A surfboard with one fin, as compared to boards with anywhere from two to five fins. Surfboards were finless until Wisconsin-born surfing polymath Tom Blake attached a low-profile metal boat keel to the back of one of his paddleboards in 1935. "Never before had I experienced such control and stability," Blake later wrote.

The surfboard fin didn't catch on right away (most surfers thought it was too dangerous), but from the mid-'40s to the early '70s, with a few exceptions, all boards had one...