mea culpa

Posted by LeeG on Jun 25, 2004

I'll take the blame for that one. Unlike a composite panel with a hard skin on both sides of a core and being subjected to a three point bend the hatches have a 6mm rib holding the curve of a 4mm hatch with specific forces terminating at the ends of those ribs that are different than a composite panel.

At the end of that 6mm hatch rib there are two forces working on the strength of the fillet that tapers out from the rib onto the underside of the 4mm hatch. When the straps are tightened down sufficiently to compress the hatch gasket it pulls the 4mm hatch down OVER the edge of the 6mm rib. If there isn't any fillet tapering off the end of the rib the area that stress is dumped can be increased into a small area 6mm wide that is evidenced by cracking above the termination of the rib. Extending glass between the rib and hatch TO THE EDGE of the hatch that extra 3/4" reduces the bending that can occur when a 1" strap is bending the 4mm hatch down over the rib. But so would glassing the hatch. I had an aft hatch develop cracks above the ends of a rib where the straps pulled down and the hatch hole was reinforced, the hatch was not glassed. The other moment of stress occurs in the other directions,,when you do a rescue the hatch is pressed down and the cantilevered edge of the hatch presses UP,,,pulling the cantilevered edge away from the edge glue joint. I saw that type of delamination in about 1/3 of the demo boats prior to the existing hatch hole reinforcments where you could hold the hatch in your hand and pull the hatch edge away from the 6mm rib as the delamination was already started simply from the practice of putting on and taking off hatches with very tight straps. That may look like trying to make things break,,but there's a reason why things can break catastrophically if allowed to start delaminating. I built about two dozen hatches for the new demo boats and had a chance to see the ones that needed replacement. Now that the hatch holes are significantly reinforced it reduces the possible flexing at the hatch hole and reduces the flexing at the edge of the ribs. The strip of tape was a "belts and suspenders" solution for ensuring that flotation couldn't be compromised by a hatch that could develop a creeping delamination at the edge of the rib at the underside of the hatch which could allow a hatch to flex up/down with a heavy paddler in a rescue setting. With a tapered fillet at the edge of the hatch ribs and the reinforced hatch holes it is a belts and suspenders solution. With the original hatches that only had two ply strips at the ends of the hatch holes it was a necessity as the consequences of a overstressed hatch stiffener AND unreinforced hatch WAS a creeping hatch failure for heavier guys doing rescue practice. In worst case scenarios is where people actually have to do rescues,,where compromises in compartment flotation cannot be a consequence of a rescue/climbing over the aft deck. My experience is that hatches tend to get knocked about,,whether tethered and rolled on a rack or sitting outside the hatch on the ground and leaned against. The extra reinforcement at the hatch ribs was with the intention of making the hatches a strong point,,and not a maintenance item requiring rebuilding after heavy use. I agree it looks funny,,,but edge gluing 6mm ply expects a lot from a glue joint without a well tapered fillet. I did about eighteen aft hatches so it came from the idea that 1/3 of the heavy users should have to rebuild hatches.

In Response to: Bizarro Hatch Lamination by Laszlo on Jun 25, 2004