Old ST60 with new transducer: wind speed is reading quite high.

[jaysweet]

I had an older thread Old 2004-era ST60 wind instrument where I asked about replacing the mast-head transducer for an old ST60 wind base unit. I've done this now: I bought a "Raymarine Standard-Arm Forward Wind Transducer - E22078" (the egg-shaped guy), and it's now installed. The wind direction works fine, but I've noticed problems with the wind speed. The first day out, it was reading something like 20-23 knots, and it just really didn't feel like the wind was that strong. Yesterday, I went out for a sail, and (unfortunately) the wind ended up being very light and variable. So I did a test: motoring in one direction (44 degrees), my GPS speed said roughly 5.5 knots and the apparent wind speed was around 8.5 coming from dead ahead. So that would indicate a 3 knot headwind. I turned 180 to a course of 224, and it was pretty similar: 5.5 knots GPS SOG, but maybe 7.5 knots wind speed from dead ahead. Of course, if there had been 3 knots of headwind the first time, it should have read 2.5.

Anyway, lots of research on the web has yielded only a very small number of hits on this issue, and not one of them is definitive. (Most problems people have are with wind direction). I do have the old printed manual, which says that there's a dealer calibration screen with a wind speed calibration setting, so I guess I'll try changing that next. The thing that's a bit annoying about the whole issue is that it seems like there should be a clear answer somewhere, like "yes, if you replace the old cylindrical transducer with the new egg-shaped one, you have to change the calibration value from x to y", or even a user saying "I had to change mine when I bought a new one", but I don't find anything like that on the web. Asking chatGPT about it, I initially got the statement that

"Yes — the Raymarine E22078 is intended to be compatible with the ST60 wind instrument, so in a normal installation it should report wind speed accurately. There is not a known built-in compatibility mismatch that would inherently make speeds read higher just because you paired that transducer with an ST60. Raymarine and resellers explicitly list the E22078 as compatible with ST60 / ST60+ systems."

followed by a bunch of not very helpful stuff about why it might be wrong.

Another minor wildcard: I do have a paddlewheel ST40 speed (though the water) sensor, but I never use it, so it's not plugged in to the thruhull. In general, having the ST60 wind set on true vs. apparent never changed anything in the past (as it shouldn't if the ST40 was reading 0 for STW), but I suppose I should double check whether the ST60 is set on true or apparent wind. Just trolling for ideas here...

 

[dlochner]

It does sound like a calibration issue. The difference you found while motoring, 2.5v3.0 wind speed is not enough difference to worry about. Wind speed is never constant, so a variation of 15% is probably within the error range.

 

[brazenarticle]

It might be displaying meters per second instead of knots....

How to Change Units on the ST60
To toggle between KTS and M/S on your display, follow these calibration steps:

1. Enter User Calibration: Hold down the disp and true/app buttons simultaneously for approximately 2 seconds.
2. Select Wind Speed Screen: Press the disp button repeatedly until you reach the wind speed units screen.
3. Toggle Units: Use the vmg and tack buttons to select your preferred unit (either KTS or M/S).
4. Save and Exit: Hold down the disp and true/app buttons again for 2 seconds to save the setting. [1, 2, 3, 4, 5]

 

[JoeWhite]

I do not see how changing the anonimiter would change any settings on the display unit.

 

[CrispyCringle]

I think after installing wind, depth, water speed, anything like that, a full calibration would be a matter of course. Even more so if not replacing the sensor with exactly the same model. I would not expect it to come calibrated. I have an ST60+, so I assume its a little newer than the ST60, and the manual states:

"The ST60+ Wind instruments are set up with factory-programmed default
settings, so in order to optimize the performance of the instruments on board a
particular vessel, the procedures in this Chapter must be carried out immediately
after the completion of installation, and before the equipment is used for
navigational purposes."

And then goes through the calibration process. The only rub on the wind speed cal process, at least for the ST60+, is it will show what it thinks the wind speed is and you then adjust to what it "really" is. Unless you have your own anemometer the only way to do this easily is during zero wind and then matching it to your GPS speed.

I highly recommend doing the full calibration for the ST60 and see how it performs then.

