Planking thickness?

[Bob Ketterling]

I am building a 14’ sailing dinghy, Gannet,designed by Ian Oughtred and I need some advice on plywood for planking. The plans call for “¼” mahogany plywood for a “light boat, reasonably carefully handled. 5/16” is recommended for a really strong hull.” I think I would opt for a “strong hull” over one that needs careful handling but I can’t find 5/16” ply. I wrote to Ian and asked him type of plywood to use and he replied "1/4" ply seems fine if it is good stuff. Okoume would be too light - unless 3/8". But no builders have reported any problems. My 18' JII is 1/4" larch ply - has been well tested.” I have written back asking for clarification but while I wait for his hand written reply to come back from Scotland I though I would ask you all. Is the good stuff he refers to Sapele? Would I be better off with ¼” Sapele or 3/8” Okoume? What do you guys recommend for durable, reasonably light glued lap strake hull in a boat of this side?

[Jay Greer]

Five sixteenths is roughly eight milimeters. Since I rarely use plywood in boat construction I can't offer you a source. However, you might consider this. The equivalent stock in white, Alaskan or Port Orford Cedar would be lighter, easier to work and less fuss to maintain if you plan to finish the hull sans glassing. All in all, you would have a much lighter boat if you chose to build it of solid stock rather than plywood.

[Bruce Hooke]

I do not think it would be wise to replace plywood planking with solid wood planking without making other changes to the design (or at least checking with the designer on whether such changes are called for). In traditional construction the frames are there in part to deal with the fact that solid wood is not very strong across the grain (i.e., it is prone to splitting). In boats designed for plywood the number of frames is often greatly reduced (or eliminated completely) because plywood is strong in both directions.

[Bob Ketterling]

Jay,
I appreciate the suggestion but I plan to stay with plywood of some type. I think going with solid woood is too much of a change of the design. My previous boat projects have been built with Plywood and am more comfortable working with that type of material.

[Graham Knight]

I'm building Iain's Acorn 13 skiff using 6mm (near as dammit 1/4") Okoume (Gaboon) ply. He suggests 5mm ply is adequate or 6mm for a really strong boat, but he doesn't really specify what type of ply he's referring to, just says that Okoume will result in a lighter boat.
Well I didn't want a heavy boat, and 5mm was harder to get and expensive, so I opted for the 6mm in Okoume which is the same weight as the 5mm would be in Meranti.
I'd suggest if you can get 6mm Meranti, or a ply of similar hardness such as the Sapele which you suggested, then that ought to be adequate. I guess Sapele will be expensive though as it is over here.
I personally wouldn't want to go to 3/8", you may find it difficult to bend on a boat of that size, I know it would be on my skiff which has quite a bit of twist in the lower planks.
FWIW Iain provides a materials list for building the Acorn using traditional construction (solid timber instead of ply) and recommends using 8mm for the planking, which he says will result in a boat 20-25lbs heavier. 1/2" x 5/8" ribs are also required every 7 inches.

[Bob Ketterling]

Graham,
I understand you concern about twist in the planks when using 3/8" material. I use 3/8" Okoume in my last boat (a Joel White design - Pooduck Skiff) and I was amazed at how much pressure and torque I had to put on some of the planks. I thought for sure they were going break but they held together quite well. I am considering using Sapele. It is much more expensive and also heavier the Okoume. In the end compared to the cost of the rest of the boat and the time invested the extra cost in materials for the planking would not matter too much.