There are two SVG files: ColdwaterLongboard2_nolabels.svg and ColdwaterLongboard2_labels.svg. You’ll use the nolabels file to create your G-code. This file has the document set at 4′ high by 8′ long, for a full sheet of plywood oriented in a landscape manner on my Maslow CNC router (see Make: Volume 68, page 42). You can rearrange the parts to fit smaller routers.

The labels file contains all the same parts, but they’re labeled (Figure A); print this out as a handy guide for inserting the ribs into the spine. Alternatively, and if you have Z-axis (depth) control, you may decide to actually incorporate the labels into your cut paths; set their plunge depth to only 1/16″ or so.

The SVG files have the ribs outlined in green and the two halves of the spine outlined in red. Short vertical paths indicate the slots, which you may have to adjust when creating your G-code if you’ve chosen a thicker or thinner piece of plywood for your parts. So, if your plywood sheet is only ¼” thick, and your router bit is ¼” thick, then on the slot paths, you’ll want to ensure that the router is performing a follow operation and not an outline operation. If your plywood is thinner than your bit, then you’ll have to cut the slots by hand; no biggie!

You’ll notice that there are ellipses inside the ribs and spine; these are weight-saving holes in the structure, so these cutouts can be discarded.

Once you’re good with the SVG files, create your G-code and load it into your CNC platform; it’s time to cut! Load one standard 4’×8′ sheet of plywood into your machine and safely cut the parts out (Figure B). After cutting, and especially if you used tabs, clean up the pieces with a rasp or sandpaper and you’re on your way to assembly.

The spine or stringer of the board is made from the two parts outlined in red in the SVG files. You’ll “butt scarf” these two pieces together – glue the angled scarf joint together, and reinforce it with two small butt blocks of scrap wood as you would an ordinary square butt joint, for extra strength. To do this, lay the spine on a flat surface and place the diagonal cuts against each other to check for fit. Next, place an overlapping piece of wood behind it, glue all three pieces, and then glue an overlapping scrap of wood on top, using a weight as a clamp. Use a good, waterproof wood glue such as Titebond III and let the assembly dry for at least 8 hours.

Once the spine is assembled and the scarf joint is dry and strong, start gluing the ribs into their respective slots, using waterproof wood glue in each joint (Figure C). You’ll want to do this on a table, in two phases — first the forward (bow or nose) half of the spine and then the rear (stern or tail) half — because the bend (rocker) of the spine makes it difficult to glue all the ribs perpendicular at the same time. To glue them all in one pass, you could make short supports and temporarily hot-glue them diagonally between each rib and spine.

Figure D shows all the ribs in place; on top is my “sight batten” — just a long, thin strip of material I use to align the whole board or to help make fair curves.

Rip 6 long strips of plywood 8′ long by ½” wide. You can find this wood at the top and bottom of the plywood sheet you used to cut out the ribs and spine. These don’t have to look great; they’re placed on the outer edge of the ribs, and help support the foam rails that will come later. You can zip-tie them or wire them to the ribs while the glue is drying (Figure E). Align these rail stringers toward the top third and bottom third of the rib edge, and overlap them so that each rib has at least two rail stringers on its edge (Figure F).

If you’re able, run some twine through one of the holes in the spine near the bow and another piece of twine through a hole near the stern, and hang these from a rafter or ceiling. Now hoist the skeleton up to about eye level (Figure G). This is a good way to see if the frame is balanced.

If you need to tip the board one way or another, tape a small piece of wood near the outer end of a rib that must come down (Figure H); you can move the weight inboard (toward the spine) to fine-tune it. Once you’re happy, glue it to the inside of the rib where it won’t interfere with planking or rails.

Lofting the frame is a momentous occasion: You can see that this is really a creature made for a liquid medium. You can imagine how it will maneuver and feel. Sit back and meditate on how your board will look.

In most all cases, your planking will be 3″-wide clear pieces of lightweight wood, such as cedar. Decide now whether you’ll do it in straight strips that run from bow to stern (longitudinally), or diagonally. You could also try bending large sheets of wood to the curvature of the board in both directions, but this can be quite tricky.

You’ll plank the bottom first, and then the top. Planking can be done either off the frame or on the frame. Figure I shows a board I made with a plywood bottom skin, and a diagonal cedar top skin, planked on the frame.

