There are 3 types of center fins used on iSUPs: 1. Slide & Snap; 2. Surfers fin; and 3. Double-fang fin (the worst of the three).
Slide & Snap fin - this fin is the easy to install and can be done when the board is inflated or deflated. It's also easy to remove when boarding over very shallow water, as in lakes or rivers.
Double-fang fin - this photo shows the two "fangs" of the fin that need to catch onto the board. The board MUST be deflated in order to put on the fin. A fang may snap so be careful.
Surfer Fin - this fin is very sturdy and readily available at any surf shop. The fin can be installed when the board is inflated or deflated. Surf shops will also have different sizes and colors.
Slide & Snap - Step 1 - clear all sand and grit. It doesn't matter if the board is inflated or not. Begin sliding the fin backward, pushing on the bottom of the fin (not the tip).
Slide & Snap - Step 2 - begin sliding the fin back through the groove. Be aware of sand that may block the groove. It should slide with a little force and should be snug.
Slide & Snap - Step 3 - insert the tethered piece into the groove. It should be snug but not impossibly tight. If it is too tight to slide then you need to push the fin back more or clear sand.
Slide & Snap - Step 4 - inspect to make sure that the tethered piece is snapped all the way in place. You are all done and on your way. This center fin design is definitely the latest and best.
Surfer fin - Step 1 - towards the back of the fin is a brass knob on either side. Drop this into the opening on the long groove. You may have to push the knob on one side to center it on the fin.
Surfer fin - Step 2 - slide the fin backwards. The brass knob should slide snugly. If not, clear any sand. A hammer may be necessary to tap the fin back. Be careful not to crack the fin.
Surfer Fin - Step 3 - line up the screw with the flat nut that sits down in the groove. The nut slides in the groove so take your time lining them up perfectly. Push the fin down to connect them.
Surfer Fin - Step 4 - tighten the screw and nut. Make sure they connect and the screws catch. Don't over tighten. Pull back on the fin to make sure it's secure. Now you are off to SUP.
Double Fang - Step 1 - insert the back of the fin into the back slotted knob. Remember that the board must be deflated as much as possible.
Double Fang - Step 2 - push down between the two knobs. The idea is to bring them closer together. So, you are essentially causing a crease/bend between the knobs.
Double Fang - Step 3 - while the the knobs are closer together, catch the front fang onto the front knob. Make sure it catches, then release the board so that the knobs separate.
Double Fang - Step 4 - put your hand under the fin and give it a little tug to make sure both "fangs" have caught and are secure. Reverse Steps 2&3 - making a crease between the knobs - to remove.
The Slide & Snap fin is definitely the easiest to install and remove. When paddling into very shallow water, like in a river, the pin can be removed in seconds.
The Surfers fin is the easiest to replace. Surf shops have dozens of them. The downside is that you need a tool to install or remove, and it takes a bit of time.
The Double Fang fin is a pain. It's difficult to install and remove. The board must be deflated. Finally, the fangs can snap off easily.
Parting words - practice at home before you hit the ocean, lake or river. When purchasing, look at what type of center fins they sell. Happy iSUPing.