DN NA Class

    Last night our club built a plank for our Opti. We used the materials suggested by Ken Smith  ie. 1/4" ash and 1/2" redwood for the construction. We didn't have any figures for spring back (the amount of over bend put into the plank while gluing it up to get the desired crown when out ot the jig) so I took the figures for a DN two piece ash plank that was in an old article and did some recalculations that I won't bore you with. To make a long story short for an 8ft DN plank you have to over bend it by 33% ( to get 2" of crown, prebend it to 2.66"). To get a 2" crown in an Opti plank over bend it by  62% or 3.24" to get 2" of crown. It actually worked out to be 59% with the redwood/ash plank but I believe an all ash 2 piece plank would be closer to 62%.

I think Paul Goodwin might have a computer progam that will calculate plank crown or spring back.

The spreadsheet is at: http://idniyra.org/articles/plank_design.htm

   The numbers I got for the DN plank came from an article written by John Jombock back in 1991. The problem here is that it's for an 8 ft. plank. Because values are cubed when you talk about different lengths, a shorter plank will spring back considerably more and a longer plank less. When I figured this out for the Opti I used John's values and took the length difference into consideration.
                                                                      Bob

Both articles on the IDNIYRA website (one article by John Jombock (Demystifying Plank Stiffness), and the Plank Stiffness spreadsheet (written by John Bushey) deal only with calculating plank stiffness, not springback.

Bob --- if you have a copy of the article from John Jombock with information on plank springback I could see about adding it to the IDNIYRA articles.

  The June 1991 IDNIYRA Newsletter has the springback article by John Jombock.