Boat structures 101

[FLexpat]

Reading Jeff’s post started me thinking about something I ran into when I started on my 23 redo... The original hatches for the fuel tank and the forward fishbox in my 23 Sceptre are bowed. The cores don’t appear to be wet and they are not super flexible so it took me a bit to figure it out; the edges of the cores in the hatches had not been properly finished.

Like Dave mentioned, cored construction creates something kinda like an I beam. When it is supported on the ends and a weight is in the middle (like someone standing on a hatch), the top of the laminate is in compression and the bottom of the laminate is in tension. That is great if the lower laminate is rigidly tied to the upper one at the edges – near zero flex or bending. My factory hatches have a balsa core that just fits inside the shoebox-lid shaped hatch and a thin glass lower laminate that only touches the upper laminate along a 3/16” edge – or it did when it was made. As a result of the lower laminate being in tension whenever someone walked on it, it broke loose along the edges. The result is a pair of hatches where the upper and lower laminates are not tied together anymore – and took a set with a shallow dish shape over the last 40 years.

So what does this say about cores? The edges of the core and the structure have to be properly finished with the upper and lower laminates being tied together. It was done properly on the hatches for my stern wells; the core stops an inch or two from the inner edge. It is tapered to the underside of the top laminate (~30-45 deg angle) and the lower laminate covers the core AND is wrapped fully into the bottom of the upper laminate. That means the tension from the lower laminate gets spread across a 1-2” wide joint to the upper laminate which is in shear.

This cartoon shows the way the hatches are and the way they should be.


So that is what I need to do to my larger hatches. I also need to properly finish the edges of the core (Core-cell) when I replace the decks. If I just have a 90 degree angle on the edge of the core in the deck and don’t tie the upper and lower laminates together properly I will cause the Corecell to fail in shear on the edges. Then the deck starts falling apart and delaminating from the edges. And I will have to redo it.

There is a good bit of discussion with diagrams about this in Gerrs book and some in Bruce Pfunds article on core failures.