How to Build Rudder Blades & Centerboards
by J.R. Watson (Gougeon Bros, WEST Systems)
Here ➥ http://www.mothboat.com/CMBA/Building/foils.htm
In the above tutorial, J.R. Watson makes a good case that:
is not a good choice for cantilevered structures such as rudder blades and centerboards. This is because plywood
is susceptible to rolling shear, shearing forces that roll the structural fibers across the grain.. Plywood's unidirectional wood fibers are laid in alternating layers, approximately half of them are oriented 90 degrees to the axis of the loads. Like a bundle of soda straws, which resist bending moments quite well one way, they simply lack cross-grain strength laterally and can roll against one another and fail under relatively low stress, especially in a cyclic environment
. Therefore, when anticipated loads are primarily unidirectional, it is ideal to use a material with good unidirectional strength. Since only half of plywood's wood fiber is used to advantage, a plywood rudder blade or centerboard
going from tack to tack (reverse axial loads) will fatigue much more rapidly than one built as described in this article...
... Western red cedar and redwood are good choices of wood to use for rudder blades and centerboards for boats up to 25 feet. Both of these woods bond very well, are generally clear and straight grained, have good dimensional stability, are easily worked and affordable. Cedar is just a little heavier than the foams used for rudders, is much stiffer and has far greater shear strength values. On larger craft, a higher-density material like African mahogany is a better choice. Oak is not a good choice.
Buy flat-grained 2'x6s or 2'x8s, and then rip them to the designed board thickness. Turn every other ripping end-for-end to neutralize the effects of any grain that does not run exactly parallel to the blank, and to reduce tendencies to warp or twist ..”