Centreboards, on my designs, are mainly made of plywood or steel (hot dip galvanised or stainless steel). I have been asked how to shape the centreboard. The sketch below gives the answer. Note that the aft edges have to be sharp and not rounded.
It happens that the centreboard vibrates, generally at high speed. This is due to a Karman vortex street at the aft edge. These vortex may also cause rig vibration when the wind is strong (mainly with aluminium mast!). These vortex may be avoided with a chamfered end as shown on the left sketch. Such an oblique cutting is to be made on all metal centreboards. For a plywood board, do it only if there is a vibration problem.
The same profile principles apply to rudder blades.