RE: Tectonic for isonic
Tectonics can be a good fin choice for the iS101, especially in the small/mid sizes (both of fin and sail combo). How "good" depends to some extent on your rider weight, sailing style (very heavy fin loader, or lighter style, gliding) sizes.
Sailing angle (pure downwind or reaching/Fig8/upwind) also influences the choice of fin for a given rider/board/sail combo.
For medium "all round" conditions on the wider tailed iSonics, many riders prefer a liftier tip fin than the (relatively) fine tipped classic Goldwing. From Tectonics, you might find the Falcon F-8 an even better Tectonics option for the iS101in larger sizes or lighter winds.
[ In general, as the tail (and overall) width goes wider on the newer boards, the fin can be put into higher load situations because of the increase in stance leverage, plus in harsh powered situations the rider can push on (where on a traditional -narrow/er - slalom, they might start to back off).
So as the fin length gets longer, the way the fin behaves hydrodynamically under load (usually governed by construction/material as well as foil profile/thickness) becomes more of an issue.
This is not just a pure flex issue, it is how the flow over the foil performs under load, which creates twist as well as flex. Quite a complex situation in itself. Obviously, at what point in any fin type/model size range (of any fins) excess flex starts to lead to loss of foil efficency and or control (and the fin starts to become "less ideal" ) depends significantly on the specific fin design and construction and in turn clearly relates to rider load (on that fin in those conditions).
In molded carbon fins, the manufacturer can locate specific layups to control and influence twist as well as flex, and in really specific designs can vary that to suit size/load/rider etc. In machined materials, like G10, the material really is more generic ; the flex/twist character can only be modified in the panel material (raw state) and not within an individual fin.
With smaller fin lengths, G10 (and similar) works well to take moderate to high loads, but as fin lengths - and load leverage over the fin) increase, the foil thickness on modern (fast) fins doesn't get thicker at a corresponding rate, and sooner - or later - you reach a point (combo of load and length) where this starts to become more of an issue than other constructions.
So, in summary, using G10 or any construction, a 36cm used by a lighter rider in "gliding mode" in lighter wind will behave considerably differently to that same fin (board/combo) used in massive overpower mode by a huge, powerful heavy loading rider on a wide tailed slalom.
Finding the "ideal" match is the key. . ]
Cheers ~ Ian