Friday, July 22, 2011

Distributing point-load stresses in plywood boats

A very common fault seen in plywood boat construction and design is that of having structural components terminate in the middle of a plywood panel. The most common case is that of thwarts attaching to topside panels, but there are many others. Sometimes the designer or builder will have allowed for this by incorporating other stiffening structures on the outside of the boat, such as spray rails, but often the structure is inadequately supported.

What frequently happens in such cases is that a crack eventually appears where the hardpoint in the structure terminates against the flexible panel.
Here is an example of what I'm talking about. The two vertical strips of bare plywood on the white topside panel are where I've been stripping paint in preparation for gluing some reinforcing frames on either side of the thwart.

In the photo above, the boat had very flimsy plywood hull panels, and the the main thwart (painted grey) terminated against the topside panel without any framing to distribute the stresses. Eventually, cracks will appear around the edges and corners of the thwart, because the plywood can move and flex, but the thwart is relatively immovable. This is a very bad example of design. What I was doing as part of this repair job was to strip paint on either side of the thwart so that I could glue and screw in a pair of frames on each side of the boat to distribute the point loads out into the gunwale and the chine joint.

Here is the result with the new frames in position. I do not like this boat, but the panel cracking problem has hopefully been solved.
In my own designs, I try to avoid any point loads from structural components which terminate in the middle of panels - here are a couple of examples: -

This is the thwart structure on First Mate. One side of the thwart is supported by the midships ring-frame and the other side is supported by half-frames which run from the inside of the gunwale down to the very strong glass-taped joint between the topside panel and the chine panel.
The stresses are gently distributed out into the structure, and both ends of the half-frame coincide with a very strong part of the boat's hull.
This is the internal structure of Periwinkle. Note how once again, the thwart is supported by either a ring-frame or a half-frame with terminates against the gunwale and a strong plank lap joint. Note also how the side deck knees run down to finish on a strong plank-lap joint.
Side deck knees in First Mate, tapered and running down to finish on the very stiff joint between the topside panels and the chine panels. The two cross-braces are just temporary, and help to hold the line of the topside panels fair until the side decks are attached. 
Modern plywood hulls can be wonderfully strong and yet still be light, but the flexibility of the material must be understood if the boat is going to last a long time.

1 comment:

  1. Rossco,
    That blue boat looks awefully like one of those dreadful $15 designs!
    A humane end might well have been the best result but it appears Dr Lillistone is working another miracle!