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Chine Corners

Some plywood boats (like the W17) do not use a wood corner member at the lower chine, preferring instead to make the corner with a large radius of epoxy filler that permits easier interior cleaning, improved continuity from side to bottom and better sealing against water intrusion. But during construction, this means that the lower plywood edge is unsupported and this can cause a problem of fairness, especially when 3-ply sheets are used—as would be the case with most ply under 6mm. Check out this photo of an AMA using 4.5mm plywood and you'll see the potential issue. Although the ply lies fairly on the stringer and gunwale, the free edge is left to warp, depending on the stresses and wood used in the plywood itself.

Clearly, it would NOT be smart to lay on the bottom panel in this state … so, what to do?

While laying on these side panels, the builder correctly clamped on some thin but supportive battens over the full length, but these were then removed, not only to show this image but also, to fair the lower edges and free the space of clamps in order to add the bottom panel.

So here's what I suggest to do in the W17 Build Manual. Add temporary full-length battens to the outside of the bottom edges, just low enough (about 20 mm down) that one can still plane the ply edges and also clean up the excess epoxy from the joint.

To hold these battens close to the plywood, I use steel clamps temporarily while I 'hot-glue' the batten to the exterior of the ply, tacking in short lengths from both above and under. Even the standard glue stick seems tough enough to hold thin plywood, unless you are working in a very hot climate that might then require the higher strength glue stick.



Once set, the clamps are removed except for the end ones that hold much of the tension as these can be arranged to clear the bottom panel. The closing panel can then be pre-fitted and then epoxied in place. As for the bulkheads, I would also bevel off the edges of the sides, so that there's little risk to squeeze out all the epoxy filler in error. Once cured, the hull can be turned over and a large 10 mm radius fillet applied to the inside (after sanding the corner out) and a bias cloth tape laid over it of adequate width.

Once that fillet has cured, the exterior can safely be finished without risk of disturbing the bond.

NOTE: As the added batten has also proven useful for holding the duct tape used to bond the bottom panel, the underside needs attachment more than the top. So only ½" (12 mm) long tacks seem to be needed at the top. This also makes them easier to remove—I use the back of a small chisel for this. See more on this under Building a Narrow Ama.


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