twist cut fins - do it yourself

[holden]

hi folks, i had a great idea recently i'd like to share with you. (viewtopic.php?f=159&t=2363952&start=0). i heard in the meantime that the idea is not new but anyways. i was not able to find the according thread so please someone post the link. i always hated the turbulances leeward finns tend to cause close to the board. you can feel how this holds back the board. to angle the fins towards the tip of the board for a few degrees is no solution in my opinion since it trades the turbulances close to the surface with a bad angle of attack at the rest (90% ?) of the fin. e.g. if the windward fin's aoa is, lets say 10 degree the leeward fin's aoa is 16 degree if you angle each fin for just 3 degrees.

i participated in the kte race in austria with my modified twoag raceboard last week. ok, the results where not ..., you know ..., but i was able to see the potential. almost no turbulances and fins parallel. the way to make them is rather simple. i cut the fins with a jiggsaw and milled the front outside and the back inside to sharpen the edges again. take a good look at the picture. this way the aoa close to the board is pointing towards the tip of the board a bit.

what do you think? as for myself, i need to get more time on the water with my racebaord to get better. right now it is definately me who needs to get into better shape - tacks and downwind practice.

h.

[BWD]

viewtopic.php?f=1&t=2360033&hilit=twisted+fins
is the thread on twisting plastic fins to have different AOA or toe in from heel to tip.
Your idea is cool too, an option for bigger, glass fins....

Another option to change toe, I thought of for surf type fins,
if you use futures or similar strong-based fins, reshape the base to change the toe-in...

[holden]

ahh, thanks for the link! i was just looking in the kiterace forum. of course all directionals have the same issue. i wonder when kiterace fin producers will offer the first twist fins. whatever, i have to concentrate on my tack and downwind training.

[BWD]

One thing to consider is they may not need to alter the fin towrds the tip much at all,
with the length and leverage of the race fins, they probably flex on their own in use, like windsurf fins do... pre-twisting them too much could end up making more problems if overdone....
What seems smart about your approach is that it reshapes the foil in the area near the root where the fin is too thick to flex and may need a different alignment for efficiency....

[holden]

instead of tack training i went to the workshop again

this time the board is an old modified ahd kite directional. three fins. 31cm each. twist cut helped a lot. i tried the fins before cutting, milling and sanding and after. check out the polar diagram of the recorded track.

h.

[pouch]

less than 50° is a great performance !
I saw that you have also put the front fins more away from the rail
did you feel bette with this setting, what distance did you choose ?


cheers

[holden]

thanks pouch!

i did not test the former front fin positions in "race mode (long fins and so on)" so i can't really tell the difference. the original fin boxes where angled a few degrees, too. the space between the rails and the front fins is around 6cm.

i added a third back footstrap just in front of the two existing. this will help to get better control downwind. i also moved the front straps forward by 5cm.

when everything is in place and i can go 20 knots downwind and 10 knots vmg upwind i will try to figure out a freeride setup. a setup with which a race directional first timer can have fun right away. and this setup is for me to have fun in any condition, even high winds and high waves.

and then i will try out the upwind booster on the freeride setup
hehehehe.

h.

[pouch]

You have 2 options for going upwind :
Either you can go fast but have an average upwind angle (60°)
Or going slow (10 to 12 knots is an optimum) and reaching good upwind angle (50°)

I am sure you can go faster downwind, but I perfectly know that this is quite difficult and painful for back legs. A target can be 25 knots.

But I am sure you knew it

cheers

[holden]

it is like baseball. eighter hit the ball well or run quickly.

25 knots would be great. i didn't have too much strain on my back leg. and it is definately not the issue of the board producing too much friction for being fast. it is just that the board is not balanced good enough for hardcore downwind, yet. once, when i went rather fast downwind it hit a wave and jumped sideways pretty violently. i flew off and while i was flying i noticed the board also flew next to me turning like a bullet - therefore very stable in its flight. it was scary. when i stepped with my back foot just in front of the back straps i had the feeling that the balance was better. this is why i mounted another strap in that position. an i will add "real" footpads to the frontstraps to get better connected to the board.

h.

[kitezilla]

I hate to be a "Debbie Downer", but do you give any thought to the potential danger of this type of "racing fin" to the people involved in the race?

I mean SHARP edges and foot and hand (and line) snagging NOTCHES everywhere.

It would be one thing if these fins were to be used by one person at a time on a race course...but its another thing to have 30 of these things whizzing around buoys in close quarters, at high speeds.