Factory fin settings are "over-pitched" so improving Drive performance is relatively easy by loosening the outhaul clews. There is a limit to what the screw adjustment will allow though -- if you could loosen the fin even more that would continue to improve low end torque and allow a higher cadence at cruise speeds. If anyone read Dr.SteelheadCatcher's link to the end, you saw that siravingmon came up with a clever use of zip ties to lower the pitch even more without bending masts and gain yet additional improvement.
viewtopic.php?f=11&t=10176&p=241430&hilit=turbo+fin+adjusting#p241720IMO, the zip tie "extension" works particularly well with the new (2015) GT Drives, which have a slight reduction in fin slack or twist ability (a necessary evil to accommodate the sprocket roller bearings).
How far can you go? When the fins feel like they're freewheeling at speed (when you back off) you've gone too far. It doesn't take much.
2015 marks the 10th anniversary of the Turbofin. Although it looks very similar to the original, there have been many improvements along the way. Lots of things have been tried -- some more successfully than others. Effective use of the "search" feature will yield a bounty of cool facts. For instance:
Differential fin tightening would work in theory, but in practice there is no significant difference in performance. The more unbalanced the fins are...(see below).
An imbalance between the fins induces a rocking motion in the boat -- sometimes a telltale of a bent mast. Combining a large and small fin on one Drive does this even more while reducing performance.
The Drive works perfectly well on one fin, although at a greatly reduced thrust level and a very "spirited" rocking motion on the boat. Good to know if you ever lose a fin that you can still pedal home! (hint, looser fin = less rocking, greater forward thrust -- it does require getting used to. Try it!).
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