Summer STEM Challenge: Sailing down a wire in the air
Image credit: Neil Downie
Why not delve into some more engineering-related experiments when you have some free time? This week, Neil Downie shows you how to make a yacht that sails down a wire.
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 angles to the line, this should give you a good speed in both directions. A sea breeze, often about right angles to the beach, is perfect.
If you are trying your cable yacht in a stiff breeze, wear protective gloves just in case it biffs you. Yachts can go faster than the wind that drives them. This is famously true of ice yachts or iceboats, which can reach 100mph (160km/h)! How? It’s all to do with angles and windspeeds, which brings us to the phenomenon of ‘apparent windspeed’.
You may have noticed looking at a passing yacht that the weathervane on the top does not point in the wind’s direction, it is instead indicating the direction of the apparent wind, the wind direction experienced by someone on the yacht. Similarly, if you go on a yacht, you will see that the windspeed gauge reading is not what the weather bureau is saying – it is the apparent windspeed. True wind W is the wind measured by someone on the stationary ground, while apparent wind A is the wind that someone on the yacht feels. With boat speed V, you can calculate the apparent windspeed A according to:
A = √(W2 + V2 + 2WVcosα)
where α is the angle from dead ahead to the wind W.
* You can easily test this formula out: e.g. if the true wind W is dead ahead (α = 0, so cosα = 1, and so A = √(W2 + V2 + 2WV) = (W + V); if W=10 and V=10, A is 20. The wind W just adds to the boat speed V.*
Finally, how long a cable yacht run can you make? Can you find rocks or cliffs to stand on to suspend the line higher? Can you make a mast to support the middle of the line in such a way that the yacht can be made to pass over it, as long-distance cable cars do?
If you liked this, you will find lots more fun science stuff in Neil Downie’s books, like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org. In line with this experiment, Neil’s current work includes developing a new ventilator system to support people with breathing difficulties – get more information on this great project at Exovent.org.
There is a back catalogue of STEM-related challenges from the past year to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM.
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