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Old 08-06-2015, 17:28   #1
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How do you calibrate wind speed?

I have a new Raymarine e7D and displayed wind speed is way low. I used the calibration adjustment to increase the speed reading about 60% to at least match my iPhone wind speed app, and an adjacent boat's reported wind. But thats kind of the blind leading the blind.

Driving the boat on a dead air day and calibrating to GPS SOG, could give me a "known good" of say 6 knots, but how does one calibrate higher speeds, like say 6-50 kts for example.

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Old 08-06-2015, 18:18   #2
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Re: How do you calibrate wind speed?

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Originally Posted by Journeyman View Post

Driving the boat on a dead air day and calibrating to GPS SOG, could give me a "known good" of say 6 knots, but how does one calibrate higher speeds, like say 6-50 kts for example.
Is it possible that once your instrument knows how hard 6 knots blows it will be capable of calculating, by proportional calculation: 12, 18, 24 knots?

And so (IMHO) you have answered your own question.

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Old 08-06-2015, 18:42   #3
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Re: How do you calibrate wind speed?

There's the option of dropping the hook near a weather bouy which downloads to NOAA/the Internet in real time. The dropping the hook bit, is so that both you & the bouy are both in pretty much the same wind/weather cell & gust/wind variation pattern. Plus are both in clear air.
Although both download lag time, & differing pitching variations & how they affect what your respective sensors "read", will cause some inaccuracies. Ditto as to what type of instruments they're using, their accuracy, & date of last quality calibration.
As odds are, a bouy wont have a calibrated, air mass flow sensor, akin to the one under the hood of your car.

That, & I'd suggest contacting both the manufacturer of the gear which you have, as well as say B&G, and NKE. Plus doing a bit of manual downloading.
Especially as you need to know how to calibrate your gear for upwash & the like, generated by the sails (or even just the bare spar rigging) when you're underway.
Which of course will vary, based on; the TWA, AWA, TWS, AWS, boat speed, sail/sail combination, sensor height & offset from the masthead... and a dozen+ other items.

Though, the most vital, accurate calibrations, are at the lower end of the spectrum. Given that changes in wind speed generate pressure, load, & vector changes, based on a squared function.
And doing your cal. the same way every time, is critical as well. So try & pick both a venue, & a proceedure (set of them) which are relatively easily repeatable.

Also, keep in mind that unless you climb up to the masthead with a (calibrated) handheld wind instrument, the data which you're reading at deck level will always be significantly different than what the wind's doing up high.
Pull up a chart, & it'll tell you the differences in wind speeds, based per heights, so that you can get a fix on such.

For other weather info knowledge, some small airports, & aviation schools have a wonderous volume of; knowledge, instruments, learning/study materials, & wisened pilots & instructors on the/related topics. Including... calibrating skills & tools.
Ditto on serious racing fleets, especially one design, trans-oceanic type.


And this may sound goofy, but it may pay to read/search through back issues of the "Lee Helm/Max Ebb" column in Latitude 38. As well as see about getting in touch with the authors of/technical contributors to said monlthly "info piece".
If you do, would you please be kind enough to post what you find out.
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Old 09-06-2015, 00:00   #4
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Re: How do you calibrate wind speed?

I have had the same question for many years now, and have not found a good answer.

Before I retired I had access to very accurate air speed instruments (hot wire anemometers), and contemplated carrying one up the mast and doing comparisons. However, the real world environment at the masthead consists of very unstable air flow (just look at how speed and AWA flip around, even in a fixed location), and the difference in time constants in meter response make such comparisons useless. Similarly, such conditions make comparing your instrument to your neighbors kinda futile.

Taking the masthead sensor to a laboratory where real air-flow measurements are made could get you a stable flow and the possibility of an accurate calibration but that always seemed like too bloody much trouble . One could take your instrument in your car on a really calm day and stick it out the window and use the speedometer to check it at higher speeds. Mounting it on a long stick so it was out of the entrained and turbulent airstream near the car body would help here but might worry pedestrians!

So, stymied, I reasoned that absolute accuracy wasn't all that important. Repeatability, so that you could know to reef when it gets to x knots, is important, but knowing that "x" is 23 knots rather than 25 is not, except for bragging rights. In the long run I compared meter readings with beaufort scale sea state descriptions, with knotmeter readings at low speeds on calm days, with other folks instruments , with handheld instruments at deck level and so on. When they agreed reasonably well, I said "it's calibrated" and pressed on with life.

As a retired scientist this lackadaisical approach appalls me, but I'd rather go sailing than obsess about it. YMMV

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Old 09-06-2015, 01:03   #5
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Re: How do you calibrate wind speed?

https://youtu.be/93jTPOeNVQQ
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Old 09-06-2015, 05:12   #6
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Re: How do you calibrate wind speed?

