Build yourself an ice-skating sail

This is a quick description of an ice-skating sail I have built and used.
The design is almost trivial, so this description is mostly to encourage people to see how easy it is, rather than declaring some great design.
This first example is very small (~2m2), and I expect to scale up.
It cost around $10 in materials, and took half an hour to build.


A tarpaulin, 1.5m x 1.5m.
3 wooden broomsticks 1.5m x 2cm 1 wooden broomsticks 2m x 2cm
6 cable ties
A 3mm drill


Skate-sail diagram

Drill 2 holes in each of the 4 broomsticks, 2cm from each end. Then take two of the shorter poles and drill a third hole 50cm from one end of each, take the third short pole and drill a hole in the centre, and finally take the long pole and drill that 75cm from one end.

Fold over two adjacent edges of the tarp (A-B and A-C) about 10cm and sew down so as to produce pockets into which two of the shorter poles can be poked. Leave the ends of the pockets open.
(The red lines in the diagram represent the stitching of the pockets).

Poke a broomstick into each of the pockets, locate the holes in the sticks, and make corresponding holes in the tarp. Attach the ends of the third short pole to these holes with cable ties to make a cross-beam, and attach the middle hole of the long stick to the middle hole of the cross-beam.
Starting to look like the diagram, huh ?

Now make a hole in the tarp at the loose end of the long stick, and use a cable tie to fix the stick there. (Or if you're lucky there was even a loop already on the tarp there). Use some more ties to keep the ends of the 3 sticks together at A.

Now, unless you have a Volvo, you'll have trouble getting this to the ice, so what we really want is either a supply of cable ties so that we can cut lose at F and E when we want to fold it up, or a piece of string and a knot there. Be creative.


Here's a picture of one I made earlier. Notice that I couldn't find a 2m long broomstick, and so lashed two short ones together.
It's effectively just a kite, but the construction (and especially the pockets), is such that the tarp can be pulled very tight to give a nice flat sail for tacking high into the wind.

Skate-sail construction Skate-sail in use Skate-sail in use

On the ice

It works ! I was lucky to take it out on a day with smoothly varying winds from 20-40mph (10-20m/s), and although slightly overpowered, I could cruise comfortably at 15mph (20kph), with bursts over 20mph (30kph). This top speed was definitely not limted by the sail, but by my survival instincts. In fact I didn't dare reach or run, and limited myself to tacking about 20 degrees upwind to limited my speed, and skated home under my own power with the sail folded up like an umbrella. On smoother ice and with more experience I would expect to get up to 30mph. There again, 40mph winds are fairly rare here in Uppsala, and I suspect that with less than 20mph this sail will betoo small (real skate sails are around 8m2, I believe).
The sail needs to be held upwind of the body, and I chose to have one arm down the horizontal bar, and the other out in front, steering. Most of the force was taken by my body, against which much of the sail was pressing, so I was leaning into the wind.
The balance felt good, and steering was achieved by small shifts of the sail forwards and backwards, much like in windsurfing. The difference was that excess wind could be spilled by tilting the sail about either its horizontal or vertical axis, giving more flexibility in trimming.
Surprisingly, the cable ties were not strong enough to withstand the forces (particularly when feathering and changing tack), so I progressively replaced them with carbon-fibre reinforced sailing string, until my fingers were too numb and I gave up, after 6 km of sailing.

By the way, I have derived the following equation for the maximum speed of a skate sail in km/h : Vmax = (3.2*105 )/ IQ2
This is consistent with my being of average intelligence and limiting myself to 32kph.
You can calculate your own maximum speed by following this link. ;-)

Update 2001 :
Firstly, I got up to 52km/h, this time limited by the wind. This suggests that my IQ has now sunk below 80.
Secondly, I build a new sail from a shower curtain, which was much prettier than the tarpaulin, but stretched rather more and was thus less efficient.
Update 2002 :
More evidence that top speed is limited by IQ and not sail design - I borrowed a real skate sail with an area of 8.2m, and still only got up to 35km/h. Mind you, I got there in less than 5m/s wind.

© Mark Harris 2002, but feel free to distribute without changes.

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Here is a picture of a real skate-sailor, and of a skate-adapted windsurfer. Heh, and why not an ice yacht too.

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