Calculated Wind Speed Way Too High

[Foswick]

We've got a Garmin anemometer/vain tied into our plotter. (I will look up the model when my wife wakes up and I can get to our manuals.)

The only thing we really use is the ability for the system to estimate true wind speed and direction. Kind of handy when we are motoring along at hull speed and wondering if there is enough wind to sail.

Today, we were motoring almost directly into the wind at about 6.5 kts over ground. The while the wind vain was pointed directly into the apparent wind, and the plotter showed the correct apparent wind direction, it was reporting 12-14 kts true wind speed. The actual speed was definitely lower - probably in the 8 kt range.

At 6.5 kts over ground with a 12 kt head wind, the anemometer would have had to experience about 18.5 kts. It was spinning more like a good breeze. We stopped to see what winds were actually doing and, like I said, they were at about 8 kts.

So, the mystery to me is what would cause it to overestimate a head wind? I can understand underestimating - like if the anemometer was gummed up and turning slow. Or, if we were going downwind and the anemometer was running slow, that could cause an overestimating.

Very confused. And, this is new. It worked correctly (or at least not nearly so far off that I would notice) just a few months ago.

Any ideas?

 

[BrianQ]

Sounds like your boat's speed indicator wasn't working or not feeding data to the other instruments or plotter.

 

[Davidasailor26]

Garmin probably uses speed through the water, not speed over ground, for the true wind calculations. So if your speed log is calibrated wrong, or you were going against a significant current, the speed through the water would be unusually high, adding too much to the TWS.

Check your speed through the water compared to speed over ground to see if there’s an odd discrepancy. You could also check the reported TWS when going downwind, to get more data points to troubleshoot.

 

[Foswick]

I don't think the chart plotter is even connected to our knotmeter. But, that might just be an assumption. The paddlewheel is pretty come-and-go, as they tend to be. But, if the plotter was using that and not GPS, that would explain everything pretty neatly.

I'm looking at a full day of 10-15 winds tomorrow, so I'll have plenty of opportunity to look at this.

 

[Joe]

The knotmeter versus the gps is an option when setting up the instruments.... you could always re install the device. Also, even though I'm a B&G, guy I've sometimes had conflict between the tiller pilot and the chartplotter when I switch into "sail to wind" mode. The AP software in the chartplotter and the Navico tiller pilot seem to want to compete.... weird. Most time's it works perfect... but your issue is interesting...my first thought was the wind device had slipped in to apparent wind mode, similar to my tiller pilot experience. Hey... it's probably something super simple... you'll get it sorted out.

 

[Scott T-Bird]

Not knowing what instrument models you have ... my guess is that you may have been reading apparent wind display and only thinking you were on the true wind display. I can't tell you how many times I've done something similar. (When you stopped, true wind and apparent would be the same - so of course it keeps you from seeing the display for what it might have been). I only mention this because my old RM had a button to toggle between True and Apparent and if you didn't notice which setting it was on, you could easily be led down a confusing path. Just a thought ....

I also doubt that any difference between boat speed and SOG could have been that large to explain the discrepancy.

 

[Hello Below]

I also doubt that any difference between boat speed and SOG could have been that large to explain the discrepancy.

Scott this was taken not to far from Foswick's sailing grounds. Not through a pass or narrows where current can run much higher than the 5.4 kts experienced here.

 

[Scott T-Bird]

Scott this was taken not to far from Foswick's sailing grounds. Not through a pass or narrows where current can run much higher than the 5.4 kts experienced here.

I suppose. But he said they "stopped". I don't know how you can "stop" in 5 knot current and not know that you are moving enough to significantly affect apparent wind. Also, he says they were motoring at 6.5 knots. If he was motoring against a 5 knot current, wouldn't that be something significant to add to the mystery? Who does that anyway under normal circumstances ... motoring against a 5 knot current on a sailboat? I suspect that is something to avoid. I'm assuming they were motoring in negligible current. But, of course, not knowing the habits of sailors in this location, I could be misunderstanding.

