Roger: Weedfins and iSonic 115

[cmeisner]

Hi Roger

I know that you have extensive experience with different weed fin designs. Have you suggestions for what will work OK on and iSonic 115?

Currently have 2 Lessacher weed duo's in the sizes 28 and 32 cm's. Problem is that I think they are too small. Downwind the 32 is sort of OK, on half wind or upwind it fells very small even when well powered on a 6,0 MS2 free race sail (that's about 2o knots of wind).

Balance does not feel good - have to work a lot with the back leg and jibing has to be done carefully and in a wide arc when powered or else ends in not so elegant slides (grip just disappears).

The MS2 produce very little sideways pressure. Have noticed this on my formula 155 where a Curtis 62 cm carbon fin previously felt big (old sail) but now is absolutely perfect (less lift and perfect balance).

Any suggestion to get the iSonic to work with a weed fin? (Would prefer not to use a weed fin but unfortunately impossible where I most often sail)

Best regard,
Christopher

[Roger]

Hi Christopher,
I've had very good results with my Lessacher 32 cm Duo Weed on sails from 4.8 m2 Huckers to 8.5 m2 NX slm race sails.
Perhaps you could try a more vertical progressive rake fin like the Tekknosport Race weed. Should be available to you as Germany is nearby.
I've also had good results with the Tangent Dynamics Reapers (similar to the Tkknosport Race Weed).
I've used 32 cm weed fins on Isonic 101/115/122.
I'm trying to think if there is some other tuning factor at work here, but can't come up with any good suggestions.
Where are you running your mast foot (front...middle...back) with the 6.0 m2 MS2?
Running your booms a little lower might help.
Hope this helps,

[cmeisner]

Hi Roger

Thanks for the help. Maybe I should try another fin just to get some hints. I have been puzzled by this as so many have written positively about the lessacher 32 cm fin and because I find the iSonic very easy to trim and sail with the standard fins. I have tried (with my 7,5 m^2 MS2) to run the mast foot all the way back but that did not really cure my problems. My usual trim position is in the middle of the mast foot box which seems to balance things nicely with the standard fins.

I guess that with the weed fin the correct position is to run the mast further back to compensate for the further aft lift from the fin.

The only thing I can think is that with my weight (93 kgs) and with the very little sideways pull from my Maui sails (they really are well designed) I need to be more powered on the lessacher fin and/or need a little larger fin?

Best regards,
Christopher

[Roger]

Hi Christopher,
I think you have a pretty good analysis of this issue.
If you have some other sailors that could lend you a fin for test purposes, I'd try a 36 cm Lessacher Duo just to see if the 32 isn't working for you somehow, and then try some other designs, specifically weed fins that have progressive rake leading edge angles and are set further forward in the fin box (I.E. they have ovrehang past the front of the fin box cavity.
Make a good gasket for the overhang and this should bring the resistance of the fin forward enough for you to get the balance you want.
Wolfgang Lessacher was working on a Duo with overhang a few months ago, so you might want to check with him and see if he's completed that design.
Hope this helps,
P.S. I'm still a little perplexed as to why the Duo 32 cm would change the characeter of your board so much.
I can run almost 50% larger and 30% smaller sail sizes and have good results with the 32 cm I have.