Demystifying Plank Stiffness
by: John Jombock, DN 1513
November 29, 1989
I believe that the DN Class would benefit from a standard definition of runner stiffness. I suggest that we begin to describe plank stiffness as the number of pounds required to deflect a plank one inch (a metric definition t be kilograms per centimeter). The higher the number, the stiffer the plank. The test load would-be applied at midspan and the plank would rest on supports 95 in. apart or on the bottom edges of the outside chock faces.
As an example, a plank with chocks is set on two wood blocks and a skipper who just weighed in at 170 lb on a decent bathroom scale steps on the center of the plank, deflecting it 1-3/4 in. The stiffness is then 170 ? 1.75 = 97 lb/in. The ends of the plank should be free to slide in the lengthwise direction of the plank, perhaps by setting the outer faces on Teflon pads or wax paper.
As a second example, say a plank without chocks is supported on a 95-in. span with a short length of 1/2 in. pipe under one end to prevent lengthwise restraint. Four 5O-lb test weights are stacked at midlength and cause a deflection of 1-5/8 in. The corresponding stiffness is equal to 200 ? 1.625 = 123 lb/in.
A standard way of describing plank stiffness should eliminate some of the trial and error in building and tuning DN planks. Measurements of the planks of boats performing well could be correlated with skipper weight to establish a table or list of desirable plank stiffnesses. Such a table could be included in the new officia1 plans.
A standard method for defining mast stiffness might also be desirable, but the .definition and test procedure would probably be more difficult to agree upon.
Last modified March 24, 2005