Playing With Boats

As I mentioned in the last post, I’ve started to explore whether the 1893 sailing canoe design Isalo would be suitable for re-designing for construction in modern materials. One of the first big questions to is how to build the boat.

If I follow through with the idea that this cruising canoe and the 16-30 are “bookends” that between them cover a nice range of sailing canoe history, then it would be best if Isalo was rendered in stitch-and-glue plywood. This has worked well for the 16-30s, proving to be light and durable and reasonably quick to build.

The body plan for the 16-30, showing its single chine hull made up from four separate stitch-and-glue panels.

The original 16-30 was a perfect choice for stitch-and-glue construction, with a single hard chine and a V-shaped bottom. The only difference from the original to the new version was that the plank keel was replaced by a filleted epoxy joint.

An interior shot of the original 16-30, showing the top of the plank keel. The bottom planks converge in a slightly rounded V-joint on the bottom of the hull.

The interior of the plywood 16-30, showing the filleted and taped seam on the interior bottom.

Our original 1893 canoe, however, has an arc bottom, much like a Star-class keelboat.

Body plan for Isalo from the original 1893 drawings.

You could likely make the plywood conform to this shape, but not in stitch and glue. In order to get the compound curve in the bottom panels, you’d have to introduce some interior framework, as on the 15 1/2′ sailing canoe Zephyr, and this would take us away from the idea of making both canoes as similar in construction as possible.

The arc-bottomed sailing canoe Zephyr, published in Yachting in 1925, was designed for traditional batten-seam plank-on-frame construction.

There are (at least) three possible ways to tackle the bottom of the new canoe, starting with the original lines. Here’s a sample section.

The original body plan at midships, with an arc bottom.

Option one is to take all of the arc out of the bottom and connect the chine and keel with a straight line, as on the 16-30:

Bottom section straight from chine to keel, with the dotted line showing the original arc section.

As you can see from the sketch, this takes a fair bit of volume out of the bottom. It might be ok, but I wouldn’t know until I did some hydrostatic calculations. Another option would be to introduce a second chine below the waterline.

The straight section from keel to chine has been broken into two parts by the addition of a second chine, with dotted line showing the original arc section.

This option preserves more of the original volume, but we’re now up to six hull panels, which will be more work to construct. Another way to do this is to give the canoe a narrow flat bottom panel:

Introducing a flat bottom panel equals or exceeds the original volume of the arc-bottom hull.

There are a few reasons why I like this option: it only needs 5 panels; it will make the boat easy to beach; and finally, the flat bottom will give a good solid anchorage for the centreboard trunk and mast steps without the complications of fitting them over a filleted centre seam. I built and sailed a little canoe with a hull like this for several years in the 1990s and it worked quite well. It was designed by John Bull, who used to own Solway Dory in the UK. He’s since retired, but the company continues, though I don’t believe they offer this design any more. The only drawback is that you need to install some floorboards so that you’re not sitting in any water that happens to come aboard!

Next step is to lay in the flat bottom on the original hull, re-draw the lines and do some calculations.

Until then. . .