The process of planking the hull, or bottom, of the paddleboard is similar to planking the deck but poses a few extra challenges.
Because the deck has been installed, there is no way to use the shrink wrap plastic to hold the strips in place, so you’ll need to get a little creative in holding the strips together as the glue dries. The instructions suggest using a combination of weights to hold the strips down against the deck, and clamps to hold the strips together. When you get to close to the edges you will need some really big clamps! When we couldn’t use clamps anymore we used packing tape to hold the strips against each other. Another possibility is to make a bunch of “C” shaped pieces out of plywood and clamp them against the ribs to hold the strips both against the ribs and against each other, as shown in these images from my cedar strip baidarka build.
Since you will be working with the board lying free and not secured to the workbench, possibly on soft blankets like we did, it is easy to introduce a twist to the board if you are not careful placing the weights evenly and making sure the board is balanced side to side.
Another concern is although the hull is “flat” in the sense that is does not have any side to side curvature, it does have a lot of rocker. The strips will need to be pre-bent with a heat gun to follow this curve, especially near the nose.
The length along the curved bottom of a 12 ft board will necessarily be greater than 12 ft, so your 12 ft strips will come up a couple inches short. This extra distance will be made up for by the nose and tail blocks.
A less resource- and time-intensive way to finish the bottom is to use plywood. I would probably use 1/8th inch luan doorskin that I could get readily at a big box hardware store, and scarf it with butt joints to make a 13 ft x 32 inch panel. Painting it instead of leaving it bright-finished would hide the joints. Plywood will lack the beauty of cedar strips but there are unlimited possibilities for painting designs on it, or even gluing on a patterned Hawaiian fabric under the fiberglass. The nose end would definitely need to be pre-bent, maybe by soaking for a few days with weights at the end. The instructions suggest this as an alternative to stripping but I decided against it because I was uncertain about the practicality of pre-bending the plywood.
Fin Box Support
After the board is finished we will install a fin box about 7-8 inches from the tail. In order to mount the fin box we will need install a solid base for the box. Katya decided to use blocks of balsa from a hobby store for this. They are glued to both sides of the central spar in the three chambers between the 1st through 4th ribs, using epoxy thickened with wood flour. The blocks are planed to match the curve of the bottom of the board, and the strips glued directly to them.
Many paddle boards also feature a notch in the deck which serves as a hand hold to carry the board under your arm. We glued 1 1/2 inch thick blocks of scrap red cedar along both sides of the central spar and against the deck in the middle of the board’s length. Like the hole for the fin box, the notch will be cut into the blocks using a router after the board is glassed.
Bottom Rail Strips
Remember the rail strip we installed on the top edge of the ribs? Before we start stripping the bottom we need to install another one along the bottom edge. The rail strips are 1/4 in x 1/4 in strips that run along the bottom edge of the ribs. They define the edges of the board and provide support for the ribs while stripping. Since the distance around the edge is greater than 12 ft, the rail strips need to be scarfed from 12 ft stock, with the scarf joint in the middle where the curve is minimal. Just as before, we glued them to the ribs with superglue and taped them in place, then removed the tape after the superglue dried and reinforced it with Gorilla Glue. The instructions say that the rail strips are not “structural” supports. But knowing what I know now, that the rail strips potentially can contribute to the structure of the rails, I would have probably made them thicker, like 3/8th inch x 3/8th inch.
Just as in planking the deck, the first strip on the bottom is installed in the center. The nose end needs to be pre-bent to follow the curvature of the bow using a heat gun. Gorilla Glue is applied to the ribs and central spar after the surfaces are moistened, and the strip is positioned and clamped at the nose and the tail. Several weights (books, cans of paint, concrete blocks, etc.) are placed along the strip to hold it against the ribs and central spar.
After the first strip is installed, strips are added to the sides of the center strip, after pre-bending with a heat gun. The strips are clamped together, and weights placed to hold them against the ribs. As the stripping progresses toward the edges and the distance across them becomes to big for the clamps, you will need to use tape to temporarily hold the strips together as the glue dries. Katya said that stripping the bottom took much longer and was much more challenging than stripping the deck. It takes a lot of weight to keep the planks in place. In the video below she talks about installing the first central plank.