How the fin works

Question
Hi !
I'd like to gain a better understanding of how the fin works, in terms of basic physics/hydrodynamics  principles.
I can easily see how forward movement is generated as a result of the interaction of two opposite forces :  the wind pressure on the sail and the lateral resistance of the fin  , but I still fail to understand the vertical lift component. Why so?

Thank you

Franco

Answer
Hey Franco,

A fin is very similar to an airplane wing in the way a wind lifts the plane. The thicker part of the wing is close to the body for strength but also to provide more lift to that area. But a wing provides lift by being horizontal.

A fin will provide lift in a vertical position because of the taper of the fin. Water is a much stronger force against the fin and because of the taper of the fin, the fin pushing through the water causes lift. The faster the board speed, the more force is applied to the fin and creates more lift for the board. If you use too large a fin for the wind, you can get too much lift or the fin will want to turn onto it's side like a wing. This makes the board very difficult to handle.

Also, as board speed increases, the wetted surface of the board decreases and soon the board is planning. When a board is on a plane there is usually only about 12-18" of board on the water. If the wind gets too strong for the sail, board speed can get so high that the fin will literally push the tail up out of the water. This is commonly referred to as 'tail walking'. Some folks like to ride right on that edge of tail walking and barely in control.

Hope this helps.
Keep on sailing,
Windlover