Plywood and lumber

Plywood

Plywood is the most critical component in a stitch and glue boat. There are various grades of plywood and international standards on how they are graded. "Marine grade" plywood is the one you must use for boatbuilding because it has fewer "voids" between layers which makes it more resistant to water. Dutch marine plywood is universally accepted to be the best followed by British plywood. Professionals also agree that plywood conforming to British Standard BS1088 is better than those conforming to the equivalent US "AA" standard. Cheaper marine plywood from Israel and southeast Asia is also said to be internationally available but I have not read anybody's experience on these.

Some people argue that "exterior grade" plywood will also do, at least for small boats, provided you religiously seal it using epoxy resin. Exterior grade is one grade lower than marine grade. Considering the cost difference between marine grade and exterior grade, I am inclined to think that exterior grade is suitable for dinghies. Build anything bigger, I would not take any risks and go straight for marine grade. Marine grade also has many subdivisions like fir, okoume, sapele, etc. depending on the origin of the wood but this is too much detail I think.

This is what the theory says, now comes the practice. Not surprisingly, I was unable to find marine grade in Ankara where plywood, if used at all, is consumed only by the furniture industry. I therefore had to examine what was available and opt for the best quality plywood I could find. About 3 months after buying the plywood I learned that marine grade has become available. Of course, which standard it conforms to, or even whether it conforms to any standard at all are question marks. What I bought was okoume plywood, seemingly similar to exterior grade. Since then, I have left a few pieces of scrap plywood out in the open to see how it performs. It has been almost 6 months now and the plywood has been exposed to sunlight, rain, snow and temperatures between -5 and 40 degrees Celsius. It is still holding well (no checking), so I suppose the plywood was good enough.

The plans of the D4 ask for 1/4" plywood for the bottom and side panels and 3/8" plywood for the bulkheads and seat tops. The closest I could find were 6 mm and 8 mm, respectively. Both have 5 layers. In general, of plywood of the same overall quality and thickness, prefer the one with the greater number of layers.

One problem probably unique to Turkey was the dimensions of the plywood sheets. The international standard calls for dimensions of about 4 ft by 8 ft. (I think 125 cm by 250 cm in metric speaking countries). The sheets that I bought had dimensions of 170 cm by 220 cm. While the larger width causes no problems, the smaller length is a potential problem source. The reason is that all designers base their designs on the standard sizes. You might have noticed that all small plywood dinghies (including the D4) have lengths smaller than 8 ft, because their panels are cut from a single sheet of plywood. Otherwise, you have to butt-join or scarf-join plywood sheets to obtain the required length. The forrmer is not aesthetically and functionally pleasing, and the latter is painful, to say the least. The side panels of the D4 are close to 240 cm in length as can be observed in the plans sheet shown. To avoid joining plywood, I had to draw them diagonally on the plywood sheets (thanks to the larger width). This resulted in some wasted material, but solved the problem without much pain. Overall, 3 sheets (170 by 220 cm) of 6 mm and one sheet of 8 mm plywood was enough for the sailing version of the D4.

Lumber

Dimensional lumber is required for the gunwales, seat stringers, breasthooks and skeg in the rowing version of the D4. For the sailing version, you additionally need lumber for the mast and spars. I think choice of lumber is not very critical for a boat of this size, and you can use anything you get hold of. Try to find clean grained wood and be sure to avoid any knots in the mast and boom (it can break there) and gunwales (it will bend awkwardly there and create a hard spot). I used chestnut for its water resistance. If I were to build the same boat again I would use pine because it is cheaper. Since the dimensional lumber on a boat is usually bright finished (varnished and not painted), color of the wood may also be a decisive factor in your choice unless functional requirements dictate otherwise.