Step 1: Frame
When you build yours, either go with a triangular structure or the same Tee shaped frame I have done, but with hollow box beams of larger section than I have used here or just better timber. While this frame seems adequate for my weight and mild conditions, the torsional stiffness of both the longitudinal and lateral members is slightly lacking. It is mildly disconcerting to have your seat twisting around as you sail along. I reckon probably the best way to go for most people would be a triangular structure welded up from steel RHS.
Step 2: Leeward Forks
I built my forks from scrap 20x20mm galvanised RHS. A couple of extra pieces got drilled and welded on to the forks to take the pivot bolts. The uprights that the tillers are attached to need to be a comfortable length, depending on your size and the geometry of your seating arrangement. For axles, I simply welded some pieces of ø16mm steel bar into notches cut into the ends of the forks. This was simple but means the wheels are not easily removable.
I got lucky and found some steel bushes in my junk box. I welded these to some pieces of RHS, which I had drilled to allow bolting to the frame. Pretty rough, but good enough.
To make use of the way the wheels and bearings were designed, ideally I should have arranged things so the tops of the forks were inclined to windward.
The flexible linkage between the two tillers is a piece of ø25mm PVC electrical conduit I had lying around. It's about the right stiffness.
Step 3: Windward End
The fork is pretty simple, just some more 20x20mm RHS with some little notched plates for the axle to sit in. This axle was threaded like a bike wheel so attachment was pretty easy.
Step 4: Go Sailing
A few things I've discovered:
-Using these wheels on this sand in these wind conditions, I found after a while that I could steer very adequately just by moving my arms fore and aft. This makes one of the leeward wheels slide more than the other but it's easy to control how much. It feels sort of like steering a large windsurfer.
-Making radical carving turns to windward is fine, it feels like the whole thing just pivots around your body. In the other direction however, all your weight is on the outside of the turn and it is surprisingly easy to flip the whole thing over with painful results. I heartily recommend the use of helmets, wrist pads, protective footwear, kneepads and any other safety gear that seems appropriate. It's easy to go dangerously fast and sand is hard.