Brain hurts - Working out course to steer

[Jim Cate]

How far is that trip?


It must be added that on a long enough passage, which covers many tides, CTS becomes easier because the currents are averaged out.

rough distance is 850 miles. The only places where there are tidal currents of consequence are exiting Southport (or Moreton Bay) and entering the pass in New Caledonia. These tidally affected bits are short, between 5 and perhaps 20 miles total, and are normally traversed in a fraction of one tidal cycle... and most of that is ahead or astern, not transverse.

I'm not sure why Alan brought that particular passage up, for it does not really relate to your discussion of CTS calculations.

Jim

[Alan Mighty]

Originally Posted by Jim Cate

I'm not sure why Alan brought that particular passage up, for it does not really relate to your discussion of CTS calculations.

Crossing the East Aus Current from W to E (or E to W) is a case where CTS can be important (more for crossing from E to W if you want to end up in good position for, say, entering Moreton Bay by the North-East Channel and avoiding Flinders Reef).


The EAC is in some seasons and in some places negated by the tidal stream. Just happens not to be at currently at the location of that particular wave rider buoy (which is the closest such buoy if you were wanting to enter the NE Channel into northern Moreton Bay from outside - I'll add a chartlet clip to show you the problem).

Think of the EAC as a river flowing through the Pac Ocean offshore of the E coast of Aus (it can get v close at times, especially near some points that stick E, making at times N-wards voyages a pain).

The width of the EAC changes from time to time. And the biggest clue to its direction (if you know the current drift of the EAC and perhaps the state of the tidal stream too) and exact location is to look at the wave profile. Let's say the EAC is setting to the S, the tide is falling (i.e. tidal stream set to N), and a SE wind is blowing (and an SE swell). Then the wind wave in the EAC will tend to be sharply peaked in contrast to the wind wave mixed with the swell outside the EAC.

[cyan]

Fascinating topic. It’s surprising to see many experienced sailors think that the shortest distance between two points is the rhumb line, in the simple case of zero current/wind. Not true. (The earth is not flat.) This false assumption is close enough in lower latitudes, though... so carry on...

[requiem]

Originally Posted by SeanPatrick

Well, I just tried working an example using this solution (which I probably should have done before posting it) and realized that it actually doesn't quite work that way.

The only error was connecting the final current vector to the destination point; the sum of the current vectors is for a particularly time period and you must make the length of your final course to steer vector reflect the same duration.

If you assumed each vector represents an hour's worth of current, and are summing three of them, then the length of the final vector must be drawn to the distance traveled over those same three hours at your assumed speed. It won't intersect the destination unless by pure chance.

(This is also not quite exact, as discussed in that other monster thread, but is close enough for most.)

[Dockhead]

Originally Posted by cyan

Fascinating topic. It’s surprising to see many experienced sailors think that the shortest distance between two points is the rhumb line, in the simple case of zero current/wind. Not true. (The earth is not flat.) This false assumption is close enough in lower latitudes, though... so carry on...

So you're advocating sailing a great circle route across, say, the English Channel?


Here's the difference:

[parkstone bay]

A Great Circle South from Needles Fairway Buoy takes you, as near as dammit, right on the Eastern edge of Passe Ouest into Cherbourg. Now, all we want is a day without any tide in the Channel for 12 hours! This navigation lark is a doddle.

[Dockhead]

Originally Posted by parkstone bay

A Great Circle South from Needles Fairway Buoy takes you, as near as dammit, right on the Eastern edge of Passe Ouest into Cherbourg. Now, all we want is a day without any tide in the Channel for 12 hours! This navigation lark is a doddle.

A great circle course, 60 miles from anywhere to anywhere, takes you right over the rhumb line, plus or minus a few meters.

[Uricanejack]

If you are going due South.
It’s a great circle.

[Dockhead]

Originally Posted by Uricanejack

If you are going due South.
It’s a great circle.

Got it in one!

[Stu Jackson]

Originally Posted by RayRodriguez

Crab angle is the term used in aviation. I am sorry that I applied that to boating.