Our Raymarine wind was over reading when it was new. I took the iPad while wirelessly connected to the plotter and went to 4-5 surrounding yachts in the marina. I compared the average readout and adjusted ours down 20% to fall within the average. So instead of showing 30kn we showed 24kn. Like a barometer, the exact windspeed isn't critical, but the closer you can get the more sense it will make when comparing to weather forecasts. If the display data is higher than actual wind you might be safer by reefing earlier, but also might be worried when you see 40kn when it's actually only 33kn of wind.
Sailing between islands last week we had an enjoyable sail with the wind showing 20-24kn and after arriving we discussed the wind with another yacht that was sailing next to us. I was surprised when he said they had 14-18kn the whole way..
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Old 09-06-2015, 05:50   #7
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Re: How do you calibrate wind speed?

Try using the Beaufort Scale! Works great and emancipates you from digital slavery.
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Old 09-06-2015, 05:52   #8
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How do you calibrate wind speed?

Ed,

The wind cups generate a series of pulses at they rotate. I believe Raymarine publishes a chart of pulses per second (or minute) vs. wind speed. If you can't find it ask over at raymarine.ning.com. With an oscilloscope it is possible to see these pulses and count the number per second (or minute) thus you can know the approximate wind speed. Then calibrate your display accordingly. The pulse signal should be available on the back of the wind display. Its easy if it has the little push on connectors. Also, you may find the pulse signal inside the junction box near the mast step used to disconnect the wires when unstepping the mast. Finding a scope can be a challenge but many electronic or computer shops have one.

A really good shop will have a pulse generator. With that you can give the display a "fake" wind signal of any speed you like. Calibrate the display to that and you will be as accurate as any method.

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Old 09-06-2015, 06:19   #9
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Re: How do you calibrate wind speed?

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Originally Posted by farshore View Post
Try using the Beaufort Scale! Works great and emancipates you from digital slavery.
Zakery... sort of... if you need an instrument to tell you when to reef you have a long way to go in getting to grips with the life aquatic.

The Beaufort scale was introduced so that the Admiralty could make some sort of sense of all the weather logs they had to deal with.... one man's gentle zephyr being the next bloke's full gale..
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Old 09-06-2015, 06:59   #10
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Re: How do you calibrate wind speed?

True dat. But the more accurate the info displayed, the more accurate the reconciliation with forecasts and predictions. It can be easier to tell what the wind is doing by monitoring it closely in a smaller scale. Also maybe more relevant to a catamaran where the need to reef might not be judged as easily by the heel of the vessel. Also a bit more accurate for crew instruction and obviously sailing angle has a huge effect on the apparent wind.
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Old 09-06-2015, 07:46   #11
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Re: How do you calibrate wind speed?

Its funny how many accounts of races and rallies and storms I have read about, where the reporter says something like "some boats saw less than 20kts, while others saw 30+kts". With my ~ 60% inaccurate windspeed experience, I will understand such reports in the future to merely be commentary on typical calibration errors between vessels.

Dan. the problem with the pulse counter approach is if there is undue friction in the wind cup bearings, you won't be seeing real wind speed.

I think I'll just motor at 7 knots on a calm, flat water day with no sails up and use the GPS SOG as a known good to calibrate to, and hope its linear at higher speed.
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Old 09-06-2015, 08:01   #12
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Re: How do you calibrate wind speed?

I wonder if motoring upwind in 7-10k wind, then motoring downwind at about 6kn SOG both ways and adding and subtracting SOG would be just as accurate as well. At least the accurate setting should be around 15kn instead of 7kn. Or maybe not. My head hurts...
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Old 09-06-2015, 10:31   #13
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Re: How do you calibrate wind speed?

As a cruiser, being close is close enough-I don't need accuracy to the 0.1 kt. I bought an inexpensive hand-held wind speed meter on ebay(Chinese, w/digital readout), hold it in as undisturbed a wind condition as I can find in a given condition, and play with the calibration factor on my RayMarine wind speed until it reads close to the handheld. After doing this a few times, in different wind conditions, I think my masthead meter is fairly close. I know the wind at the masthead is different than that at deck level, but I'm in the ballpark. And like I said, I'm cruising, not racing.
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Old 09-06-2015, 10:37   #14
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Re: How do you calibrate wind speed?

Pick a day with a strong wind, and no tide. Motor directly into the wind, and then directly downwind. The difference in measured wind speed should be double the boat speed.

For example, boat speed is 6 kts. Wind speed is 25 kts. You should measure 31 kts upwind, and 19 kts downwind.
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Old 09-06-2015, 10:41   #15
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Re: How do you calibrate wind speed?

Google "calibrating raymarine wind instruments" Had them on a sailboat. Don't remember the exact steps but you put it in the calibrating mode and motored around in a circle several times. Somehow its little brain would add and subtract the differences in wind speed on the different headings and give you an answer. Was it accurate, I don't know. I do know that when it was blowing hard there was a big number and when the wind was light there was a little number. Larry
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