Technology of Power surfboards

The actual amount of rocker in inches depends upon length, the planshape and other factors, but the basic rule I follow is that the rocker should not increase towards the tail.

There is a very good hydrodynamic reason for this, namely that a flat rocker has less drag and makes less lift, whereas a more curved rocker produces more lift and more drag. So, just like a powerboat the board should have a lot of rocker in the nose to provide lots of lift when moving slowly, as it accelerates it should lift this high lift high drag section up out of the water and use the flatter low lift low drag areas toward the tail. This happens progressively as the board accelerates, and it also allows the board to reduce overall wetted surface area as it accelerates ( less area is needed as the board accelerates as it produces more lift per square inch of wetted surface area. This kind of board naturally trims aft with a nose up attitude as it accelerates.

As far as the actual shape of the rocker goes, the very simplest shape which is a decelerating rocker ( i.e doesn't increase towards the tail) is a circular arc rocker. I use these sometimes, they have an even curve from nose to tail. An increase in efficiency can be had by progressively decreasing the rocker via elliptical nose rocker and circular arc tails, or an elliptical or parabolic rocker section throughout the board. . . they all do a similar job

One of the first flashes of inspiration I had regarding rocker was that it is in many respects like an underwater foil section . .. .. . one can't go wrong using a foil section for a rocker.

So if we take any good foil section it will make a good rocker. . .. and not only that it will make a good planshape.

Foil cross sections can be used to design the entire board. Look at the cross section, there's your fin cross section. Look again, there's a planshape curve , look again and there's a rocker curve. Of course the rocker will have to be proportionally flatter than the planshape, but the same shape will do beautifully for both.

Foil sections have a widest point, this is usually between 20% back from the leading edge and 50% back from the leading edge. Since it is best to match the position of the wide point and the rocker apex ( we call this 'matched foils' ) it is convenient to use the same foil curve for the same board. Of course with the rocker it isn't necessary to use the entire nose section as the board isn't operating underwater, basically a bit can be lopped off so that the tip of the nose doesn't wind up pointing vertically up. . . . . no harm if it does though