Cross Channel Tides

[HappyHunter]

Does anyone have or know of a spreadsheet or other program for PC or Palm that simplifies working out the tides for a cross channel passage. It is a very time consuming thing to do manually, and I'm sure someone must have tries to automate the process.

 

[bedouin]

I think most of us reckon that it takes 12 hours to cross the channel, so the tides almost cancel out. Start out heading on the rhumb-line course and then adjust when you get a few hours off the far side!

 

[robp]

www.neptune-navigation.com

2 mouse clicks and it's all calculated and laid out for you. S/P £65. Using the "Optimimum Departure" function, it's surprising what time differences you can achieve. I'm pleased, it's a good planner.
(No connection)

 

[Robin]

In the strong tides between Solent/Poole and France the tides rarely cancel each other, even over 12 hours. We cross in around 9 hours and can do 8 and the offsets then can be as much as 10 mls, must be even more for a powerboat crossing in say 6 hrs.

However, crossing from Needles or Poole to Cherbourg at least the tides are more or less at 90 degs so a simple add/subtract east versus west gives an offset which is easily applied since 1 mile at 60 miles range is 1 deg +/- on the course. That way you do not risk being downtide or downwind or both and a significantly longer trip.

 

[bigmart]

I used to do the crossing regularly from Christchurch to Cherbourg/Alderney. Voyage time 12-14 Hours. A direct course worked right every time.

 

[bedouin]

You obviously sail a bigger boat than I! I do passage planning at 5kts which makes pretty well 12 hrs Needles or Bembridge to Cherbourg/Alderney.

With the strength of tides off Cherbourg in particular, a difference of an hour over the predicted passage time can make a significant difference to the course. This is a particular problem since if your passage is slower than planned you will inevitably end up down-tide of the destination!

My technique is to set off on the rhumb line and then do a more accurate calculation when about 15 miles from the French coast, adjusting to ensure I keep slightly up-tide of the destination. That should not require a change of more than 5 degrees. Doing the sums shows that even if it requires a 10 degree alteration it only adds 400 yds to the journey - not enough to bother me!

 

[iangrant]

I'm with Bedouin on this one - 12 hour crossing 6 one way 6 the other - works for me - ended up heading about 4 miles West of where we wanted to be on the way back last time but we corrected by a few degrees and so it made little difference.

 

[bigmart]

Of course it's a standard navigational tecnique.

Set your course to arrive a few miles uptide and correct as necessary. At least you know which way to turn when you get there.

In this age of electronics we seem too obsessed with arriving within 4Ft, sorry 1.22 Metres, and 10 milliseconds of the allotted time.

The attraction of sailing for me is to escape from timetables & regulation.

Mind you I also like playing with the latest toys.

 

[jimi]

Also agree with Bedouin, I aim straight there and after about 6 hours work out based on distance covered what tidal offset to use to put me about 1 mile up tide & then recalculate after 2 hours to put me 0.5 mile uptide & then keep an eye on it.
Very rarely do I have to amend course

 

[LadyInBed]

I'm with you - point & go.
The only other calculation is to get to the Casquets at HW Dover, to get the run of the current down to St Peter Port.

 

[Chris_Stannard]

I use Reeves Foulkes, or is it Foulkes Reeves, tide tables. I never can remember.

I once did the exercise of adding up all the tidal fglows for a single point which showed an overall movement across twelve hours of about 3 miles at half springs,

If you add to this the observed fact that as you approach Cherbourg the tides get stronger the chance of them cancelling becomes less and less. However at six knots if you leave the Needles on the first hour of the ebb that does appear to be the case. In other cases 10 miles can be the error, for example leaving the Needles on the last of the ebb. (This is all from memory so no doubt someone will say I am wrong but I did my plan for Friday twice today, first for a 6.30 start and second after SWMBO told me she was going to get her hair done before we left. Current ETD1400.)

If you are aiming for Alderney missing by 10 miles, or indeed being down tide at all can be very embarrassing.

Agree with the system of only bother about the East West. I work out the inital correction for right across on that basis, add a mile for tide at the finish (Never arrive downtide of the mark) and work out the course. Takes about 20 minutes. I make a small table of this in my planning book. Incidentally on the tidal atlas I have a visiting card with 6 knots advance on one side and 5 knots on the other. It makes working out the tides quick and simple.

Having worked out the Course to steer I use that to calculate EPs at 2 hour intervals, which I put into my GPS so I can keep track of what is going on and make corrections if necessary, usually about two hours out. It means that in the middle I can put my feet up for a couple of hours whilst SWMBO remains convinces that we are not going to Venezuala.

I prefer to do it this way as I then know what is going on.




Chris Stannard

 

[Twister_Ken]

Some very cruisy answers below, but if you end up with a two-sided beat, lee-bowing and a wind shift predicted, you need to navigate a bit more proactively than point-and-go. Which means reassessing the situation increasingly frequently as you near the other side. Of course, GPS makes this relatively easy. One thing GPS is really good for is spotting the actual time the tide changes rather than the predicted time from the tide tables and stream atlas. So if you're leebowing you barely lose a yard.

If you need the intellectual stimulation, try running two plots, one a GPS record, the other trad DR. It helps keep techniques up to speed.

BTW, sorry I've no idea about tidal programs.

<P ID="edit"><FONT SIZE=-1>Edited by Twister_Ken on Wed May 29 10:12:58 2002 (server time).</FONT></P>

 

[Robin]

We passage plan at 6.5kts, sometimes 7kts and can average 7.5 so make the same minor adjustments as required from say 10 mls off.

 

[Robin]

Leaving from Christchurch you have limited times available for departure, ie around high waters, from Poole we can leave at any time of the tide and there are therefore more variations, especially since we cross in 8.5 hrs +/-. There would be occasions when we too could cross on the direct course, but it is not the norm.