Babylon
Well-Known Member
Can anyone remind me how to calculate a CTS which pre-accounts for the mid-passage alteration to cross a TSS at 90 degrees before altering back again?
Pictures will help.
Pictures will help.
CTS allowing for TSS?
- Thread starter Babylon
- Start date
marklucas
Well-Known Member
Well this is how I would do it
Work backwards from your destination.
Work out your optimum heading to arrive at the destination from the destination side boundary of the TSS. This will give you a CTS and therefore a target exit point on the destination side boundary of the TSS.
Your CTS in the TSS is defined as 90 degrees to the TSS, so work out where you need to enter to exit at this target.
And now you just have one more CTS to calculate from your origin to the entry point.
A diagram would make this clearer, but hopefully you can make some sense of this.
Work backwards from your destination.
Work out your optimum heading to arrive at the destination from the destination side boundary of the TSS. This will give you a CTS and therefore a target exit point on the destination side boundary of the TSS.
Your CTS in the TSS is defined as 90 degrees to the TSS, so work out where you need to enter to exit at this target.
And now you just have one more CTS to calculate from your origin to the entry point.
A diagram would make this clearer, but hopefully you can make some sense of this.
snowleopard
Well-Known Member
Sound advice from Mark. All I would add is that it is best to go right round the TSS if you can. Obviously there are cases where that would make the trip too long. Bear in mind that some national maritime authorities take a dim view of yachts going into a TSS at all for example I have heard of one being fined for sailing within a mile of the end of the TSS in German waters.
BlueSkyNick
Well-Known Member
Not always - there is little benefit in sailing across the end of the TSS compared to sailing through it - it's all the same ships going in or coming out.
Babylon
Well-Known Member
Thanks, I can certainly picture it, but it seems to me that your 'optimium heading' from the destination side boundary of the TSS involves a degree of guesswork - and this would become more guessy the more oblique the angle of the main passage is to the flow of the TSS (if that makes sense).
Wouldn't it be easier to notionally shift the TSS right up to the departure point (keeping it parallel to and as wide as the actual TSS)? You'd then plot your 'initial' heading at 90 degrees to this shifted TSS, thereafter you've got just a single line (with no guesswork) from the exit point to the destination, to which you can then add all the tidal vectors to get the resulting single CTS to steer at all times before and after the actual TSS. Does this make sense?
marklucas
Well-Known Member
This could work
but assuming an hour or more to cross the TSS then conditions will always be different either side so you may have a discontinuity in tide etc.
There is no "proper" or "RYA approved" method - all that is required is a sensible and methodical approach.
And the tide and wind will always be different to any forecast used in a passage plan, but at least you have some initial courses to steer which you can then amend as your fixes are made.
As has been alluded to, many merchant vessels treat the whole of The Channel as a TSS when it isn't - and calling them up on VHF rarely raises a response unless you use DSC to call their MMSI.
Good luck.
but assuming an hour or more to cross the TSS then conditions will always be different either side so you may have a discontinuity in tide etc.
There is no "proper" or "RYA approved" method - all that is required is a sensible and methodical approach.
And the tide and wind will always be different to any forecast used in a passage plan, but at least you have some initial courses to steer which you can then amend as your fixes are made.
As has been alluded to, many merchant vessels treat the whole of The Channel as a TSS when it isn't - and calling them up on VHF rarely raises a response unless you use DSC to call their MMSI.
Good luck.
BlueSkyNick
Well-Known Member
Don't count on it, I had one ship cancel the DSC call twice as well as ignore calls on Ch16.
john_morris_uk
Well-Known Member
With respect, why would you want to call up the ship anyway?
snowleopard
Well-Known Member
To find out his intentions. For a start his ARPA is a lot more accurate than anything you are using. Has he noticed you? How is he planning to avoid you - or not? It saves you screwing up his avoidance plans by making unnecessary alterations of course.
snowleopard
Well-Known Member
But the rules are different. 'Thou shalt not impede' doesn't apply.