Planking off the frame is easier. You’ll want a large, flat surface where you can glue all your planks together for the bottom, and then again for the top.

Regardless of how you choose to do it, make sure you can clamp pieces together tightly enough to get a good bond; this may mean doing only a few boards at a time. Long bar clamps are great for this, as are “come-along” ratcheting straps. Packing tape can even be used if you get it tight enough.

Planking can even be done with strips of plywood, but be aware that when plywood is sanded to match the curvature of the frame, it will create “striping” where the high points are sanded down. Still, the interesting look of sanded plywood high spots may do it for you; I think it looks fine!

Once your bottom boards are all glued together edge-to-edge, you’ll need to make a decision: fin box or no? A fin box is a plastic box with a slot that accepts a standard removable center fin. The box is placed in the underside of the board via a slot cut from the outside. Since the spine of the board runs down the center, you’ll have to cut into the spine, or get creative and mount the box off-center.

If you don’t want to go through with a fin box, you can make a nice fin out of scrap plywood and simply epoxy it on later.

TIP: Plan to make your fin while you’re covering the board in fiberglass and have a day to spare.

If you’ve created the bottom skin off the frame, now’s the time to glue it on. Take a look at the board. Whether you’re planking piece-by-piece on the frame or in one go, you’ll want to make sure that the board is not developing a twist. To knock out twist, prop up the ribs with support blocks on your table while you clamp the ribs to the planks, and place large weights on top as you’re gluing the planking on.

A hollow surfboard needs a vent to let air escape and equalize with the outside air, during temperature or altitude changes. The normal vents are small plugs about 1″ wide and 1½” deep. Cut a small block of scrap wood about 3″×3″×1½” deep and drill out a hole the size of your vent plug. You’ll put this block on the inside of your top skin. First lay the skin on the board and measure where the vent box will go — about 24″ forward from the tail and close to the spine — then glue it in place.

It’s time to put on the top skin. If you’re planking on the frame as you go, keep on going (Figure J) — just remember to put in the vent box when you’re ready. Again, you want to ensure that no twist is developing in the board. Place large weights on the top skin to iron out any warps or wrinkles, and prop the bottom with blocks, to alleviate the dreaded twist (Figure K).

When the glue is dry, clean up the skin by carefully cutting it back to the outside edge of the stringers and the first and last ribs (Figure L).

The nose block is thick, about 1½” or so after shaping, 24″×3½”×2½” thick to start. If you have a big enough piece of wood to use, that’s wonderful. Otherwise, clamp and glue up smaller scraps (cedar is great). The sensuous curves of the nose and tail pieces look amazing with laminated pieces of wood, so I always go that route.
Glue the nose block to the first rib on the bow end of the board.

Now we’re getting to the heart of my NeverSink method. If the board suffers a terrible crack, these rails will keep it afloat.
We’re using 2″-thick extruded polystyrene (XPS) foam, commonly found as “pink” foam in home improvement stores. Wear a mask when you cut or burn this material. Cutting XPS is not that bad with a sharp hand saw. A hot-wire setup works great, though the fumes are pretty noxious. A table saw is the best way to cut it, however.

Rip four 4″ wide strips of XPS, each 8′ long. Then cut a 45° angle on one end of each rail (looking down on the 2″ edge). This is where you’ll scarf the rails together, amidships. Stand the rail pieces up against the rail stringers just for orientation, with their 4″ faces vertical and their 45° cuts at the middle (Figure M). This is your basic layout.

You’ll notice the bow portion of each rail has quite a bend to it. To alleviate this and ensure that the foam rails don’t crack when we’re torturing them to this curvature, we’ll use kerf cuts. Using a sharp saw, knife, or hot wire, cut notches about 1″ deep from outboard to inboard, about 2″ apart, from the nose to about 30″ back (Figure N). These kerfs allow the foam to bend without over-stressing and rupturing its skin. You can use this technique on wood as well!

Now you can glue the rails to the rail stringers.

Just as you crafted the nose block, now craft the tail block. You can place this on the board before bringing the rails in, or after bending them around the stern-most rail curvature. Glue the block tightly to the last rib; you can run ropes or straps along the long axis of the board to cinch it tight.

