Crossing a Traffic Separation Scheme

[simonpickard]

Hello all,

Quick question, and I'm sure I'm missing something silly here but in the RYA training guides it says to cross a TSS at a right angle heading, even if the course over ground, due to say tide, means you're "sliding" through the water thereby taking longer.

Why wouldn't you adjust for any current, tide, etc, so your course over ground is as close to a right angle to the TSS as possible?

Regards,
Simon

[Oceanride007]

I believe the intent is to get you across quickly. Nobody will nit pik with you what influence the tide/current has.

[simonpickard]

That's my issue with the RYA books though. I would have thought the best way across would be the quickest. At a right angle from A to B. Not keeping the heading at 90 degrees and ignoring wind / tide, currents, etc.

Can't work that bit out.

[carstenb]

The reason for this is that if you compensate for the current in places where there is a lot (say 4-5 knots crosscurrent) you will "appear" to be sailing against the traffic in one of the sides of the separation.

sailing against the traffic is a big "no-no" and will get you fined big time.

[Oceanride007]

I would think quickest is good. I'm a ships engineer many a entertaining time I've had in busy sealanes just observing. There is nobody watching how neat your track is but what would get ships officers upset is your dawdling with a less than adequate transit. In commercial ships compliance is regulated because a licence could be jeopardized. Maybe not yours but you have been told by RYA or whoever it is, what you are required to do, as they have. Do your best particularly when roadstead is occupied.

[goboatingnow]

The reason is to present a correct aspect to the ships in the lane. Ie to clearly demonstrate you are crossing. Of you tried to do a right angle course on areas of strong tide , you could have a very different heading and could look to approaching ships that you were joining the lane not crossing it

Dave

[goboatingnow]

Originally Posted by Oceanride007

I would think quickest is good. I'm a ships engineer many a entertaining time I've had in busy sealanes just observing. There is nobody watching how neat your track is but what would get ships officers upset is your dawdling with a less than adequate transit. In commercial ships compliance is regulated because a licence could be jeopardized. Maybe not yours but you have been told by RYA or whoever it is, what you are required to do, as they have. Do your best particularly when roadstead is occupied.

There is no specific requirement to cross in any specific time. You can take all the time you need.

Dave

[simonpickard]

Originally Posted by goboatingnow

The reason is to present a correct aspect to the ships in the lane. Ie to clearly demonstrate you are crossing. Of you tried to do a right angle course on areas of strong tide , you could have a very different heading and could look to approaching ships that you were joining the lane not crossing it

Dave

That makes a lot of sense actually, thanks for the replies everyone.

[Asheim]

Simon, in addition to the above mentioned reasons, the perpendicular heading is the one that will get you across quicker.

Asheim

[Dockhead]

Originally Posted by Asheim

Simon, in addition to the above mentioned reasons, the perpendicular heading is the one that will get you across quicker.

Asheim

Actually not "in addition", but this is the right answer.

Crabbing along the rhumb line might be the fastest way to your destination, if the TSS is not so wide that the tide changes during your passage of it, but it will be the slower way across the TSS, leaving you in the TSS obstructing traffic for longer.

For example -- if the TSS is 10 miles across, and you are sailing at 6 knots across a perpendicular current of 4 knots, a perpendicular passage across along the rhumb line means adjusting your heading by 34 degrees to compensate for the current, which means sailing 12 miles through the water, taking you two hours to get out of the TSS. If you sail a heading perpendicular to the TSS, you will sail only 10 miles through the water, which will take you 1 hour 40 minutes, reducing your time in the TSS by 20 minutes.

Showing the correct aspect, and not appearing to be sailing against traffic, are certainly significant plusses, but speed across is the main reason. A 34 degree deviation from a perpendicular heading does not have a huge effect on your aspect, and will not make you appear to be sailing against traffic, but it will have a significant effect on your time across.

[Dockhead]

Originally Posted by simonpickard

That's my issue with the RYA books though. I would have thought the best way across would be the quickest. At a right angle from A to B. Not keeping the heading at 90 degrees and ignoring wind / tide, currents, etc.

Can't work that bit out.

Please refer to the calculation above.

Sailing the rhumb line might well be the fastest way to your destination (if the current is constant), but it will be the slower way to get out of the TSS. The fastest possible way across the TSS is on a constant heading exactly perpendicular to the TSS, and at your best speed.

[Tellus]

On a running oder tidal water I would tend to make a dogleged curve, calculating a rate action to get from A-B

[Dockhead]

Originally Posted by Tellus

On a running oder tidal water I would tend to make a dogleged curve, calculating a rate action to get from A-B

I would be interested to see how you calculate that. Never heard of such a technique.

[Dockhead]

Here's a useful diagram:

Traffic Separation Schemes - How to cross them safely

[carstenb]

Originally Posted by Dockhead

Here's a useful diagram:

Traffic Separation Schemes - How to cross them safely

I believe we have beaten this animal to death in the CTS thread. Seaworthy Lass won that round by developing anew method to cross currents effectively.

So we actually do have a mathematical answer to this.