STEM Challenge #58: Cable yachts

Sun sparkling on a blue sea, waves lapping on the shore, and triangles of white sailing across the scene. It’s lovely! But… you’ve had an hour or two to admire it and you want something else to do. Why not try making a yacht for the beach?

Now a model sailing vessel is a tricky thing to sail through waves breaking on the shore. And a sand yacht needs a big area of flat sand and big wheels to cross the sand, so that’s tricky too. Instead, try out the Cable Yacht, a yacht that sails down a wire in the air…

You’ll need pulleys, some wood or Meccano, 20 or 30 metres of washing line or other thick string, a small weight, and something to make a sail with. Mount the pulleys so that if the yacht bobs up and down a lot, it’ll stay running on the pulley: you can even use a pair of pulleys, one above the other, to do that.

The sail can be the traditional triangle of a flexible material like cloth with a mast and boom, or it could be an aerofoil cross-section conveniently made from foam. Simpler, as used here, is a piece of plastic signboard, the lightweight kind (often seen on houses for sale) with a corrugated sheet inside. Reinforce the leading edge with a thin piece of wood or Meccano strip taped to the board. Whatever you go for, make the top and bottom similar, so that the yacht will stay roughly upright in the breeze.

Mount the sail at 45 degrees to the direction of travel or make it adjustable. And to keep it upright, add some weight to the bottom of the sail, or on a side-boom. A pleasant feature is a sail that can be switched by 90 degrees when changing the direction of travel so that you and your co-pilot can send the Cable Yacht sailing back and forth easily.

All finished? Time for a test voyage or two! If suitable trees or posts are available, you can tie the cable to them. Alternatively, you and your co-pilot can just hold the cable. Make sure that you tension the line enough to keep the yacht from touching the ground at the halfway point. If you put the wind at right…