Efficient Jibe with few cards?
Post by iconPost by lorenzo | 2019-10-29 | 21:22:28

Hi folks, can anyone suggest an efficient way to handle (without tons of cards) a jibe situation similar to the pic below?
When maybe you go in few hours from a TWA of 120 to 45 and then jibe.
Using Programmation or Waypoints? Or mix?


commenticon 11 Comments
Post by iconPost by toxcct | 2019-10-29 | 22:18:06
Stay awake... 0 cards :)

Post by iconPost by SlyStarLeRetour | 2019-11-04 | 03:36:34
excellent answer :)
Post by iconPost by YourMomSA | 2019-10-29 | 22:18:53
If that isn't an option, it gets expensive on cards... start with a straight compass course... then spend one to get your VMG TWA, then one to gybe to the new VMG TWA and then a third to go to a new straight line. Maybe more depending on what needs to happen next.

The cheaper option would be two straight lines (using one programmation or two waypoints), but then you have to accept sailing beyond the optimal VMG angles.

Post by iconPost by lorenzo | 2019-10-29 | 23:09:59
hey toxxct I've long since passed the point where I sacrifice hours of sleep for VR, for sure a couple of kids ago 

YM: exactly! I was looking for something like 2 or 3 straight lines, and yes ready to lose the optimum.
I was wondering if anyone did some maths and came out with something like “you use 3 straight lines cutting VMG at this angles and you lose x% in performance/time. If you use 4 lines at this angles you lose y%"





Post by iconPost by toxcct | 2019-10-30 | 06:05:05
My reply was ironical, but the only way to do it the cheapest way though (as even YM mentioned ;) )

I have kids too, and I also preserve my nights for sleep, so I use programming cards, and when I don't have any, I buy a 5 cards bundle for 150 credits. I seems fair enough...
Post by iconPost by bmertens | 2019-10-30 | 00:53:22
The notches on the TWA curve represent hours. So, you can line that up with the optimal course, count the notches and compare that to the time you are supposed to be making a maneuver at that point. At least then you will know how much time will be lost with your simple route.
Post by iconPost by lorenzo | 2019-10-30 | 10:09:46
toxcct I know you were, I did not want to sound harsh, all good

Setting down waypoints and see the time lost could be a way but I do not trust too much the engine that in VR predicts the arrival time

Probably the way is to play with the polars and some geometry. I will try to give a shot next occasion

Post by iconPost by YourMomSA | 2019-10-30 | 12:03:23
It depends on how fast the shift is occurring. If it's going through fast, you can use two straight lines because ideal won't require VMG TWA very long. If it's going through slowly, then the sub-optimal straight line will hurt more, especially upwind where you'll be pinching for a while.

I think if you're considering more than two straight lines, then at least one leg should be VMG TWA. In the image at the start of this thread, straight, then VMG, then gybe straight into a straight line. It can work... but then timing is critical.

Anyway... since the speed of the shift matters, I don't think there's any clean way to lay it out.

It also depends on the boat's polars (ie, how sensitive is the boat, in those conditions, to sailing beyond the optimal TWA). One trick that can help is to look at the red line in Zezo... If its dots line up ok with the isochrones, then sailing a straight line to the gybe won't hurt much. But if the red lines' dots fall behind, then sailing a straight line to the gybe (or tack, where it's usually worse) will hurt. That trick doesn't work as well exiting the maneuver though.

Post by iconPost by marcusbelgicus | 2019-11-01 | 15:38:05
Hi Lorenzo, with my PDD boat, I usually use the procedure described by Your Mom. So the following approach :
- Straight line until you reach the VMG, then TWA until gybing.
- TWA until you reach the angle of the second straight line.
That would be 3 progs. I tend to put the 1st prog a little bit earlier than achieving the VMG and the 3rd later just to make sure the boat never have an angle below the VMG, which usually will decrease the speed rapidly.
Post by iconPost by lorenzo | 2019-11-01 | 18:01:01


Optimum IMOCA
Optimum
TWA VMG
43 6.34 kts
142 7.89 kts

Here I would go with straight line as in pic until 120 TWA
then?
I mean, whatever you chose to do after the point TWA 120 (which is not every single TWA change) then guessing where you will be at which point is quite hard

Post by iconPost by mcmgj | 2019-11-04 | 13:56:50
Hi Lorenzo,
In first you can't use programmation and WP on your future route so you must choise between one of these options. the best (for me) is programmation (you can use them as direct line)
in second way, on your graph you cut a wind line, so, after this line your programmation can be false ...
on your graph, you can use only two programmation,
for the first, use red line to know the better CAP reach the 6th point, and give this CAP at your boat, then look CAP and TWA at this 6th point and next point to know the best CAP or curve to goal the jibe, put on your game your fist programming and adjust parameters.
for the second prog, put on your game your second programmation with the same parameters, advance this prog along the line to the jibe point (time), and verify that the cap (about 185 / 190) match with the graph.
If this is out of sailing, cut your route in two steps with a third programmation.
Enjoy
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