Planks and stiffness

[Ken Smith]

A friend asked about a stiff plank and building a new one.  Below is my reply, but I welcome other opinions and comments.

Plank stiffness has been favored as stiffer and softer over the years.  Currently, Ron recommends 1-3/4 deflection with pilot and 30 pounds on the plank.  He presumes one of his carbon masts.  My now 12 year old favorite was built to deflect 1-1/2 inches for me, and now deflect 1-3/4 inches.  But it was the skipper who adjusted trim for the plank! 

I personally believe that mast-plank combinations make the boat easier to sail (soft-soft), fastest speed but lots of hard work to sail (stiff-stiff), or any number of compromises (stiff-soft, medium-medium, soft-stiff).  A Sherry  carbon mast is medium, a whip is soft, and most Kent's are stiffer (he makes a variety).  All combos can be very fast, but the sailing style has to adjust to match.  I'd consider 1-3/4 plank medium.

[DNUS1006]

 What would be the easiest / fastest way to stiffen a soft plank?

  Nick
  DNUS 1006

[JOHN BUSHEY]

What I have done is add a thin lamination of wood to one side.  I had a soft DNplank and added three strips of 1/8" oak mullion to the top of the plank, only under the center, not full length so it doesn't interfere with the chocks or top plates.  It is installed 6" wide by 32 wide in the center and tapered down to a point on each end for a total length of 64".  I feathered the edges so they blend in to the old top surface and glued them down with WEST epoxy after sanding off the old finish.  I can't recall the change in stiffness, but it was significant.

[Ken Smith]

Another alternative is to add unidirectional glass.  I made a plank that was too soft (1.28 inches/100 pounds, or 2-3/4 inches with my weight).  I added three tows of 3 inch unidirectional glass, according to my notebook, to the bottom surface only.  The glass was from Wick's Aircraft Supply. 

I added to the bottom only because the higher modulus materials there move the neutral axis closer to the bottom, making the plank act thicker.  The glass is actually more effective at stiffening the plank when used this way.  Also, no hardware need be remounted (not an issue in my case), and the rougher surface is not visible to the average joe observer. 

The bottom line, post glass addition, the plank stiffness was .817 inches per 100 pounds, or right on my target of 1-3/4 inches.

[Separate note:  I have two "books" that I keep.  One  is for the shop, where I record all the projects and internal details in case I need to later measure or add something, I know whats under the skin.  I also recored plank and mast deflections, construction tricks, lists for work remaining for the season, or required for next season, etc.  The composition book is now 12 years old, and full of personal history data.  The second book is a miniature composition book.  It fits in my shirt pocket.  In it I recored daily sailing notes.  I can tell you shroud lengths, mast step locations, sails used, etc for boat tuning.  I usually make the notes while warming up after shedding ice clothes in the car before starting home.  I know, "get a life."]