We use four to six 2” to 2.5” deck screws with 2 in the flange holes below the angle and 2 to 4 in the angle itself.
If you are attaching the KayaArm to a 1.5” thick piece of 2×6 that has been fastened to a metal or plastic dock, you may only need 1.5” deck screws to avoid having the screws bottom out against the metal or plastic.