Now you can cut down and shape the nose, rails, and tail to the profile of your choice using a fine handsaw, then the Surform rasp, a hand plane (Figure O), and/or varying grits of sandpaper. If you use a hot wire to shape the foam, beware of slicing away too much!

Perhaps the easiest and best rail profile is a D-shaped cross-section that has the same curvature on top and bottom (“50-50” rails). Of course you can try alternate profiles such as a “pinched” rail, but these are for specific performance aspects.

I won’t go into all the wonderful traits of epoxy, or exhaust the many ways in which you can lay up your board. Suffice it to say, you need at least one coat of epoxy on the board to seal it. First, patch any gaps in the planking or rails. You can mix some fine sawdust with epoxy to create a patching compound the consistency of peanut butter.

To seal the board, apply the epoxy with a foam roller or carefully with a brush. Beware of drips, as these will cause sanding headaches later!

Optionally, you can put a layer of fiberglass on the top (deck) and/or bottom of the board for extra abrasion resistance, but you’ll have to fully “wet out” the fiberglass cloth and then add at least two or three layers of epoxy to fill up the weave.

The XPS foam rails must be covered with fiberglass for strength. First, you can fair the rails (spackle any gaps) with simple drywall joint compound. After fairing, coat the rails with epoxy and then lay the fiberglass cloth on the wet epoxy. Rails, nose, and tail blocks should all have a layer of 4oz cloth that overlaps the joint between rail and skin; I recommend glassing the entire deck while you’re at it (Figure P). “Wet out” the cloth fully with epoxy, smooth out any bubbles, and clean up any drips.

When this first laminating coat is dry (Figure Q), you’ll want to brush or roller on at least two more coats of epoxy on top of this, to completely fill the weave and create a smooth surface for sanding. Plan on a few days.

The surfboard fin or skeg is a simple creation that you can cut out with a jigsaw or your CNC. Typically, these are about 8″ long for a board this size and there are many shapes; search the internet for “center fin” and you’ll see the dizzying variety. For a good all-around fin, a large “slanted D” style, which looks pretty fish-like, is fine (Figure R). Make this out of two pieces of ¼” plywood laminated together, and sand down the leading edge to a graceful, blunt curve. Sand down the trailing edge to a sharper profile, but don’t make it knife-sharp, as that will actually hinder performance.

Cover the fin in a coat of epoxy, or optionally, fiberglass it too.

Epoxy the fin on the bottom of the board, about 8″ forward from the tail (Figure S). For strength, create a fillet from epoxy and sawdust and apply it all around the board-to-fin joint with a gloved finger.

TIP: Come back in an hour or two and dip a gloved finger in denatured alcohol and then smooth the fillet joint; it does wonders and cuts down on sanding later.

You’re getting really close now! Since the rails are that unsightly pink foam and drywall compound, they won’t look all that great. Paint them, and any other areas you wish, with a good all-purpose paint for plastics such as Krylon (Figure T). Go slowly and sand between coats to get it smooth.

Finish work — you love it or hate it. Here you’ll sand, paint, sand, and sand some more. Make sure you’re not sanding through any fiberglass cloth! Add any extra designs, if you like.

As epoxy is inherently not UV-stable, you’ll want to varnish over it. Pick a good spar varnish, go slowly, and avoid lumps and sags (Figure U). You can varnish over the painted rails as well, though it may impart a slight color change. I sometimes add pinstripes and I varnish over these for extra protection; the coloring actually is quite pleasing.

You’re done! Sure, there may be odds and ends, and sure, the overall curvature of the board may not be exactly perfect. But it’s close enough, it’s beautiful, and it’s yours. You have come a long way and put your hand into many different techniques from CNC to artistic joinery and composite hull construction. Hooray!

Once your board is dry, you can create a paddle (I like this one) or just go out and hit the waves.

I recommend you don’t let your SUP sit in the hot sun or a sealed-up vehicle for long, if you can help it. But the vent really works well and should mitigate any problems.

Keep your board clean and keep grit from grinding into the varnish coat. If it gets a lot of use, varnish it again every couple of years.

As always, think safety. Take a class or get instruction. Wear a life-preserving device and never go too far from a safe landfall. Watch the wind and weather, and have fun!