I will ask no further questions.

Ray, I sincerely apologize, no harm intended. I am not an aviator, although many sailors here are. I did not know. But the terms are different in sailing, and I hoper you can gain access to the texts I suggested to learn therm. If I was learning to fly, I'd betcha I'd learn how to crab.

Bearing, Heading, same thing in merica.

That's why I mentioned it. It's just plain wrong.

[john61ct]

Do your canonical sources use this terminology in the same way as explained here?

https://aviation.stackexchange.com/q...s-tr/8947#8947

Obviously AmerEng vs British usage differences would not be productive.

[Dockhead]

Originally Posted by john61ct

Do your canonical sources use this terminology in the same way as explained here?

https://aviation.stackexchange.com/q...s-tr/8947#8947

Obviously AmerEng vs British usage differences would not be productive.

I think that's a very good explanation, and the terms are used precisely the way we do, including the "or 5 degrees crab into the wind".


I maintain that "crabbing", although it comes to us from aviation, is perfectly good maritime terminology. Derived from the sideways locomotion of crabs, it means getting to someplace otherwise than by heading straight for it. You head into the tide and crab across the Hamble River, for example -- commonly spoken around these parts.

[sailboatscot]

Originally Posted by boatman61

It used to be essential for Coastal Skipper and the old much higher qualified International Helmsman Certificate back in the 70's/80's.. as opposed to the vastly dumbed down ICC of today.. but hey.. it boosts RYA revenue for what is basically the old Competent Crew of yesteryear.

I just looked out my passage plan for my Yachtmaster (2017) I was set the task of getting from Gosport to St. Helier in order to arrive at such a time to get over the sill without undue delay. I worked backwards from St. Helier estimating EP based on an overall favorable set of tides all the way back to the French side of the TSS. Then working forward I did 2 CTS to get me to the appropriate point on the English side of the TSS (where one is somewhat mandated how to cross anyway) to arrive at the right time to pick up the favorable tide on the far side.

More than likely not the shortest route but that wasn’t the task set, the critical part of the task was arrival time.

[cyan]

Originally Posted by Dockhead

So you're advocating sailing a great circle route across, say, the English Channel?

Well, no. I'm advocating a bit more study for people who write things like "the shortest distance between two points is the rhumb line". Those of us who cross oceans would be more concerned than those who cross channels.
That's cool, though. I think sailing around in a spiral path is clever. It's just not the "shortest".

[wingssail]

Originally Posted by Andy Todd

If I work out CTS on a paper chart over multiple tides and hours.... its an easy vector diagram exercise. But say I do all my nav on a tab, and I can't draw a vector diagram, how do I work out average drift over the whole passage. Does anyone have a spread sheet to do it or is there a 'trick'?


It's been a long time since O level math for me!!

Apparently few of the responders to your question bothered to read or understand it. I get that you know how to do the vectors on a paper chart, now how do you do it (figure out CTS before departing on your passage) when you don't have a paper chart?. OK.

Answer: You don't need to try to figure it out precisely. Take a look at the route and look for the big tidal effects or other effects, (like leeway). Make a rough guess at the net effect of them and correct your departure heading to compensate for that.

Why don't you need to be so precise? Well, I assume you are sailing. The variations of wind speed and direction, and the variations of current set and drift from predicted, will make your route into a series of adjustments. So make a best guess at the net CTS and go for it, then monitor things as you go along (and don't try to stay on the rhumb line in an ebb tide if a few hours later you'll have a flood tide, etc.)

Now, if you have a great charting system (not a tablet) you can and SHOULD make a predicted route, from start to finish, on your electronic chart, day by day, (or for every six hours, or whatever), and for each leg of the journey, taking into account known factors like tide and great circle. Then you try to follow it (Doing this helps you from running into rocks and islands too).

Finally, in actuality, your CTS is going to be a hope, nothing more. You come out of your departure harbor, put up the sails, then see what direction you can go. Or you become a motor boat and sit in your captain's chair behind the wheel and the chart plotter and motor along your CTS.