Builders' Forum Archives
Re: Fillet Mix
Posted by Mark Camp on Mar 17, 2005
Good note, especially your real-world experiences with point-of-failure.
I suspect that today we may not be very near the goal on joint treatment: panel joints, such as chines, that are just strong enough but not too strong.
The challenge is two-fold: very little in the way of lab tests (Lee G has done some), and then: how do you relate lab results to the real world failure modes, once you have them? It is very hard, because the boats don't break that often, and we only hear the sad tale in a small fraction of the cases. And even then, and just a little bit about the specific crash event, and resulting point of failure.
I think from what Lee has posted previously that we are missing a basic point, that making the filleted and taped joint strong may be making the most likely failure mode WORSE, not better. Scot hints at that.
Lee's results indicate that the plywood away from the taped joint, but more particularly the "stress riser" where the fiberglass ends, are invariably the problem. Well, as the above note suggests, making the taped joint so stiff just increases the stress riser problem at the place that is already targeted to fail.
Also, I think we miss the point when we assume that "thinking about current practice for joint construction is irrelevant, since the joints are way stronger than the rest of the boat."
From the premise, we should come to the exact opposite conclusion: if the joints are way stronger than they need to be (thus, more expensive and heavier) then they are precisely where we should be focusing our attention on improving the design.
I'm reminded of the experiences of a FEM stress analyst for plastic computer printer parts. The engineers and designers would come to him and say "this latch was breaking in the field, so I beefed it up, but the problem didn't go away!" My friend would tell them: "you should have made the latch THINNER, not THICKER. If it were more flexible, it would not have failed." Think 'skin on frame kayak.'
These are just thoughts from someone who reads the forum, not to imply I have any experience myself in boat design or construction, and we have folks here who have a world of experience behind them. It just doesn't always get communicated.
In Response to: Re: Fillet Mix by Scot on Mar 17, 2005