Maybe if he was reading 12-14 knots True, the most logical thing to do would be to observe the apparent wind speed and tell us what that was.

 

[Foswick]

The issue of discrepancy can have to do with current (as they do run swiftly here). It is possible to be moving at hull speed and traveling backward. But, more to the point, the paddle wheel that measures water speed is unreliable. Ours usually works, but something gums it up about 25% of the time.

So, I rarely even look at it. I usually just look at the water to sense how fast we're moving. It didn't occur to me to look at it to see if it was stopped when we were experiencing this phenomenon. Will be looking today ... when the winds come.

 

[shemandr]

I don't see how TWS can be calculated in tidal waters from SOG.
If there really is/was a 5 knot current the SOG could vary from 10 knots going with the current to 0 knots SOG going into the current. That variance is 200% of the assumed boat speed (STW) of 5 knots. Even at a current rate of 2.5 knots the range of SOG going upwind or downwind would be 100% of boat speed. How could the software account for that?
Yes paddlewheel are inaccurate. But if kept clean they are close. Even if your sense of speed is as or more accurate than the paddlewheel, how do you tell the processor what you think the boat speed is?
You could get a transducer that doesn't have a paddlewheel.

 

[rgranger]

Assuming the issue is not current or an incorrect toggle on the display... you can check the calibration by waiting for a calm day and then comparing your GPS speed to wind speed.

 

[Foswick]

I don't see how TWS can be calculated in tidal waters from SOG.

You can't. I just hadn't thought about it, so just just assumed it was using GPS. If I'd thought about it a second, I would have realized.

But ...

We had a good sail today. When we started out, motoring out of our anchorage, we were moving along at about 5 kts and GPS and knotmeter agreed on this. The true wind speed indicated 5 kts. Looked up and the anemometer wasn't moving. So, spot on - that's the TWS it should have calculated.

We changed point of sail in every direction in conditions ranging from 5-20 kts. The TWS always appeared reasonable and the GPS and knotmeter were always in agreement on speed (with the expected current effects).

So, today proved nothing because I couldn't replicate the problem. But, also, the knotmeter seemed to be working fine all day. So, that hypothesis still looks strong.

I did try to disconnect it, but that was more hassle than I was willing to exert today. And, I kind of wonder if disconnecting it will just take it offline such that the plotter recognizes it is offline - like disconnected is not the same as registering zero.

 

[Davidasailor26]

And, I kind of wonder if disconnecting it will just take it offline such that the plotter recognizes it is offline - like disconnected is not the same as registering zero.

Yes, probably. Simrad/B&G just shows dashes if it doesn’t have all the data needed to do a computation.

 

[Foswick]

Well, we sailed or motored another 50 NM or so over the past few days, and the phenomenon never recurred. I hate that.

 

[jssailem]

And non-sailors wonder why we refer to our boats as She/Her (women).

 

[Scott T-Bird]

So, when you were motoring along at 5 knots and the anemometer wasn't moving but TWS was 5 knots that could only mean that you had a following wind at exactly the same Boat Speed (or SOG depending on your settings). Did you look at the AWS and direction? AWS should have been reading 0 and I suppose the direction arrow could be moving erratically. You should describe your instrument display so that we would know what you are looking at (we don't know at this point). My displays (B&G Triton) have numerous windows and I can customize the information presented. I can read TWS, TWD, AWS, AWD so that I can see all simultaneously in various formats as I like to see it. Your observations would be far more helpful if you described the apparent wind characteristics.

I had the ST60 wind display and I know that I had to toggle between TRUE and APP. The little square tells you which one is being displayed. It would be easy to be blind to it if you had it fixed in your mind that you are reading one instead of the other. Your description that TRUE wind speed was about 4- 5 knots higher than it should have been (motoring upwind no less!) simply screams out to me that you were looking at APP wind. Well, you even said that the wind vane was pointing consistently with the CP APP wind direction. So, if your plotter is showing APP wind direction, why would you expect the instrument display to be showing TWS?

