Hi, just wondering what would be the effect of changing the stab foil from a flat section to one with turned up or down ends? Ive had huge improvements changing the original flat front foil on my manta foil to the larger wing with turned down ends, the stability is much better and doesnt feel like side slipping in a turn as the flat one did. Not sure if a stab with turned up vs turned down would make any difference. Please enlighten me..... or just guess !!
The curved tips on foil wings are there to eliminate the tip vortices as the water flows toward the tip of the foil wing by placing a "wall" in its way where it would usually "fall off" the tip so to speak. The winged keel on Australia II was an extreme version of it, if you understand how that worked.
The stabilizer is generally tweaked to provide up lift, so when you think about the flow hitting it, it will be wanting to "fall off" the tip from the top surface of the foil, downward So it makes sense for the tips to have an up turn at the tip.
as to why some foils have down turn tip on the stab, not sure.
Some other considerations. Tips on airplane wings are there to reduce induced drag (vortices) but are also used because making airplane wings longer (high aspect) another method of reducing induced drag can either have other less desirable effects (added weight, parasitic drag) or be infeasible (limited space at the airport gate).
Boeing 777X folding wingtip Higher aspect and raked wingtip (similar to AFS R1000) for low drag in flight - folded for ground use
www.boeing.com/777x/reveal/video-777x-Folding-Wingtip/
Wing tips and stab tips do add some tracking and yaw stability. Going between a foil with has both to one that has none and are similar in longitudinal (fore/aft) stability, I definitely can feel a bit of wiggle on the flat tipped foil especially when, say, initiating a jibe. It's not huge but it's there. Note that most foil wings are tipped down resembling Hoerner tips rather than the upswept tips you usually see at the airport. Hoerner tips are also noted for reducing Dutch roll (yaw instability) and increasing roll stability. en.wikipedia.org/wiki/Wingtip_device#Hoerner_wing_tips buyplaneparts.com/the-hoerner-wing-tip-design-story/
Im no engineer, so cannot enlighten you about that. However, I think you will not get a clear cut answer, as with a lot of things with wings, in the end we (well, me at least, but that is the general feel I always get when talking to people who are supposed to know) do not know why it works.
In the end F-One opted for twist / washout in the frontwing, but big wingtips for the stab. The big wingtips on the stab add tracking stability (as you say). For the frontwing its a slightly different story.
As far as I see now there's 2 trends which seem to work for higher end foils. Either big wingtips, which add traction, at the expense of control (compared to the other option). Frontwings with swept wingtips inevitably (at least currently it seems very hard to incorporate swept wingtips with an efficient twist pattern to reduce active surface area at lower angle of attacks.) result in a foil with which the power shifts forward as you accelerate (which you sometimes want ex. In up down racing), but they are easier to lock at a certain roll angle. The other option I see a lot is the flatter wing with twist / washout at the tips. These foils ride more flat, with less windward heel, however, as a bonus the position of the power is more stable through various speed range.
Ok, thats an indepth topic 
, Ill assume changing the rear stab ....may.... improve tracking...Thanks for the replies.
Yep, done it, with a Manta.
Stab with upturned tips, noticeably more lateral stability.
Maybe partly due to area also?
Manta stab is 174cm2 and 365mm wide x 60mm chord
Test was 232cm2 and 380mm wide x 70mm chord
Test was molded off an F4 stab.
