DIY Retro Twin Fin, Part 3: Rails & Tail

Special care should be taken with the nose and tail. For the most part these sections are much thinner and will break with a heavy hand. I took about a ¼-inch of foam off the thickness at the middle of the board but more than an inch at the nose, leaving it only a ¼ inch thick at the very tip, so sanding took some time. I planed the stringer down first, taking care not to snap off the end, then sanded down the foam using uniform perpendicular strokes from rail to rail, leaving a nice smooth, lightly curved surface.


When it came to the tail, I cut a deep 6-inch swallow tail. I planed the tail down to about an inch thick, tapering down to a blunt point at the end, and hand-shaped the rails on the inside of the tail. I kept the rails pretty routine, rounding the top but leaving a pretty hard edge on the bottom. I carried the overall curvature of the tail from rail to rail, almost giving it a domed shape.

Working the rails.

Once I was at a comfortable spot on shaping, leaving still a bit to go, I cut the rails with the Fred Tool. The Fred Tool is essentially a block of wood with a 30-degree blade off one side. I determined how deep I wanted the rails to come—roughly an inch and a half on the top. Marking small pencil marks along the board every six-inches or so. I then took the Fred and ran it along the board, cutting to those points. I then flipped the board and marked about an inch in from the edge, tapering off about two-feet shy of the tail. I then ran the Fred, cutting to the pencil marks. Once the rails were cut, leaving a triangular/trapezoidal shape, I began rounding the rails by hand with some 120-grit sandpaper, tri-folded. Walking nose to tail with long continuous strokes, I gently shaped the rails, following the contour of the board, making sure to only round the top of the rail of the last two-feet, hardening and flattening the bottom.


Even with a light touch, there will be some wonky spots along the rails. To correct, a long block sander with flex is key. This will span the dips and smooth everything flush while following the general curve of the rail. The rails are one of the most difficult parts to shape, especially for first timers. A simple tip is to count out the strokes when sanding—sand ten times, flip, sand ten-times. It is not perfect, but it will help keep the rails symmetrical. And if one were so inclined, a simple sharpie line the length of the rail—dead in the middle—if done smoothly, can be quite helpful, showing the dips and imperfections. Smooth out and lightly sand away the line.

Finding imperfections under light.

When the board looks good to glass, it probably isn't. A good way to find the imperfections and hollow spots is place the board over some lights shining upward. Bring the board inside on a rack, then set some fluorescent lights below, just a few inches below the board, along the rails. Step back and get low, equal with the board and it is pretty easy to see all the imperfections. Mark them with pencil, flip and repeat. Sand and shape, throw back on the rack and flip the lights. Repeat as needed.


For some hollows, simply running a hand along the board is all it takes to find the spots. It helps to feel the board without looking, using just the sense of touch.

Sanding the channel.

Also, a second or third pair of eyes would not hurt either. It takes some time to notice the imperfections put into the board by your own hand, and it can be a bit of an ego blow when someone points something out immediately, but in the long run it is worth the minor embarrassment. So don't get defensive!


As I mentioned in an earlier post, I added a groove underneath to try and channel water for added propulsion and stability. I sanded on both sides of the stringer until I got to a decent depth, about a half an inch, and feathered it out toward the rails. I ran this channel about two-feet up, tapering to even (essentially like a half cone, flattened). I used the Surform Pocket Plane to bring down the stringer, and cleaned everything up with some 220-grit sandpaper.


Once the board has been shaped, run a level perpendicular with the stringer in places where it is intended to be flat. Smooth any dips found by lightly sanding with a block. In spots where more blending needs to be done, especially on the deck and the bottom, do so with a longer block sander that has some flex. When it comes to rails and tail, do so by hand with an extremely light touch—try to let the natural curve of the board works its way out. The tail was particularly difficult for me, taking off more on one side, then the next. I remedied this with the florescent light, then hand sanded until the tail was uniform, touching up with the large block sander.

Working the tail on the twin fin.

In the end, when it comes to shaping there is not much technique involved other than be conservative when eating into the foam and be mindful of the next step, it will help keep from whittling the board down to a tooth pick. And take your time. The smoother and cleaner the shape pre-glassing, the easier it will be to glass later, and the better it will ride in the surf. Remember, it's all about the ride, enjoy the Shore Life!