And as I said, in some jurisdictions you aren't allowed to enter a TSS.
john_morris_uk
Well-Known Member
I suppose if you have AIS you can positively identify the ship and make sure you are calling the right one.
As a general principle I seem to remember that calling ships on VHF as part of collision avoidance used to be positively discouraged (although it didn't stop a few people trying...) and was mentioned in several MAIB investigations following collision as a contributing factor in the accident.
Perhaps its just me being old fashioned and taking bearings while I carry on saving for a plotter, AIS and transponder.
Out of interest I sailed across the channel twice in the last week and every ship that was on a collision course altered for us. (I was plotting bearings visually and off the radar and could see what they were doing.) I wasn't so impressed with the one that passed 0.65 miles ahead of us - but there was lots of other shipping about and I think he was a bit constrained by how much he could alter. It was night time and I would have altered myself if his bearing hadn't been changing so rapidly...
Ex-SolentBoy
Well-Known Member
For a short TSS crossing it is simpler to ignore it for calculation purposes. Just assume your desired heading until you hit it, point the boat across it until you are out of it, and then recalculate.
For a longer crossing I do this differently.
1. Draw a line from departure point to destination.
2. Find the midpoint of the section of line in the TSS.
3. Draw a line at right angles to the TSS.
4. Make a rough estimate of the tidal flow when you cross the TSS
5. From the TSS midpoint, using the required heading make two calculations to determine your entry and exit points from the TSS.
6. Then treat the pre and post TSS sectors as separate plans.
For a longer crossing I do this differently.
1. Draw a line from departure point to destination.
2. Find the midpoint of the section of line in the TSS.
3. Draw a line at right angles to the TSS.
4. Make a rough estimate of the tidal flow when you cross the TSS
5. From the TSS midpoint, using the required heading make two calculations to determine your entry and exit points from the TSS.
6. Then treat the pre and post TSS sectors as separate plans.
johnalison
Well-Known Member
All this makes a good exercise in chartwork, but as sure as anything, by the time you've got to the other side of the TSS you won't be exactly where you expected and will have to re-calculate your last leg.
I usually make a rough guess at the right course to start on, bearing in mind that I may wish to "save" a mile or two against a possible wind shift or a strong tide at my destination. In these days of GPS it is not as necessary to be as accurate as in the past, and a straight-line route may not lead to the quickest or most comfortable journey.
I usually make a rough guess at the right course to start on, bearing in mind that I may wish to "save" a mile or two against a possible wind shift or a strong tide at my destination. In these days of GPS it is not as necessary to be as accurate as in the past, and a straight-line route may not lead to the quickest or most comfortable journey.
snowleopard
Well-Known Member
I can't remember when I last plotted a CTS. I just select the waypoint, press GOTO on the GPS and Track on the autopilot and let them sort it out between them.
OK so I might get there sooner if I played the tide a bit but I enjoy sailing so doing a few more minutes of it isn't a hardship.
OK so I might get there sooner if I played the tide a bit but I enjoy sailing so doing a few more minutes of it isn't a hardship.
lustyd
Well-Known Member
Cunningly they have put the TSS in the channel parallel to the channel itself. This means that even with a tidal flow of 2kt (don't have my tidal atlas on me right now but this would probably be high for the channel?) if it takes an hour to cross you'll only be 2 miles out when you get where you're going. This would be taken care of by pilotage anyway in good vism and in poor vis you were aiming a couple of miles to the side of the destination anyway so no big deal.
I would work out which way you'll drift when you get to the TSS - if that helps you to go to the side of the port you were aiming for then great, ignore the TSS and aim straight for the port in your calculations safe in the knowlege that you'll be the correct side. If it moves you to the "wrong" side of the port then you'll need to recalculate and aim further to the "right" side of the port.
I would work out which way you'll drift when you get to the TSS - if that helps you to go to the side of the port you were aiming for then great, ignore the TSS and aim straight for the port in your calculations safe in the knowlege that you'll be the correct side. If it moves you to the "wrong" side of the port then you'll need to recalculate and aim further to the "right" side of the port.
