Convinced that there is no reason that 90 degrees should be the optimum fin angle for starboard speed runs I built my first prototype. Waiting for wind.
It's canted 18 degrees. Under conditions where you may need 20kg of lateral lift, the fin will have to work a little harder and produce 21 kg of total lift. But there will be an associated 6 kg of vertical lift. That should be a win. 6 kg less work for the hull, and only 1 kg extra lift required of the fin. (Hard to believe, but check the sin and cos of 18 deg) And the fin is more efficient at producing lift - a double win.
So much for theory I'll let you know how it goes, I'm hoping it will feel pretty good on starboard and very sluggish on port. If I can't tell the difference .. I'll let you know anyway.
No one variable at a time, it is still upright and still aligned down the centre of the board. It was originally a Tekknosport 35 trim box. Used to go well in a Tiga 125.
Interesting .
Fin lift is more than 2x more efficient than board lift. So should be less draggy but I can see the tail jumping out if you go too fast and get too much lift.
for 40+ knts
for 40+ knts
I remember years ago when Boogie was just getting started he made a really bendy fin to get much the same effect. I think he almost broke his ankles as the board tried to roll as it was getting so much vertical lift but only on the windward side of the board. Your tilted fin may have the same problem.
i just recently had an exchange with Serge in Holland about this.
they have made and sailed various prototypes along these lines.
even some that work on both tacks by being able to flip from side to side.
i'll send him a link to this thread.
i clearly remember my experience with that super soft fin Yoyo mentions, but i would say it is a slightly different issue as the lift is changing dynamically depending on the load and so with the flex a lot more than with a fixed angled fin which is relatively stiff.
but that changes nothing about the fundamental challenges this configuration faces to work properly. i mean what are you trying to achieve? go faster, right?
one of the main aspects to think about is the location of where the lift is generated in relation to centre of gravity of the system.
the fin is naturally located in a lateral aspect where it is needed to counter the sideways force of the sail, but this point usually is behind the centre of gravity.
which means that when you start creating serious lift from the fin behind the COG your nose will drop.
the force the fin generates is largely dependent on the square of the speed you are doing and the angle of attack on the fin. so the change in lift will change a lot and be very unstable as the board lifts up at higher speeds or you sail in choppy water.
remember that what makes a windsurfer work so well in rough water at speed is a high enough longitudinal stability due to a balance of forces. length of wetted surface area and mast foot pressure to name a few.
so if you start lifting the board out and take away the longitudinal stability due to the wetted surface, then you might theoretically have a more efficient set up but not really faster due to control issues.
just a bit of food for thought...
Boogie
www.C3-fins.com
You're siding with the Bonominator Boogie? I'm too old to be a test pilot.
Fin lift increases with the square of velocity for a constant angle of attack but windsurfers do not sail with a constant angle of attack. The fin's connected to our feet. When sailing in a straight line we subconsciously vary angle of attack all the time to balance the lateral lift provided by the sail. The lift that the sail can generate is limited by the righting moment that the sailor can provide by hiking out. I came up with the 20kg approximation to fin lift by looking at the moments about an axis running lengthwise thru the board. If the board is kept level and the fin doesn't flex much, the vertical lift of the fin canted 18 deg will be in a fixed geometric ratio to the 20kg lateral lift. It shouldn't exceed 6 kg even as speed increases.
The lift provided by the hull is centred towards the front of the wetted surface. Taking 6 kg off the front and moving it back will alter the centre of hull lift by not much more than the shift you get by taking out a wider board. You just have to lean back a bit more, or stack on a few kilos. Look at the turning moments about a horizontal axis across the board's centre line.
I'm pretty sure i won't be launched by uncontrollable fin lift, whether we're all sailing flip flopping fins in 10 yrs time? that's a long shot.
20 knot NE forecast for Gerroa on monday, no waves forecast, hopefully give it a try then. Maybe I should duck tape a spare fin and screwdriver to the nose of the board - it's a long swim down there if something breaks.
am i sensing that for a canted fin, you really want an offset fin box, to reduce the ankle snapping leverage effect??
Looking into my crystal ball...
The extra vertical lifting surface, which has a fixed location, will dominate over the board planing lift (as planned). But... now you've disabled the main pitch stabilising element- the effect of wetted length and trim angle which is "self adjusting" with changes in body orientation, board speed, and apparent wind on the nose. Not only that, you are now coupling the board trim angle to the leeway angle by canting the fin. With a vertical fin, these axes are orthogonal and therefore independant. So when you hit a bit of chop and the nose comes up, what happens to the leeway angle (=torque on the tail)? Its coupled to trim angle now... I'd be wearing some serious ankle strapping to take the pounding of the chop impact being transmitted into heel pressure.
And on the topic of pitch stability, with the LCOG now located further back, and determined by heel pressure, what is balancing the pitch now? Are you relying on the hydrodynamic lift around/in front of the mast track? (that would defeat the purpose methinks).
I'm curious to see what happens. My guess is the board will stick, and the nose will veer and roll to leeward when you hit chop. You might even spin out if the rail lifts enough to ventilate the fin :)
Will be interesting to see how it works out 
Don't forget the on-board video Ian.
Icesurf (Michael H, NZ) has designed some really radical speed fins that are only 15cms deep. I 'd hate to steal his thunder (and the fact I have very little technical knowledge in respect to fluid dynamics) so I'll let him post if he wants to share.
Kudos for trying something radically different guys. A world without guinea pigs would be disappointing indeed (although somewhat not as smelly in our back yard)
Finally got a bit of wind. Comfortably powered up on a 6.2 Hellcat, Sonic 90. Short, sharp Lake Illawarra chop. Did 30.5 knots on the canted fin and 30.05 knots on the control fin - (a 35 cm "Fins" brand reboxed to a 38 degree weedie.) That's probably about as fast as you could expect me to go in those choppy conditions no matter what I was on.
Interestingly you could probably put someone on it, without showing them the fin, and they probably wouldn't pick up on anything being particularly out of the ordinary, just like sailing someone else's board.
On the starboard tack it is a little prone to spinout in chop, but once you adjust it's fine. On the gust where i did the "top speed" I was bouncing off the chop, getting a bit of air, but putting it back in the water OK.
Going back upwind beating into the chop on the "wrong" side it was very solid, didn't look like spinning out. The chop angles were wrong to try bearing away. And I'd thought it may not even plane on this tack!
Easy to trim on both tacks, wet between the footstaps, just shows how adaptable we are.
Came back after a dozen or so runs and put in the weedie. Back to back you can notice how nice the decoupled feel of an upright fin is when picking a course thru chop. A bit like a RWD vs front wheel drive car. The weedie felt a few knots faster when I got a gust to bear away in but the GPS shows it wasn't.
It is easier to trim the upright fin on the tail... not much in it, maybe just used to upright fins.
So it is sailable. What's the theoretical gain of losing 6kg off the hull? That's probably only associated with a few percent of total drag. Halve that for potential increase in top speed. Probably lucky if there was an extra knot in it after all the appropriate tuning and getting fully comfortable with it.
If you've got plenty of wind and worried about spin out - sailing on the "dig in" side might aid control?
Conclusions. If you are reboxing a fin and don't get it exactly 90 degrees - don't worry about it, there's nothing in it.
Likewise if the fin is a little loose in the box.
