Larger fins generally have more hold, drive and control. Smaller fins are looser but you sacrifice drive and control.
A longer base equals more drive, acceleration and draws out turns. Shorter fin bases increase manoeuvrability but decrease drive.
The greater the depth the more hold and vice versa. The smaller the fin, the more release.
The amount of backward curve. More rake longer turns, less rake (amore upright fin) equals quicker tighter turns. More upright rake, means more pivot and looser feeling.
FLEX (bendiness in the tip):
Stiff fins for quick response, speed and drive. Flexier fins are whippier through turns – slow at first then they whip through on release.
The angle or splay of the side fins. Less cant faster acceleration and a stiffer feel. More cant more manoeuvrability and looser feel.
Curve on the inside and outside of the fin. Very complicated! The fin foil is the aerodynamic cross section and generates lift. Side fins are generally flat or curved on the inside of the fin with a foil on the outside. Center fins have 50/50 foils. Flat sided fins can work well on deep concaved boards as they stiffen them up, inside foils can add drive through turns.
The toe of your fins is the angle the fins point inwards an creates different pressures on the outside and inside foils. More toe increases the responsiveness of your board. Less toe faster speeds but a stiffer feel.
FLAT SIDED FIN VERSUS INSIDE FOILED FIN
Flat foils break free sooner and are better for controlling speed, use them in fast powerful surf where you don’t need to generate additional speed. Flat foils will feel closest to glass ons.
Vector foils will allow you to maximise speed and drive hard in your bottom turns, the fins feel fluid rail to rail. Futures produce a series of V2 fins which feature the speed generating characteristics of the V (vector) at the base transitioning into flat foil at the tip, for controlled release in critical turns.