Sunday, October 09, 2011

Ama: tortured but alive

You know why they call it Tortured Plywood right?... Because sometimes it dies.

Fortunately the starboard Ama (on which I am now working) underwent severe torture today but the plywood told me what I wanted to know so I let it live. All in all a very satisfying shape achieved (more pics tomorrow). Below is the challenge. Turn flat panels into the wave piercing round bottomed form of a fast ama 1/2 shape at left. After determining what 2d shape I wanted to start with (part, geometry, part "maybe this will work") I joined the panels with a finger joint and stitched them together.

A pictoral journey of the events that followed can be seen below. I knew there would be some compromise in my desired hulls shape something I learned on the main hull but I wanted to really push the tortured shape to the limit of what I thought was possible and I don't think I could have gotten any more out of it. Then again, it didn't crack so who knows.

It was obvious to me that my desired bow shape was going to be a problem so I cut two splits in the bow section 40" long following an isoline from the tip of the bow and curving up along a line of max curvature. By re-stitching these, I could set them at an angle and help the curvature process. It also let me lock in the shape in that area so I don't end up with too fine a bottom near the bow. I have never seen this method used with torturing so I guess I'll call it "split-tortured plywood". The rest of the keel was glassed together over a generous fillet of 3.75" diameter with 20 degrees of deadrise. This angle is crucial to making it work. The smaller the deadrise you set at the get go the more compound curvature your asking for. Unlike the main hull, the ama is made of just two panels.

While the seams were curing I whacked out the bulkheads of 4mm ply on the shopbot. Sooo much faster than hand fitting. This worked great as you can see in the pics. The discrepancy between the bulkhead and the hull in the 1 photo where it is obvious occurs on the way to the bow where the deadrise angle must increase. Eventually at the bow, the angle is 90 degrees so there has to be some compromise there. The rest of the bulkheads fit like a glove.

At the start of initial torturing 2 cracks alerted me to a problem which was recognized at the finger joints. One of the fingers opened up on the outside as tension was placed on it. Work was put on hold until the joints were glassed in the area of maximum curvature. This is the joints weakest bending axis. They are strongest in axial loading such as overall compression or tension of the entire ama structure. This problem was not seen in the main hull presumably because the whole bottom panel was glassed before bending.

Once the patches were kicked, the torture continued. In the end, I was able to get all of the bulkeads glued and filleted and the stem joined tonight, just 3 and a half days after cutting the panels.

Tomorrow, I plan to turn the ama upside down and apply boiling water soaked towels to the outside of the skin in hopes of relieving some of the stress that is built up in the sheet before epoxy encapsulating.

1 comment:

anonymous said...

I think the skins can compress more than they can stretch. Maybe hot h20 inside would be good too?