Edited to add:
Originally Posted by foolishsailor
See attached drawing. The dotted red lines are your laylines and your tacking angles. I am assuming a tacking angle of 90degrees.
If you calculated a CTS and short tacked the CTS line you would not be sailing an optimal course by a long shot as once you started entering the middle of the channel every port tack would be dead into the peak current.
A much faster route
would be to sail on starboard tack the whole way across the channel until you reached near your layline and then tack so that now you are in lesser current for the entirety of your port tack .
Scrap all the comments below. Incredibly stupid of me. Penny dropped, in fact it was more a cannon ball that dropped LOL.
CTS only applies for variable current over time, not variable current in different spots on the course. Of course you would head straight over to spend min time in adverse current . Red faced here.
Well, I would fail miserably as a tactician LOL. I am at the stage where my head
is spinning looking at this problem, but I think I would do the exact opposite to what you suggest and I know you are an experienced racer
. Doubt a sticky is ever going to be on that trophy if you follow my strategies given we are poles apart (I am at least getting the hang of the lingo, so there is some hope).
Let me preface this by saying it is the first time I have done any planning for a journey tacking with variable current.
My plans are sketchy without the profile of the current (eg is it 3.8 exactly in the centre and over a widish area, decreasing rapidly on the two sides as the depth
varied, or does each segment take up about 1/3 of the width?) or without knowing the expected speed close hauled with a 15 knot
wind wind in flatish water (flatish as wind is roughly with current), or without knowing if the wind is different near shore etc etc. But at least I can give you a general plan.
A few conclusions based on the data given:
is directly into the ground wind.
The current is roughly divided in thirds (no specific info given).
The strongest current is in the first 2/3 of the journey.
The average current is about 1/3 of the likely boat speed (8 knots??) and is 4O odd degrees clockwise of the ground wind.
The CTS is therefore displaced clockwise of the ground wind.
Longest on port tack (a bit less than 2/3 of the time?) following the shortest track through water.
Here is my rough strategy based on the info given:
You would want the port tack (the one lifted by the current) to be in the portion of the strongest current to gain the most lift
, so it needs to occur roughly for a bit less than the first 2/3 of the journey. So start on starboard tack briefly (better tactically anyway), tack to port and continue on for a bit less than 2/3 of the time. Tack back onto starboard tack for the rest of the trip. I know I am very hazy about the details, but that is the rough plan.
Why I don't like your strategy:
You nip over the channel on starboard tack in fact being knocked by the current the whole way so your sailing angle is worse and on top of this you end up a long way down stream and very likely will end up not sailing the last leg close hauled, as it would be EXTREMELY difficult to judge when to turn onto this last tack and lay the mark. You leave yourself no margin to take advantage of good shifts in ground wind. You may end up sailing a very long leg with a lot less wind if it is close to shore.
I really don't get sailing in conditions where you are knocked badly initially, to then have the least lift
from the current that whole long final leg, which by the way is dead into current. At least the current is lifting me for 2/3 of the trip.
How much faster is it actually doing what you suggest? I am still puzzling how your technique works.
Edited to add: See my post #353 for the rest of my comments.