Wind sensor error fix needed

[Dr. D]

I was racing yesterday and I thought the the TWA/AWA numbers were a bit odd on a long beam reach. At the first mark I tacked and it appeared the wind shifted as I tacked; well, the instruments said so. After a while I noted a wind shift favoring a tack so I did. After the tack, the wind appeared to have shifted while I was tacking, by some 20 to 30 degrees. However, my sense was there was no actual wind shift.A couple more tacks showed that the wind sensor, or the electronics connected to it, were shifting the wind at each tack. As I was sailing solo, I decided to ignore the instruments for the rest of the race.

What failure of the wind sensor or the electronics might cause this error? Could the wind sensor have been mechanically damaged?

 

[kappykaplan]

Any chance your speed-through-the-water input was messed up?

 

[Davidasailor26]

Speed through the water miscalibration could definitely cause that. It’s also possible that the wind vane’s dead ahead position fell out of calibration, but if it used to work that seems less likely. Once in a while I also get spider webs that restrict the vane’s rotation and cause those problems.

Check motoring straight into the wind and see if the instrument says the apparent wind angle is 0 degrees. Also check at a couple off-center angles like having wind right on the beam (while not moving) to see if the Apparent wind angle matches a flag or traditional wind vane. If the apparent wind angle looks right then everything is mechanically good and it’s just a matter of Speed Through Water.

 

[Dr. D]

Wow, I’m surprised by the STW response. My triducer was fouled with slime so I had selected “Use SOG as STW.” I didn’t think doing so would cause such an error.

 

[kappykaplan]

I had selected “Use SOG as STW.” I didn’t think doing so would cause such an error.

As you know, SOG can be much different than STW. When I was out last Saturday, my STW was consistently 1.5 kts or more slower than SOG. Normally, they are much closer. I've since lubed the paddle, so will see if that makes a difference next time.

 

[Davidasailor26]

Wow, I’m surprised by the STW response. My triducer was fouled with slime so I had selected “Use SOG as STW.” I didn’t think doing so would cause such an error.

That’s interesting. That should fix that kind of error rather than causing it. Was there much current, and is your heading sensor accurate? Those could also confuse the math.

 

[Dr. D]

My original post has an error. It was TWD that was having this extreme shift when tacking.

There was some current, not much, maybe 0.4 kt. I calibrated the Precision 9 compass a couple of months ago. Something is confusing the algorithm determining TWD.

 

[kappykaplan]

It was TWD that was having this extreme shift when tacking.

That is puzzling!

 

[Techstyle]

Your wind instrument only sees Apparent wind angle and Apparent wind speed. True Wind Speed and Direction are calculated and based on your Speed Thru Water - so based on the new info that it was TWD that was out - I would suggest, like others, that it could be to do with your speed sensor being fouled or misbehaving

 

[dlochner]

Your wind instrument only sees Apparent wind angle and Apparent wind speed. True Wind Speed and Direction are calculated and based on your Speed Thru Water - so based on the new info that it was TWD that was out - I would suggest, like others, that it could be to do with your speed sensor being fouled or misbehaving

The sensor should be using SOG not STW for the calculation. Consider this scenario, sailing in 10 pts of wind at 6kts against a 6knot current, the SOG will only be 0 knots. So the TWS and AWS will be the same, however if using STW, the TWS will be calculated to be 4 knots, 10 k AWS - 6 k STW = 4 k TWS.

 

[smokey73]

Help me understand why Speed through the Water (STW)is used for the calculation of true vs apparent wind rather than Speed over the Ground (SOG)

Take the following example:
You are sitting still in a strong 3 knot current, Zero SOG but your STW would be 3 knts (even though you are not moving relative to the bottom). At this point, wouldn't the True Wind and the Apparent Wind be exactly the same and from since it is no different than being tied up to a slip? Throwing in a calculation taking into account a knot meter reading (STW) would seem to shift true wind calculation artificially. Just having a hard time wrapping my head around why the calculation of true wind uses STW vs SOG.

Using the example above help me see what I am doing wrong.

Didn't see @dlochner post but essentially the same conclusion.

 

[Techstyle]

Depending on how old your instruments and system in general will depend whether SOG or STW is used, for instance, Boats prior to the late 90's didn't come as standard with GPS so SOG was unavailable and therefore the Raymarine ST1 stuff of that era used STW. So depending on whether his system is using SOG or STW, it could be a fouled Speed sensor

 

[Davidasailor26]

Racers concerned with checking their boat’s performance use STW for the calculation. The rationale is that you have polars that say you should be going a certain speed through the water in a certain wind speed, and all of those measurements use the surface of the water as the reference. If you used SOG as the reference you’d be mismatched from your polars if there is current.

Personally I prefer using SOG. Some navigation platforms refer to that as Ground Wind Speed, GWS, as oppressed to TWS.

 

[Dr. D]

I believe I found the reason for the drifting TWD: A fouled paddlewheel on the triducer (DST810). Here is why:

I motored out of the marina. At first, the STW was zero. After about 10 minutes the STW was showing about the same as the SOG. The main difference as I left the channel was due to current. Once clear of that effect (the current down to 0.3 kts) I began making tacks while still motoring. The TWD plot varied some, as appropriate for the wind conditions, and no large swings with the tacks.

The wind was light, 3 to 6 kts. I put up the sails and shut off the motor. Before I made the first tack the STW went to zero; probably not enough water flow to overcome the resistance. With these winds, the boat was moving at about 2 kts. So I selected the instruments to use SOG for STW to calculate TWD. Now when I tacked, the TWD would swing with my heading. From a port tack, tack to a starboard tack, and the TWD would shift significantly to port. And the other way around.

I also noticed that the instruments were saying the current was now nearly 2 kts, boat speed. What I'm thinking is the AC42 autopilot (which calculates current, TWD, and other terms) uses STW to calculate current. If STW is zero, then the boat is drifting with the current. That then throws off the calculation of TWD.

If you look at the attached image, before the 10 hack, the boat was on a port tack. I tacked onto starboard and the wind shifted the same way. After a few minutes I tacked onto port and again the wind shifted the same way.

My problems with my paddlewheel are due to too much anti-fouling. I should have only applied it to the flat section and the sides. No more on the paddlewheel.

 

[kappykaplan]

No more on the paddlewheel.

Never on the paddlewheel! I remove mine and install a blanking plug when the boat is going to sit for a week.