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I'm still betting that you were reading AWS when you thought you would be reading TRUE. But without knowing what Garmin display model you have and how it functions (or the setting options that you have), how can anybody come up with a reasonable deduction?

 

[Scott T-Bird]

As for paddlewheel performance, I find that it is just about impossible to know how accurate it may be. I've been sailing the past several days and I can say that my Boat Speed (with a clean & freely spinning paddlewheel) varied just about all the time from about .5 knots above and below SOG and often on the same tack it could be faster or slower than SOG. But there were also significant periods when they were even or within 1 knot variability. That seems to correspond to the currents we have in the bay proper (away from the main channel to the ocean). It seems that the currents are variable and unpredictable as to location on the bay.

It also seems that the angle of heel may affect the performance of the paddlewheel. Mine is at centerline in a flat section in front of the keel.

Variable currents would seem to make calibration darn near impossible. In fact, I'm happy that there is an automatic calibration function that relieves me from the stress of trying to do it myself. All it requires is a length of time motoring in calm water at a consistent speed (IIRC).

 

[Foswick]

So, when you were motoring along at 5 knots and the anemometer wasn't moving but TWS was 5 knots that could only mean that you had a following wind at exactly the same Boat Speed (or SOG depending on your settings). Did you look at the AWS and direction? AWS should have been reading 0

Of course.

I'm still betting that you were reading AWS when you thought you would be reading TRUE. But without knowing what Garmin display model you have and how it functions (or the setting options that you have), how can anybody come up with a reasonable deduction?

I am sure I was reading TWS when I thought I was.

My display shows both TWS and AWS in separate areas, and they are clearly labeled as such. The TWS reading was always pretty much consistent with the AWS as if the boat were stopped. For example, going into the wind with an AWS of 10 kts, it would give me a TWS of 10 kts, even motoring at 6.5 kts (over ground). When we saw this discrepancy, of course, the first thing I did was look at the AWS to see what it said.

That is what makes me believe the hypothesis that, while the plotter shows speed over ground (from the GPS, obviously), it is using water speed for the TWS calculation, and that water speed is intermittently way off.

So, even though the on-screen numbers appear to not make any sense, it is actually doing the math correctly.

I just found, in the manual, the place to configure whether the plotter uses GPS or water speed for wind calculations. So, when I get back to the boat, tomorrow, I can check that it is using water speed. I am betting it is, and I'm betting therein lies the problem.

The next step is to catch it in the act again. But, there is more to the mystery.

According to the manual I have for my paddlewheel water speed sensor, there is no simple way it is connected to my Garmin chart plotter. Of course, it is entirely possible that the manual I have is for a sensor that was replaced a long time ago. I do know that there is a paddlewheel because I checked it at last summer's haulout. However, it might must be a vestige and there might be a transducer installed elsewhere, for which there is no documentation. It would not be the first surprise I found on the boat.

Most of the past owners have kept pretty good maintenance logs (dating back to 1976), so it might be mentioned somewhere.

I'll find out more tomorrow.

 

[Foswick]

As for paddlewheel performance, I find that it is just about impossible to know how accurate it may be.

Yeah. I always just assume it is off ... a bit ... but not several knots.

That said, I periodically compare the knotmeter to the GPS speed and find them to be reasonably in agreement, insomuch as the difference reflects about the amount and direction of current I might expect.

There is an OK(ish) way to calibrate, though, that is pretty accurate but requires pretty much a slack tide and very little wind. If you set the wheel/tiller at a fixed angle to travel in a medium-sized circle, and reset your average SOG on your GPS, you can circle several times and record both your average SOG and your average knotmeter reading (by taking readings at more-or-less regular intervals). If your GPS track shows that you drifted during the calibration, the accuracy will be off. But, a little bit of drift actually has very little impact on the accuracy unless you are interested in getting the knotmeter within less than about 1/10 kt.

My knotmeter is an analog dial that I can only read to about 0.2 kts accurancy anyway.

The circling method is better than any straight-line calibration because you can unambiguously see if wind/current is affecting accuracy.