Calculating a route across multiple tides

[jimbaerselman]

Studying my copy of the Admiralty Tidal Atlas for the Channel, I find that the numerical data is quite sparsely given, with lots of bare arrows - which means a fair bit of interpolation or guesswork where it comes to deciding the actual tidal rate at any given position. (And unless its bang on springs or neaps, that means further interpolating a load of interpolations!)

So I've ordered a copy of the full Reeves-Faulks handbook combined with all the atlasses (less than a tenner new through Amazon marketplace), which I gather presents the data more 'flowingly' and in a fashion that's easier to pluck at any given position.

Yes. But it's based n the same sparse data. Just someone else's interpolation.

And on top of that, there will be a small wind induced current which is completely unpredictable, but could reach 0.5kts. And where there are changes in depth there are un-recorded eddys.

The whole job of predicting tidal outcomes is surrounded by levels of uncertainty, so you have to plan for that. when plotting forward, you must expect the probability that the tides you experience en-route my be 0.5kt out from time to time. Hence, plot an arrival point up-tide by this possible error, and correct things as you travel.

 

[Wunja]

Thanks for all your replies. I think I now have a better understanding of the process. I will be saving up for Reeve Foulks (however it is spelt! )

Part of the reason for the question was that I was experimenting with a program to calculate routes and generate the CTS. The GE screen shot above was from the first itteration of that based on constant course and speed. The latest I can put in the destination to let it generate CTS, but it comes up with a 4 degree error (curses!) Still a lot more functionality to add.

A note on Mr Completely's reply - I think you missed the point of my question which was that it is more efficient to ignore a constant COG, but to allow for tidal cross track error to make the best distance made good. Please correct me if I'm wrong.

 

[Babylon]

How far uptide to aim for?

The whole job of predicting tidal outcomes is surrounded by levels of uncertainty, so you have to plan for that. when plotting forward, you must expect the probability that the tides you experience en-route my be 0.5kt out from time to time. Hence, plot an arrival point up-tide by this possible error, and correct things as you travel.

That was my other question: just how far uptide to aim for?

If I understand you correctly, on say a 65 NM crossing from the Needles to Cherbourg averaging 4.5kt (small boat), the passage should take about 14.5 hrs, call it 15 hrs. So plot a CTS to aim to arrive 7.5 NM (0.5kt x 15hrs) uptide.

And later, from say a fix at 13 NM from destination (about 3 hrs at same boatspeed), plot a new CTS to arrive 1.5 NM (0.5 x 3) uptide.

Good rule of thumb.

 

[Skysail]

A forumite has designed a program for X Channel course to steer, see here:

http://www.compasscard.co.uk/tp2home.html

 

[Mr.Completely]

My apologies, I think got the wrong end of the stick there.