Center Fin Box – Origin and Patent

A common fin box, especially with single fin longboards, is the center fin box.  This fin box is called many things such as an American Box, A-Box, Standard Box, Fin Box, Bahne Box, Original Box, Chinook box, and others.  This fin box is a rectangular channel with a slotted channel near the bottom of the channel.  A recess in the center permits the installation of a fin with a small pin, typically at the leading edge of the fin.  Normally the trailing edge of the fin is locked into the slot with a nut plate and screw. These boxes come in various lengths and 8 ½ and 10 ½ are common sizes.  Other water sports also use these boxes, such as kayaks. 

This fin box permits a fin to be moved forward or aft to trim the stability of the board.  Some people install fins which consume the entire length of the box and thereby lose this feature of the fin box.  Alternate configurations are also used such as a thruster set-up with other fin box installations. 

The origin of this fin box is found in the original patent for the fin box by Bahne; hence one of the names of this box is after the inventor of the box William (Bill) L. Bahne Jr..  The Bahne patent, patent # 3,564,632, was issued on February 23, 1971.  You can download a copy of this patent at the US Patent Office website. 

Today there are various versions of this patent being sold by Fins Unlimited, O’Fish’l, and others.  Fins Unlimited fin boxes sometimes show the Bahne patent number inside of the box (you need to squint with light to see it, if you can see it at all), which shows that Fins Unlimited still owns the patent rights to the Bahne patent, which makes sense as that is where Bill Bahne is.

These boxes are commonly made from a relatively strong plastic which is potted into a milled slot in the board core foam.  The overlay of glass and binder secure the box inside the board and in general the box can only be removed by structurally breaking, cutting, or otherwise mechanically separating the box from the board.  In some versions of these boxes, such as O’Fish'l, the box slot is covered to assist in installation and then the plastic is removed at a later time in the board manufacturing process by the board manufacturer to expose the slot.