Course to steer for crossing the channel

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FairweatherDave

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As a theoretical exercise I thought I would do a passage plan across the channel from Chichester to Cherbourg. It is a long way from reality for me. So I have studied Andy du Port's book and want to make a nice S shaped ground track. I have a handheld GPS and paper charts. I also have Navionics on a tablet although the subscription side has lapsed. But I am trying to do it from first principles. I have been through the tidal atlas and got my hourly sets to East or West of the course. They have pretty much cancelled each other out (0.7nm East). I have a course to steer of 205 degrees magnetic. Leeway I can only factor in on the day and compass deviation I will ignore for now. So in theory if I maintain my course to steer and a constant speed of 5 knots and a 15 hour passage I'm there. My question is what use is there for the GPS other than for keeping an hourly log? Waypoints not required other than one of the compass rose to help with the paper chart position and one of the Cherbourg entrance (purely for interest not to navigate to until the end?) Do I draw an estimated course, ie an S shaped plot and watch how far off I deviate (and adjust course accordingly). Or do I stick with the plan and then do more serious calculations say 3 hours out from Cherbourg. I also guess this is where having a functioning log paddle will be very useful.

I know these are elementary questions if you have crossed the channel a few times but really I'm trying to see how to use my GPS and Navionics (without subscription) to best effect when they normally give you a straight line to follow....... Pottering about in the Solent I tend to find out if the tide is helpful or otherwise and on which side and that's all I need to know.....
(Yep, I'm leaving out fog, AIS, shipping lanes, lobster pots, ferries and unhelpful wind directions....)
 
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You are right: the tides cancel each other out so you can steer a course straight for Cherbourg. You will deviate from this in an S curve but you will be travelling the shortest route. You can see how far you deviate by looking at the GPS but do not alter because of it. Tom Cunliffe explains this in the Channel Pilot amongst other places. Also, you can take sightings as far as you can see into the passage and then see how accurate you are from the GPS. Then you can take sightings when you raise any of the landmarks on the other side.

When you do come to do it you usually want to make sure that you are heading up tide from the harbour so that you are not delayed entering (soul destroying) by bucking the tide. So on raising landmark that might be when you want to re-plot your course

Hope that helps
 
If you are really keen, try doing the passage and plotting the EP's each hour using the best available information and then put your GPS 'fix' on the chart as well. You can see whether you've wondered far 'off track or not'.

In real life I have a rough idea how far the 's' will take us off rhum line and I keep an eye on the cross track error on the GPS to make sure that it roughly equates with what I am expecting. A good rule of thumb is to review your course to steer once you've halved the distance and then when you've halved it again. By then you're three quarters the way there and you can think about making sure you are up tide and up wind of your destination. You only end up down wind and down tide once on such a passage if you've got any sense at all. Beating up wind against the tide after a long passage would make me change my port of destination very quickly...

edit: I see mattonsea types quicker than me!
 
I haven't done your exact route but I've done Brighton-Cherbourg and others numerous times, both with and without GPS. I think your problem is about how best to integrate traditional course planning with the use of GPS.

I would start by following the traditional route, ie draw your course line straight from start to finish. the work out the tidal offset for the day. I do this very roughly with so many miles to the east and so many to the west, but you should properly draw the vector for each hour. You then calculate your distance travelled in the calculated time and apply your offset to this distance (not from the destination), giving a new point to draw a line to from the start. This will show you where you would arrive at if you steered directly for the destination, and the angle between this new line and the original is the angle to be added of subtracted from the straight course line to give the course-to-steer. On a cross-channel trip the result will be quite small, just a few miles or a few degrees and give you a satisfying S-curve.

I would start off on this new course and plot my position every hour or half-hour from the GPS. You should have an idea of how far from the straight line (track) you are likely to be. This might be something like 6hours of 1.5 knots tide, or maybe ten miles if you start at slack water. After a few hours, you might wish to alter your course-to-steer to make up for any major deviation from the expected course-over-ground, but usually I leave this until later.

When a few hours from Cherbourg, I make a decision about what the wind, and especially the tide is going to do, and make a 'final' course correction, aiming for a point a mile or two up-tide from the harbour, knowing that I can run down-tide in the last hour and insuring myself against the reverse. The decision is taken from my then GPS position, and my course for the last few hours may well be a straight line, and if motoring by using the track facility on the autopilot.
 
Calculate the required course over the 15 hour passage you expect. This is most easily done by drawing the direct course, then putting in a short vector to allow for the net effect of the 15 hours of tide, given your starting time. This will give you the course to steer ON YOUR COMPASS after you have allowed for variation, deviation and leeway. Ignore your GPS.

Actually you'd be foolish to ignore your GPS, since "the best laid schemes aft gang agley". But you might try recalculating the required compass course every couple of hours on passage given your present position - hopefully it will remain constant, while your course over the ground describes a nice S-bend.
 
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Great. This is all reassuring. My theoretical navigation is rusty, not my sailing. But when faced with such a long passage (twice my longest) I want to get it right.....don't want my theoretical crew getting the ferry home:)
 
It will probably be a beat.
About halfway across, decide whether you're really going to St Vaast or Braye instead.
 
To be honest I also hate being below on a passage, particularly looking at charts as I'm soon feeling green. So the easier I make the process the better.
 
Old school crossing, assume tidal offset cancels, head off on a fixed heading. Plot position as you go with dr or off gps and then adjust as needed closer to your goal.
 
Neil_Y said:
Old school crossing, assume tidal offset cancels, head off on a fixed heading. Plot position as you go with dr or off gps and then adjust as needed closer to your goal.
Click to expand...

Used to do pretty much that but would still do the vectors from (sailed from Gosport) Bembridge Ledge making that my departure point for the passage. Everything up to there, pilotage.

EP over then in last twenty miles, do a new course to steer to arrive up tide.

Was nearly always 185 magnetic from Bembridge Ledge.
 
From a GPS fix around 20 miles out, a course to steer from there is easy. Planning at 5 knots is usual, but of course speed will be up and down and dodging around the ships adds unknowns.
 
Thanks capnsensible (and all the others). I need to think in "Old school terms" as relying solely on a tablet would be foolish.....it is in a waterproof case but no sprayhood and no mount and no powerlead except below.......... My GPS is infinitely more seaworthy, as are my two spares :)
 
The answer does depend on departure time, because the tide streams are stronger in places and don't turn simultaneously.
Also the speed you plan for has quite an influence, an extra hour of 3kt tide would be about 3 degrees?
Even if you're able to sail a steady course, I've had average speeds of anything from 4 to 7 knots. In the same boat.
 
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