Justine’s plans do not give any information about the planking, save for specifying the plank thickness as 5/8″ and providing a full-size drawing of the molded sheerstrake’s cross section. I had to turn to a variety of resources to decide how many planks I would use on each side of the hull, and what the plank widths would be along the entire length of the hull.
One useful resource was Maynard Bray’s book How to Build the Haven 12 1/2 Footer, which describes building a smaller, but very similar version of the Flatfish. The book includes a table of plank widths at several stations, and indicates that the Haven was built with 10 planks on each side of the hull. I did a simple calculation: I measured the total width of the 12 1/2 Footer’s planks amidships, and also the total width of what would be needed for Justine’s planks amidships. Justine’s total width was about 30% larger, so I decided I’d use 13 planks on Justine.
I read several books about general principles for lining off planks. An important factor concerns the need to “back out” (give them a concave shape, which I did using a home-made plane) the inside of planks that fit over frames that are curved. Generally, the higher the curvature of the frame, the narrower the planks should be. Otherwise you would need to begin with overly thick planks in order to provide enough material to end up with a curved plank that was 5/8″ thick over its entire width.
In addition, even if the hull were flat, you would not want to make the planks too wide. Wood absorbs and releases moisture to adjust to its environment. It swells as it absorbs it, and shrinks when it dries out. A wide plank that is firmly fastened down can generate high internal stresses as it swells and contracts, and excessive stresses can cause the plank to crack. Narrower planks with caulked seams that are not too tight leave some “wiggle room” for the planks to take on moisture when the boat goes in the water, and dry out some when the boat is hauled out for the winter. (Plywood planking is much more dimensionally stable, so if a boat is constructed with plywood planks, the planks can be wider.)
So one needs a rule about the maximum width of a plank, and a suggestion I had recommended that planks be no wider than 8 times the plank’s thickness. For Justine, that meant no plank should be more than 5″ wide.
Because the girth of the fitted planking is largest amidships and decreases toward the bow and stern, the planks need to taper in width as they approach the ends of the hull.
With these guidelines, I began to line off the planks on the frames using pieces of string so that the lines would be plainly visible and easily adjusted to take fair curves.
The position of the sheerstrake’s top edge (on which the decking rests) was marked very clearly when building the construction molds: the top edge of the “cross spall” on each mold was placed at the sheer line. So in lining off the planks, you are deciding how to divide the distances between the keel plank (or stem far forward) and the sheer line into 13 segments of unequal width.
Even though a carvel-planked hull is smoothly curved, because the plank seams are caulked, the plank seams generally remain visible and become a significant feature of how the boat looks. So not only do the individual lines need to be fair, they need to be visually pleasing as a set of not-quite-parallel lines. Ideally, the entire set of lines needs to be “eye sweet.”
Once I was satisfied with the way the strings looked, I made pencil marks where each string crossed a frame. I removed the strings and used a batten to fine-tune the locations of the marks so that each plank edge would be very fair. I completed the lining off in July 2010. I finally was ready to “get out” some planks and “hang” them!