Wind Direction Indicator

by Dermot Tynan in Microtransat

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Posted on Saturday, May 04, 2013 at 08:00

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Now that the hull is looking solid, it's time to start thinking again about the wind direction indicator. It is possible to detect wind speed by using an ultrasonic sensor and receiver, and measuring the delay between the two. You need to account for temperature changes and gusts can cause issues, but it is fairly reliable and has no moving parts. Generally, you use two transducers offset by a distance of perhaps 20cm for the North/South computation, and another pair in the East/West direction. I think if we were using a larger hull, such as a 4m boat, this would be a good plan. But, for the 2.4m (or the 1.2m) boat, it's just too big and awkward. Also, this jury is undecided about how well they would work, over the long haul.

So, it's back to the tried and tested "arrow on a stick." A wind vane is mounted on a 4mm brass shaft with ceramic bearings. The shaft is attached to a potentiometer. Most pots will only handle around 270 degrees with the remaining 90 degrees a "dead zone." I could put the dead zone up at the front to match the "no-go zone" of a sailboat, but I suspect I can't afford to lose measurement in this critical area. As a result, I'm using two potentiometers, offset by either 90 or 180 degrees. When one is in its dead zone, the other will give a reading. They also need to be low torque pots. Most pots nowadays have artificial torque added by applying a sticky grease to the mechanism. It gives us a tactile sensation of something or other (quality? power?) when we turn the pots on a large stereo amp. If they spun easily, we might not be as impressed. The bad news for us is that most pots seem to have this artificial torque. So, I ordered a series of low torque pots from Digikey, which should do the trick. The other aspect about these pots is they are fully 360 degree rotation. They only give a reading for 270 of the 360 degree circle, but they don't have end-stops like your average hifi pot.

The pot are quite small, and I'm hoping I can embed the whole thing into a very small unit, and perhaps stick it at the top of the mast. It would also be really great if the assembly was pivoted, so that the wind vane is always vertical. Adding a pot to that pivot would also allow me to measure boat heel. My suspicion is that the boat will be pegged to a heel angle of 25 degrees for most of the first half of the trip, until she catches the trade winds. Heeling angles help to depower the sail. But they also interfere with the wind direction indicator. Not just because the thing is at an angle, but because the boat is likely to roll through +/- 25 degrees, which creates an artificial wind at the top of the mast as it sweeps back and forth. Putting the sensor on gimbals won't help with this, but it might just be possible to adjust the wind direction computed, by allowing for the rate of change of heel angle.

At this stage, it's all pretty academic as I don't yet have a wind direction indicator to play with. Once Hull 001 is ready for early tests, I can focus my attentions on this particular problem.

As the design involves depending on wind direction for steerage, it is a concern that there is only one wind direction indicator, and it has moving parts. To lose this critical piece of gear during the trip would be disastrous.

I will be adding a lot of software for testing the onboard systems, and trying to detect if all of the equipment is still working. I will also be adding software to mitigate against failure of any single system, including the wind direction indicator. That won't be trivial, I can tell you!

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