Building with a rocker bed

If you've followed my Compsand builds and Matt S's hotwiring you'll see that the offcut from the rocker cut is a duplicate of the bottom rocker.

Matt S is laying up by hand, but I am bagging all of my EPS boards.

When I purchase a the EPS billet for hotwiring a blank, I order the billet in about a 5 foot length and 6" thickness. This allows me to save the offcut to be used as a rocker bed that I can vac my board to. This gives an idea of the bed, although the blank in this photo was not cut from this block of foam.

When you hand shape, it's easy to change the rocker - you just mow foam different on the next blank. Even for Matt S where he is hotwiring each blank, he can "fairly" easily alter his cutting templates and get a new rocker. When you use a rocker bed, you can do the same thing, but I have a bit of time and effort invested in my rocker bed and I'd rather not have 5 of them hanging around if all I want to do is tweak the nose rocker a little.

Increasing nose rocker is pretty easy. All that needs to be done is insert a small piece of foam between the rocker bed and the blank and that will force a bit more rocker into the blank at that location (nose or tail).

DECREASING rocker is a bit more difficult as is starting the nose or tail rocker in a different location.

So here is a trick that I use.

Rather than cut and skin the blank in the desired outline, I skin the ENTIRE cut - which is a 2' x 5' slab. It's a little tough to tell, but this e.balsa skin is attached to an EPS foam core that has the rocker hotwired in.

So with that understanding the typical orientation would be to just "plop" the blank down onto the bed as a perfect mating surface.

Because I have used a length of 5' I can adjust where I place my outline if my finished board length is less than 5'. All of the boards I have built have been in the 4'6" range so that allows me 6" of adjustmet. I can start the outline further forward on the skinned blank to increase the total rocker OR conversely I can position the outline further back to decrease the nose rocker. I still end up with the same 4'6" LOA, but the rocker will be different depending upon where I start the nose and I have 6" to mess with. In these diagram the blue vertical lines indicate where I could start and finish the nose and tail of the board. You can see that the two different cuts would provide a different rocker.

A final adjustment that can be made is to reposition the "blank" on the rocker bed before vacuuming. The force exterted by the vacuum will allow you to alter the shape of the lightweight 1# EPS. I have cut my rocker bed out of 2# EPS, so that I can manipulate the final shape of my board without worrying too much about distorting my rocker bed. As this diagram shows, positioning a trimmed blank over a different section of the rocker bed will allow you to reshape the blank after it is skinned. It may require that both top and bottom skins be attached for it to hold this shape, but after all pieces are attached, it will retain the shape.