If you are interested in this thread, please look at viewtopic.php?f=71&t=42667
LOOSE RIVETS ON AKA KNUCKLE ASSEMBLY
started by Yakaholic
The reports there confirm the concerns expressed by Nohuhu and myself about how these loose xbar to aka joints would wear over time. The rivets go first it seems.
Think about it. These joints have to do two things. One, stop the aka/ama assemblies from moving up and down relative to the long axis of the boat. That is keep the floats rigidly extended. The other is more subtle, but probably the source of much of the wear and tear problems. Drag on the ama's wants to move them backwards and thus ROTATE the aka pipe in its socket. So this puts sheer stress on the rivets (or any screws). Looseness creates a ton of repeated impacts on the rivets--perfect for deforming them.
The same thing goes on with the aka assembly insert where it fits into the xbar. It not only has to stop up and down motion, but it has to resist rotation. Rotation wants to happen to it not only when sailing, but what we fold the ama/aka assemblies in. The whole thing wants to sag down, because the inserts into the xbar are held only by spring clips. This brings us to the problem talked about in
another thread (can't find it now), where the inserts become very hard to remove, because they have twisted and now remain somewhat twisted.
An abstract design direction (with issues to solve) to correct all this would be go to to square cross section tubing for the xbar and the aka's. A square socketed within a square resists rotation because of its shape. This all comes back to me now because I built a small trimaran years ago where the aka's inserted into rectangular cross section sockets in the main hull. I tapered the ends of the aka's where they inserted, and then built the sockets using these as a form--so the fit, when the aka's wedged in tightly, was near perfect. Only thing needed then was a piece of line to hold the aka's from sliding out.
But we still need some sort of work-around here it would seem, because that's a whole redesign, and at that the approach would cost in other ways if taken on the TI.