A properly foiled thicker fin should maintain connected flow compared to a similar fin that is narrower. At some point the increased drag outweighs the resistance to separation. I rode self-foiled thick marine ply/glass-on fins for many years. When I built my first board with a fin box, the Brewer fin I bought was significantly thicker than the slot and all the thin fiberglass single fin slabs common at the time. There’s bound to be heaps of those old Brewer fins around. What were they, 1/2” thick or more?The only thick fins I have ever tried are the ‘glass in’ ABS polypropylene Yater template ones in so many longboards at the end of that era. I assume there is some advantage but what this entire discussion means is that like 99.9% of all surfers are using less optimal fins. Crazy huh?