wind speed calibration

[Bobg]

I have a ST60 Wind speed instrument, I noticed last fall at the marina, my boat was registering 25 knots of wind speed true, and all the boats around me were clocking 35 knots of wind.  That explains why last summer I noticed the wind seemed blowing harder than it seemed according to my intruments.
Has anyone had this problem? I have read the book a few times and am hoping I can recalibrate this summer, but any input would be appreciated, the way I am interpreting  the book, it says to set the wind speed to reflect the current wind conditions, but how do we know what they are?  Thank you  Bob

[waterdog]

Bob,

This is not a calibration problem.   It's a feature for your nervous crew.   You just tell them not to worry, it's only blowing 16 knots, see.   

I like to couple this with a boat speed indicator that runs a knot high.  Yesterday we flew in to Mazatlan sailing at 8 knots on a close reach in only 10 knots of apparent wind! 

[Wayne]

I'm still laughing at Steve's reply . . .
But seriously, these things do need to be calibrated.  Mine was reading way to little wind as compared to observed conditions.  Adjust the calibration number is pretty straightforward--read you manual and it will tell you how to get into a calibration mode where you will (in your case) want to decrease the number.  Basically, you have an anemometer at the top of your mast which whirls around in the wind, faster in more wind, slower in less, and sends down to your instrument readout information on revolutions per time unit (minute?).  The calibration factor is a number that is plugged into a little computer algorithm to convert revolutions per time unit into wind speed.  Bigger calibration number = higher wind speed.  The tricky part is know how fast the wind is really blowing so that you can adjust your calibration factor.  I got mine to where I think it is pretty close to reality by observing known measurements of wind speed and adjusting my unit incrementally each time I went out sailing until I felt pretty good about it's accuracy.  Whitecaps start to occur at about 10 knots; that is a good reference point.  I observed my readings at particular times and compared those readings to nearby NOAA ground station readings for the same time period.  To make things really complicated, one must remember that the wind is more than likely blowing faster at the top of the mast than at cockpit level, possibly 15-20% faster.  In the end, your wind instrument reading is an estimate; I don't know of any way to get it set to a 100% accurate reading.

[Ron Hill]

Bob : One thing that you need to check is where the indicator is mounted on the top of your mast.
I say this as the guy at the top of mine mounted the foot behind the the anchor light with the indicator facing the bow.
 
To make a long story short we had the A-spinnaker up with the full main.  As the wind built the indicator only read 15knots, but I felt that it was much more and decided to take down the A-spinnaker.  As I pulled down the chute we started to take off like a shot, lifting me off the deck(holding on to the chute scoop line.  We had a real problem, started the engine and finally got everything under control, but hairy until we did.  The chute was actually blocking the wind so the meter read low!!  We moved the meter foot forward of the anchor light - now everything is fine.
Another C34 had the instrument at the top mounted backwards (facing aft), but it seemed to operater OK

Look how you mast head instrument is mounted, that might be your problem?  A thought

[Stu Jackson]

...noticed last fall at the marina, my boat was registering 25 knots of wind speed true, and all the boats around me were clocking 35 knots of wind.

How do you know when you're newly calibrated?  Call these same guys on a windy day!  

[steve stoneback]

One day a couple years ago there was not a breath of air moving so I decided we would motor out on the lake and jump in.  What does that have to do with calibrating your wind instrument, well if there is no wind, absolutely none and you are motoring at 7 knots your wind instrument should be reading 7 knots.  Try it, it may give you an idea if your wind indicator is at least close.

Steve