The fin box with a fin for surfboards has in the longitudinal direction of the surfboard a slot for the fin which is to be locked. The sidewalls of the fin box each have a groove which guide the rear (stern, aft) bolt of the fin shaft and a slide block for locking the fin. Arranged between the coupling lug of the fin or the fin and the slide block is a coupling and rupture joint or a coupling joint which is detachably connected thereto and breaks when a prescribed load is exceeded. The fin can no longer be damaged.
