Plugging it in will generally make a subtle difference in your surfboard’s feel, and experimenting with it is not only fun, but it will improve your overall fin sensitivity.
Put simply, fins generally feel either too stiff, too loose, or somewhere in between, for whatever you’re riding. And like most things in surfing, balancing elements that work for you is key. The Knubster is a handy little keel fin that can help you tweak your fin setups to be exactly what you like.
Added as a stabilizer to your quads, the Knubster will add a little bit of firmness to your tail overall. In particular, I noticed an added feeling of control in the drift I sometimes experience on hard bottom turns, and a boost of quickness when “regrabbing the wave” after throwing a tail blowout or finishing a cut back.
Knubster plugged into the forward slot.
One really cool feature of the Knubster is that, because it only uses a single FCS tab, it can be positioned in either of your two center fin plug slots. This actually makes a difference in the effect it has on your surfboard. (FCS should note this on the Knubster’s packaging in my opinion.)
Quick example of the sort of note/diagram I’m talking about.
If you plug the Knubster in farther back, it adds a tiny bit of drag- but it also beefs up the size of your spray, makes your pumps feel more effective, and increases hold (cutting down on drift.)
Plugging the Knubster into the other slot (closer to the nose) still gives a little bit of extra hold, but with less stiffness, and actually boosts your overall drive more noticeably- which I observed as a higher top end speed going rail to rail down the line.
Check it out: The Knubster VS from FCS.