Building Heidi

I'm set to begin building Richard Kolin's Heidi,a 12' traditional built lap strake row/sail skiff.The plans call for half inch thick timber planking.Due to difficulty in obtaining the right timber and/or cut,i've desided to cut the planks from marine ply.It will be held together as per plans(clenched nails,or in my case copper rivets) I have been told that if using ply i can go thinner than half inch.Does anyone here have any thoughts on this?

 

Should be possible.

Glued lap plywood skiffs are common. Fasteners are avoided with plywood. The goal with ply is no water ingress into the ply. Completly sealed with epoxy...no or very few fasteners

Half inch ply is robust but may not be flexible enough to bend to the shape of a small boat. Perhaps 9mm is better

Why not review the scantlings. of the many glued lap ply skiffs on the internet for guidance.
Alsp many small craft builders on boatdesign net that can help woth the scantlings

 

I can't get excited about using plywood instead of timber and fastening it with rivets or clench nails. I repaired a few of those years ago. That method allows some movement between the strakes ... a good thing with traditional planking. Not so much with plywood.

Movement between the strakes and thousands of small holes will eventually let water into the plywood where it will cause headaches. Glueing the laps would be better and converting to stitch and glue better still.

I know some have fiber-glassed the outside of lapstrake boats. I think getting glass around and into all those corners would prove very, very tedious. Well lined off stitch and glue has the most of the beauty of lapstrake, and takes advantage of modern materials.

 

What types/sources of timber did you consider ?

 

This is incorrect. You can't go smaller then a solid wood scantlings when substituting with plywood. The rule of thumb is the same size on small craft and slightly thicker on large craft. This is because plywood is about 1/3 weaker in longitudinal stiffness, compared to solid wood planking. This assumes a traditional build over frames and mechanically fastened.

Now that I've said this, if you epoxy the seams on the strakes, you can reduce the planking thickness to 3/8" (9 mm) plywood. You can also toss out 1/2 of the frames too. If memory serves me, the frames are on about 18" centers, so there'd be about 7 of them. You can reduce this to 4, with one forward, two bracketing the thwart and one aft, under the forward edge of the seat. You can do this because the glued laps (epoxy) create a longitudinal stringer at the top and bottom of each strake, greatly increasing the longitudinal stiffness of the hull. Also the glued seams create a single, monocoque structure, which is quite unlike a riveted or clenched build.

Stitch and glue is imposable on a lapstrake hull. You have to make it a carvel, then tape the seams, which ruins the look of this boat, not to mention makes it a fair bit heavier.