Hello!
I am an intermediate windsurfer who just bought the 129L Magic Ride ES (2023). I stepped down from a 151L Starboard Go. I was not wanting to go too low as I am still learning the manoeuvres and didn't go to the footstraps yet. The board came with a fin of 40 cm.

I want to use the board with a 5.0 m sail. I know it is below the 5.5 m size that is the minimum by design. Is it a real problem or it is just not gonna be optimal?
I cannot afford to have many boards. I have a second sail which is 6.0 m for lighter wind.

I would like to ask what fin size should I use? The one that came with the board (40 cm) as I understand can cover the 6.0 m sail?
What about the 5.0 m sail? Should I go for a smaller fin to not loose stability?
I was looking at the charts and they always show smaller boards and more narrow when going to 5.0m but if I am sailing in max 25knots is 129L really a problem? These charts are therefore not saying the fin size for my configuration.
It would actually be nice if the fin was fine for both 5.0 and 6.0 as I am guessing the 40 cm is also a lot for 6.0 and the one that came with the board is not G10 but softer and I have seen it easily gets sand abrasion in the spot I windsurf.

I would appreciate your help because I am a bit lost between sail size, board width and the matching fin size.

Cheers!

The problem with too small a sail on wide boards (freeride, slalom etc) is you need to get planing and hang your weight off the boom to get out in the straps without sinking a rail. That cannot happen without sail power to hang your weight against.
5m is fine if going fast and getting into the straps.


5m is also fine if you are learning and just doing 5kn everywhere (that is not you)

If you are not getting in the straps, then fin size is not yet relevant. Just use it with standard fin and when going so fast you have no control go to the straps. You go to the straps because you have to, not because it is the goal in your mind.
Then when you do that a lot and cannot hold the rail down when planing, you get a smaller fin.


EDIT - +1 for Imax

Thank you all for making me less worried about the fin. I was reading about the problems with too much lift and was worrying it can make it hard for me to learn planning consistently.I was saying I have not yet used the straps but I do use the harness for a long time and I do plane when the wind allows. I actually need to move back touching the straps to keep control but I was too afraid until now to get into them. I know it is just a mental barrier, the same I had with the harness I guess. Everyone is telling me it is safer to be in the straps

BTW I am ~85kg and I can see that with 5m i can go on a plane with around 20knots or a bit less. With the 6m a gust of around 16knots was enough.

Straps are there to make control easier. Get in them as soon as possible, even if you then fall off plane.
I've seen beginners get all weird about them and it sets them up for all kinds of problems.

Fin will be fine for any sail you want to use. You are a beginner and won't be out in extreme conditions. Try not to over think stuff. Get out there and enjoy the smaller board.