![]() Consider them as masthead weather stations. Using sonic technology, these units provide accurate information about wind direction and speed, as well as barometric and temperature information. But two companies, Maretron and Airmar, have developed wind speed/direction sensors that use no moving parts. Broken vanes, missing cups, and wands tweaked off centerline are the stuff we’ve long had to deal with to get the wind data we need. When you have a delicate instrument with moving parts spinning around at the top of your rig, there are bound to be numerous trips aloft to replace bits and pieces of your wind direction/speed sensor in its lifetime. Windspeed and direction are measure by sonic pulses passing between the sensors (explained in the diagram below).| His master's thesis on the subject of sailing instruments is also worth reading.| The Maretron WSO 100 delivers wind data without the cups and moving parts of a traditional masthead anemometer. McCurdy is the cofounder of Ockam Instruments, so it's pretty safe to assume that Ockam uses a formula similar to that for their correction calculations. Certainly two reefs in the main is going to look different to the anemometer than none at all and a big 155% genoa will look different than a 100% jib. I would assume that upwash changes depending on the set of the sails. What I don't know is how useful these adjustments are. I know Ockam can adjust for upwash with some calibration and I assume B&G, Nexus, and NKE can as well. B&G even make a 1.8m (6ft) wand to get the instrument out of the upwash. Upwash is an issue unless you can get your anemometer up and away from the sails. In addition to being able to accurately measure the apparent wind vector in 3D space and therefore definitely correct for heel in both angle and speed, it should also be able to give you a handle on upwash from the sails. Perhaps even better would be a 3D ultrasonic anemometer like this one from Wimers Meßtechnik. ![]() It would seem to me that they'd be pretty much immune. Both of which are 2D ultrasonic amemometers. I wonder how this applies to ultrasonic wind instruments like the Airmar WeatherStation or the Maretron WSO100. Gentry's paper indicates that this equation is used because of the issues a cup-anemometer has with heel: which you call cosine response. In Arvel Gentry's paper referenced above, the formula he uses - attributed to David Pedrick Richard McCurdy - to adjust apparent wind speed for heel isĪWS_corrected = AWS_measured * cos(AWA_measured) / cos(AWA_corrected) This being said, I would love to get wind tunnel results from my anemometer and program an exact correction if required. So without wind tunnel results of the cosine response of a particular anemometer, the conservative approach is to do nothing. It may even go the other way and require a correction in the opposite direction. This means that the poor ‘cosine response’ eliminates in part or totality the theoretical correction. For most designs the cups will actually spin faster for a wind a few degrees above or below horizontal.” () This means that as the wind direction changes to hit the sensor from below or above the horizontal the response of the sensor deviates from the ideal response, which is spinning at a rate proportional to the cosine of the angle of the wind above or below horizontal. “Cup anemometers also suffer from poor 'cosine response'. So I agree that a correction to the AWS would be required for an anemometer that has a near ideal ‘cosine response’, like wind-turbine anemometers. When the boat is heeling, the anemometer will see a vertical component of its own apparent wind, even if the apparent wind vs water is completely horizontal. ![]()
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