The planking on Alita is a combination of single planking (tight seam construction) for the first eight planks and double planking from plank 9 to the sheer. The single planking is made from Amendoim, a South American hardwood similar to H. Mahogany. The double planking is made from Spanish Cedar for both the inner and outer planks.
When I took delivery of this boat, I was under the impression that the planking was sound and that most of the work to be done was cosmetic – even after having the boat professionally surveyed. Unfortunately, this turned out to be not the case.
After removing some of the paint, it became clear that there was a problem, so I removed the first 5 planks (upward from the keel rabbit) on both sides of the boat. The planks from the port side are shown in the pic below.
These next three pics illustrate some of the damage.
According to a forest products professional, the (single) planks suffered from compression failure. This is not at all surprising given the species of wood and the method of construction. Consider the following:
- In tight-seam construction the edges of adjacent planks are in full contact. So when the planks swell after launching, the planks are subjected to tremendous stresses, which actually crushes the wood. This is especially true for hardwoods, which have a high modulus.
- Wood is relatively weak under compression parallel to the grain.
- Repeated swelling and drying (due to extended haul-outs) exacerbates the problem.
Because of these problems, I decided to replace or repair all of the single planks. I’m also adopting traditional Carvel-plank construction; i.e., with caulking between planks. In Carvel planking, only about 1/4 to 1/3 of the edge width makes contact with the adjacent plank. This should reduce the extent of damage due to compressive stresses.
The double planking suffers from a yet different problem. In double-plank construction, the outer planks overlap the inner planks such that the seam between adjacent inner planks is covered by an outer plank. A sealant (typically shellac) is applied between the two layers.
This form of construction is inherently water tight. Any water that might enter the seam between two outer planks will immediately encounter an inner plank covered in sealant.
As the planking swells after launching, the plank edges are crushed. This in itself is not a problem; however, if the boat is left out of the water too long (as this boat has), the planking shrinks. As the outer planks shrink they apply tensile stress to the inner planks causing them to split.
It is unclear whether or not these split planks will be watertight after the boat is launched, so at present I’m planning on replace the double planks below the waterline.