WNS (What Now, Skip?) October

[timbartlett]

Thanks to all who contributed to WNS September.
Now, in the bizarre world of publishing, we're into October.So here (below) is the next thrilling installment.
But please read the notes first.
All the best
T

• The idea is to offer a nautical puzzle, which experienced skippers will (hopefully) find interesting or entertaining, from which the less experienced may be able to learn something, and from which we can all pick up ideas.
• The WNS skipper is a fictional character. Any resemblance to a real individual is purely accidental, except that he occasionally makes mistakes, and he is not able to make time run backwards. So having got into a situation, he can't get out of it by wishing that he had done something different.
• WNS is not a competition to see who can match some hidden but predetermined solution. Of course I have an answer in mind (you wouldn't like it if I gave you an impossible situation, would you?) But mine may not be the best or only answer.
• If you think I've missed something or given confusing information please ask for clarification.
• Attributed extracts from selected posts will appear in the next issue of MBY.

Our hero is off on a week’s credit card cruising with a mate, on a 7-metre RIB. The plan is to visit all the main islands in the English Channel .

The boat is no longer new, but it’s robust and reliable, and reasonably well-equipped with a compass, log, and chart plotter, and a brand-new VHF, backed up by a hand-held GPS and a paper chart of the Western Channel. Both men are wearing drysuits and lifejackets, and and there's an epirb and a polybottle of flares on the A-frame.

The pair are now on the 100-mile leg from Ushant to the Scillies, in a brisk easterly wind that is is kicking up a lumpy sea that makes steering by compass difficult. Two hours out of Ushant, steering by the rolling road display on the plotter, they reach their half-way point, where they stop for a pee and a Mars Bar.

While using both hands to re-seal the zip on his drysuit, the owner stumbles and almost falls – wrenching the GPS antenna off the A-frame in the process. With no GPS input to the plotter, they do their best to steer a compass course for the Scillies. Two hours later, though, there is no sign of the Scillies – or, indeed, of anything else.

Taking stock of their situation, they find that the “back-up” GPS is useless, thanks to sea water in its battery compartment, and that the VHF – fitted only the previous day, after taking advantage of VAT-free Guernsey prices – has probably been responsible for a large but unknown compass error. They've got half a dozen litre bottles of water, a big box of biscuits and chocolate bars, and enough fuel for another 50-100 miles -- depending how fast they go.

What now, Skip?

 

[hlb]

You cant see the Scillies till very very close, so carry on for half an hour or so. If no result, aim for Newlyn, only 35 miles from the Scillies. You will hit the Coast some where, just maybe have to go left or right a bit, to find a harbour.

 

[strakeryrius]

I agree with HLB - in the conditions they may have mis-judged the exact halfway point - so they should slow down to preserve fuel, but keep on going for another 30 mins as the Scillies are pretty low lying and they may still stumble upon them yet.

Failing that they should move the VHF and compass apart to be able to rely on it more and set a course for 045 approx so that if they miss the Scillies they will at least hit the mainland before they run out of juice. They have to assume that the compass has taken them too far West as if it were too far East they would have already had sight of the mainland by now.

It would also be a good time to put in a call to the CG to let them know of their predicament - if they can't get a reply then they know they are definitely too far West and out of range.

 

[Firefly625]

Oh... what an adventure... but in reality being a bit of a chicken I would make a call to the CG straight away. I would advise them of the scenario, obviously not in any danger just yet but they would be able to get a fix off your VHF transmission which would allow them to give an approximate course to make landfall... not sure I would continue to Scillies... they are easy to miss the best of times.... good luck skip!


....it is the day you are pleased you read (obviously if you cannot see land you have to read the whole article!);

http://www.aztecsailing.co.uk/theory/ch1 sect 6.html

 

[Nick_H]

Well first off they better slow down so they have enough fuel to get to the scillies even if they are going in the right direction. Then they should fix the GPS aerial, as there's nothing really to go wrong with them, so most likely just a disconnected wire. If they can't fix either GPS, then call to the CG so they can monitor their progress, then proceed, heading a bit more Easterly.

Can the CG really plot your position from a VHF signal?

 

[Firefly625]

....Can the CG really plot your position from a VHF signal?

Radio Direction Finding

Some radio receivers are designed so that the aerial can be rotated, and you can tell when it is pointing straight towards the transmitter. This can be used to draw a position line in exactly the same way as when you take a bearing with a hand bearing compass.

As an early form of electronic navigation this was quite widely used in the past (and still is in aviation) with radio beacons set up and marked on the chart like radio wave lighthouses. It is less accurate, and now much less widely used, than GPS.

However in an emergency, the Coastguard can give you a position line by listening to your VHF radio transmission from his direction finding aerials (marked on the chart). You can then draw the bearings he gives you onto your chart as position lines.

 

[davedpc]

If all else fails i have never been south of the scillies without seeing countless ships coming and going from N.America entering or leaving the western approach to line up or leave the channel seperation zone. Years ago without the luxury of a gps it was common to ask a fisherman or ship to point you in tht right direction, either using the radio or hail.

 

[gjgm]

have they got alot of rope?
I dont know how that will help, but it solves every other WNS !

 

[DAKA]

I agree the VHF fix is by far the safest route but assuming the new vhf is bust then it is time to get the rope out for a depth contour reading.

Then steer the same incorrect compass course while your mate disconnect the VHF and removes it to find the compass deviation.

Then dead reckon from your last know fix (the comfort break, Flybridge owners dont do peeing over the side), the dead reckon backed up with the depth contour fix should give you a reasonable idea of where you are.

If you haven't got enough rope then you could strip it down and knot the threads together.

If fuel becomes an issue anchor and wait for the tide to run NE.

 

[nonitoo]

1. Call CG and request either a VHFDF fix or, if a fix is not available (perhaps only one VHFDF Station) then a VHFDF single bearing. That will at least give you a single position line.

2. As they have now travelled for 2 hours after their mid point break they will have a rough idea of the distance covered, the first 2 hour period was with the benefit of the plotter and so they should have ---

2a. An idea of the average speed and hence distance achieved - this can be used for the t/s/d calculation of the second two hour period

3. A comparison of the track achieved between departure and mid point and the course steered will give a rough indication of the compass error. This can be used to project forward from the mid point and the combination of distance and track with the VHFDF bearing will give some indication of position.

4. Now remove the VHF, calculate a course to the nearest large lump of land using the original deviation card, and steer this.

5. If no land seen after half the estimated remaining distance has been covered, reconnect VHF, check VHFDF bearing and, based on this, either continue or request assistance.

Tom