RE: Fins s-type 137 2007
Until you are planing, all the extra power (fin) in the world is going to be a benefit, but it's what happens AFTER you get planing that (normally) matters the most.
The R13 and R19 are definitely light wind, massive planing power fins, but in overall use too much so for the ST. These very vertical designs(R13/R19) are really optimised for Formula or Free Formula type boards with massive tail width and sailed in a very steady locked down state.
For slalom use, (or typical "blasting" sailing) fins with slightly more rake (and increasing rake as the fins get smaller) are more ideal for the optimum combination of control (rake) and efficency(vertical).
The design character/function of the ST is really on the "moves" side of Slalom, inferring an even greater tendency to rake over slalom.
Further, the the softer (bevel) rail of the ST (a positive benefit in control/comfort/safety at speed) doesn't provide the same "edge" as an equivilent hard railed slalom board (of same dimensions), which reduces slightly the ability to "set" the power of the fin against the rail of the board. (or in summary, the ST doesn't like as much fin/power as a similar sized slalom board, and obviously a lot less than for the same sail size on a super wide FW).
The 48 (rather than say the extreme 50-52cm) choice concedes a very small amount of bottom end/planing power, but returns a considerable amount of extra/better range/performance at the planing/top end. Most riders will consider it a more suitable choice to carry a little less than absolute max fin (requires maybe a little more work/effort to plane) in return for a much better overall "tuning" once they are on the plane.
The underlying principles of this apply to all brands of fins, even though here we discussed mainly the Drake models. Of course, in finer detail the differences in stiffness/twist/foil etc also have an influence on personal choice because of variations in specific fin/size/ride behaviour.
Please let us know if you have further questions.
Cheers ~ Ian