish to maintain will need to be recalculated.

It may be necessary or desirable to move between contours to avoid hazards or find marks to help you fix position. Do this early.

When crossing a channel and potential traffic , check local VTS on the VHF for any ships in the area and whether its safe to cross and then cross at heading at right angle to the shallow contour of depth on the other side, then turning onto a new heading.

Have a look at this chart and consider how you might find the refuge of  Stokes Bay to sit out the fog at anchor. Imagine we’re just approaching the exit of North Channel heading east when the Fog starts to fall. An east cardinal mark can be seen 200m ahead …..

We’ll give a possible solution next week and a consider how we might then continue on to Portmouth harbour.  Or come on one of our training courses and where we complete simulated fog exercises on the water

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Comments
  1. andrewsb2012 says:

    You’re sailing along, enjoying a beautiful afternoon and having one of the nicest Saturdays you can remember when suddenly—wham—everything changes. A thick, wet, and chilling fog rolls in unexpectedly and totally surrounds you. Now, you can hardly see two boat lengths in front of you, never mind see where the shore is. Can you find your way safely back to the dock?

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