Double Diagonal Planking

After some thought, I decided to build the dinghy with double diagonal planking instead of standard carvel planking. The reason is simple, the dinghy will live upside down on the deck where the sun will beat down on it and cause the wood to dry out and shrink. When we decide to go to shore, we would put the dinghy into the water and the seams would leak until the wood swells. Swelling is not a quick process, usually taking several hours to several days to occur. Since the dinghy would never be properly swollen, we would be plagued by a leaking dinghy every time we wanted to use it. This would mean that our feet would get wet when we go to shore for dinner!

The alternative is to plank the hull in plywood, or in double diagonal planking. I am morally opposed to the use of plywood, since you will never find a tree that grows as a 4x8 sheet! The only option left is to plank the hull with double diagonal. 

Double diagonal planking offers the same advantages of plywood, but with better quality wood. Plywood is made by skimming off layers of a spinning tree that are then glued together. As the tree is spun, voids in the tree will become voids in the wood. Plywood allows for large holes to exist in the plys, since the glue that holds them all together will fill in the voids and offer some added strength. The reason I don't like plywood is because of the glues that are used. 

Marine plywood is made with better glues than regular plywood, allowing it to survive in an immersed environment. To pass the quality control test for marine plywood, a sample is repeatedly boiled and then subjected to shear forces. If the glue is weak, the plies will separate as the wood comes unglued. 

My big gripe with plywood came when I used a scrap piece of marine plywood as a cleat in our shower on Wisdom to hold the box I built around the mast (which runs right through our shower). After six months of being in the shower, the plywood started to delaminate! The shower is a wet environment, but the wood is never subjected to "boiling" temperatures or much stress at all. The plywood is screwed to the wall, further supporting the plies. Seeing a piece of marine plywood delaminate in rather mundane circumstances made me have very little faith in plywood as a whole. While this is a personal opinion and an isolated event, it has left a lasting impression on me. With these preconceived notions in mind, I did not want a scrap of plywood in the dinghy! Everything had to be made out of real wood, wood glue (no epoxy), and fasteners. 

Double diagonal planking results in the creation of plywood. The first set of planks are laid down in one direction, while the next set of planks are laid down 90* to the first set. The planks are fastened together and a bedding compound between them fills in any potential voids that may exist. As with plywood, the wood used doesn't need to be perfect, nor very strong. The combination of the planks running in opposite directions will combine to produce a very strong skin made out of better quality wood. 

More so, double diagonal planking can be repaired much more easily than plywood. If a plywood hull rots, that entire sheet of plywood would need to be replaced. With double diagonal, only the rotten plank needs to be replaced. 

Since the planks are laminated in opposing angles with bedding compound between them, there is little chance that water will work its way through the hull as the large bedded faying surfaces will provide a good seal. Polysulfide bedding compound used between the laminates will remain elastic, allowing it to move with boards as they shrink and swell. This will let us place the dinghy in the water and hop into a dry dinghy as we make our way to shore.

The other advantage of double diagonal planking is the planks run in different directions with strength along the direction of their grain. Since the planks are running in different directions, the strength also runs in different directions. Carvel planked hulls will have diagonal strapping cut into the frames to offer a similar benefit because the horizontal planks do not provide this support. The diagonal planks in double diagonal planking do, allowing you to actually use thinner planks to create a thinner skin than with carvel planking.

The biggest downside to double diagonal planking is that it is more time consuming to install. The hull needs to be planked twice, and a lot more bedding compound is needed as well. Bedding compound is rather expensive, but it's hard to put a price on comfort! 

The second biggest downside is when repairs are needed. A rotten strake on a carvel planked hull is simple to repair, simply remove the plank and replace it with a new one. On double diagonal, if a plank on the inside begins to rot, all the outer planks that cover that board need to be removed just to gain access to that board. This adds considerable labor to the process, and the costs go up as well as new bedding compound will be needed to re-bed the entire area that had to be opened up.

In the long run, there is no simple option and no magic bullet. Every choice has its benefits and drawbacks, its just a matter of choosing which option works well for you and your needs. For us, with a dinghy that will live on a deck most of its life drying out and only going in the water for short periods at a time, double diagonal planking fits our needs and we are willing to work with its drawbacks.

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