Shim the Rear Wing
Shimming the rear wing is necessary to control the amount of downforce that the rear wing (stabilizer) creates. Downforce is actually necessary on any foilboard to keep the foil balanced.
When any foil goes through the water it creates drag, drag is a force opposite the direction of travel (IE Backwards force). Drag force pulls the foil backward against the direction of travel. And because this rearward force is acting below the board, and creates a rotational force. The rotational force is because the board is traveling forward with no drag whilst the wings are being pulled back by the drag. This drag on the front wing causes the board’s nose to tip forward and downward. To compensate for this, the foils rear wing is designed to fly downward. When the rear wing flies downward, it pulls down on the back end of the fuselage. This is what we call “downforce”. Because fo the long fuselage, and a little bit of downforce can easily counteract the Rotational (“pitching”) force of the drag on the front wing. The result is that with both wings operating together the complete foil system will provide a straight and level lifting force under the board.
What is a shim? The Shim is a wedge that is placed in between the fuselage and the rear wing. Shims can have different shapes to create different angles between the wing and the fuselage. The angle is measured against the angle of the front wing which is usually said to be set at Zero degrees because the relative angle between the front and the rear wing is what really matters in this case.
There are different ways to shim the rear wing depending on how you want to the board to ride. Many boards come set with a Zero AOA rear wing setting. Depending on the rear wing shape, it may already create downforce at a Zero AOA. For example, asymmetrically foiled wings do this.
To change the AOA of the rear wing a shim can be added. And by increasing the downward angle of the rear wing it will, in turn, increase the amount of downforce that the rear wing creates.
Diagram of the Foil system showing the Shim and wing angle