A-Cat Polar

[Erwankerauzen]

Dear Anarchists
It seems that there is little information available regarding A-Cat polar.
I think it could be helpful to gather information on a dedicated thread.
So whatever is your experience with a floater or a foiler, do not hesitate to post.
There are 2 points which deserve special attention imho:
1-Windward on the edge of foiling
2-Downwind minimum TWS for foiling
Any contribution is welcome
Cheers & Merry Xmas

 

[can-UK]

PERFORMANCE COMPARISONS AND HANDICAPPING - MyWiki

scmultihull.org

very old and out of date.. but maybe some useful info here.

 

[Erwankerauzen]

Thank you can-UK,
It will take me time to chew these info, at first glance I don't understand very well, I ll have to do some homework.
Fair winds
EK

 

[revintage]

Hi EK,
Have you tried the polar question at Boatdesign? Even if it has become less good, my guess is that you could get more answers there.
Cheers
Lars

 

[Erwankerauzen]

Thks for the suggestion Lars,
On SA I was expecting more sailors than architects.
So more real world datas than datas from the spreadsheet
Cheers

 

[maxstaylock]

Try asking on the A Class Cat Tech group on Facebook?

 

[Lost in Translation]

From my sailing experience, 12 knots upwind or so is the boat speed pre foiling. Get to 13 and you can start foiling at 16 to 20 or so. Not very much breeze is needed for this. Maybe 12 knots is comfortable and useful for many sailors to reach high upwind foiling speeds.

For downwind, the same take off speed and downwind speeds are not that different, just a little faster.

The speeds are dependent on wind as a traditional polar would suggest but even more so on waves. Waves can highly limit downwind speed in particular. For this reason not that much wind is needed for very high speeds up and down.

 

[Erwankerauzen]

Thank you very much for the figures LIT,

So far, I used to rely on my foil lift calculations, and the boat speed I got as a minimum required to achieve foiling, was around 6.5 m/s.
Windward I used to assume the boat speed =1x True wind speed
and downwind I used to assume boat speed<=2x True wind speed.

While I do not foil with A-Cat, I guess the transition from windward floater mode to windward foiling mode brings significant change in apparent wind speed, requiring significant adjustments in tuning the main sail, that is probably an actual challenge?

Thks Maystaylock, but I am not a digital native and as a result I am not on Facebook.

A good understanding of apparent wind gradient & wind shear is imho, of paramount importance to understand how the A-Cat rig is actually working.

Happy New Year to all Anarchists

Erwan

 

[Lost in Translation]

The take off speed you have looks right. The sails have evolved to be flatter and the masts more durable. The rig tension is tighter. The mainsheet power is higher too now. It is amazing it all works as the apparent wind is so much higher and the force in the mainsheet much more. When you are foiling upwind it feels very windy! All of this is enabled by the differential rudders which show so much effect in the A and SailGP and now the Nacra 17 too.

Maybe 1.8x wind speed upwind and 2.5x wind speed downwind is a good assumption in the best conditions now? The downwind speed potential degrades faster as it is more sensitive to wave contouring.

Take a look at the videos on YouTube from the 2022 world championship to see some video above the mast for current sail shapes.

 

[Erwankerauzen]

Thanks for the ratios LIT, anyway, it is probably wise to consider a bracket instead of a single point, and with a spead-sheet, it is not much extra work, and your figures will be very useful. By the way I am very positively surprised by the 2.5 ratio, I would not have dare to go that far without your suggestion.
What is interesting computing the apparent wind gradient & shear, is that the wind shear (so the corresponding sail's twist) is decreasing when the boat speed ratio is increasing.
and the problem with the sail's leech is that the tension much hold less twist for the same lift (or for same righting moment).
To get it you must have much more tension on the leech.
It is basically the same equation as those used for an horizontal rope in tension where you suspend some weights concentrated or evenly spread, and check the angle (alpha) at the attachement points.
AFAIR the tension at the attachements is a factor of 1/sin(alpha)
Thanks again for your contributions
Cheers

 

[Lost in Translation]

Yes, for sure leech tension has increased. You can feel it in your hands and and see it in the purchase systems and increased ratchets. It's amazing the sails hold up but they are built more heavily as well with lower stretch materials.

One could argue I am pushing the wind speed multiples, but I think under the right conditions they are being achieved. It's much harder though when it is blowing 20 where the wind speed multiple curve goes down to 1X to 1.2X upwind and 1.5Xish at scary-best downwind though some have hit a little higher.