 

[Ward H]

Was there any current running while doing your test? If so I'd expect the current is causing the up current / down current readings to be different.

 

[jaysweet]

It does sound like a calibration issue. The difference you found while motoring, 2.5v3.0 wind speed is not enough difference to worry about. Wind speed is never constant, so a variation of 15% is probably within the error range.

Right. What I was noting was that heading into the wind, I should observe 5.5 + x, where x is the true wind speed, and 5.5 - x heading 180 degrees from the wind. I was seeing 8.5 and 7.5, which doesn't seem possible if properly calibrated.

 

[jaysweet]

It might be displaying meters per second instead of knots....

How to Change Units on the ST60
To toggle between KTS and M/S on your display, follow these calibration steps:

1. Enter User Calibration: Hold down the disp and true/app buttons simultaneously for approximately 2 seconds.
2. Select Wind Speed Screen: Press the disp button repeatedly until you reach the wind speed units screen.
3. Toggle Units: Use the vmg and tack buttons to select your preferred unit (either KTS or M/S).
4. Save and Exit: Hold down the disp and true/app buttons again for 2 seconds to save the setting. [1, 2, 3, 4, 5]

That's a good idea, but since 1 m/s = 1.94 knots, it would be reading slower, not faster. So that can't be it. I also just checked at the boat, at it was set for knots.

 

[jaysweet]

I do not see how changing the anonimiter would change any settings on the display unit.

Yeah: I agree. I've had the anemometer on and off over the years, and now put a new one on. Nothing should have changed. It was working with the old one last fall, before installing the new anemometer. So that seems to point to the new one requiring a different calibration (wind speed) factor than the old one.

 

[jaysweet]

Was there any current running while doing your test? If so I'd expect the current is causing the up current / down current readings to be different.

Since I'm measuring apparent wind, current is irrelevant. I'm just comparing speed over ground (from the GPS) with wind speed detected at the masthead (from the anemometer). Neither of these knows or cares what the current is. Now if I had the water speed paddlewheel plugged in and I was reading "true wind", current would be a factor, but I wasn't.

 

[CrispyCringle]

Since I'm measuring apparent wind, current is irrelevant. I'm just comparing speed over ground (from the GPS) with wind speed detected at the masthead (from the anemometer). Neither of these knows or cares what the current is. Now if I had the water speed paddlewheel plugged in and I was reading "true wind", current would be a factor, but I wasn't.

I dont think thats right. Say you were going into 10 kts of current and 10 kts of wind, both right on the nose. And if you could measure it in this test, you had 10 kts of speed through water. You would have 10 kts apparent wind and zero GPS speed. If you were to drift perfectly with the current in this test, you would have zero apparent and 10 kts ground speed. So, GPS and apparent cannot always be relied on alone for this type calibration. Thats why I was saying earlier to calibrate with as close to no wind as possible. You remove one major unknown. Maybe early in the morning, and if you have significant current in your area do two runs 180 deg apart. That should be very close. I dont have much current in my area, so I wasnt thinking of that earlier.

 

[jaysweet]

So I'm actually a mathematician by trade, and I think the math here is pretty straightforward, so let's write down the equations (which I should have done for myself earlier). Suppose there were a scale factor of k between the actual (apparent) wind at the anemometer and what the base unit shows for apparent wind. If it were dead calm (0 true wind), we'd be done. I was moving at 5.5 knots through still air, so the apparent wind should be 5.5 knots, but the meter read 8.5. So the equation would be
k*5.5 = 8.5
and I could just solve for the scale factor. But I think there was probably a small (but unknown) true wind x. I get one equation from motoring into the wind and one from motoring 180 degrees away from (with) the wind:
k*(5.5 + x) = 8.5
k*(5.5 - x ) = 7.5
Even though these aren't linear equations, you can still solve the system for x and k, and you get k = 1.4545 (repeating) and x = 0.34375.
So I'd want to change the calibration factor (which is currently 1.0) to 1/k = 0.6875, since the meter was reading out larger numbers than the actual wind speed.

Of course, if the wind was really that light and variable, I don't know whether or not it changed during the period I was doing a 180 with the boat, and I wasn't all that careful about noting the GPS speed and the wind speed as measured on the base unit. In fact, I was pretty sloppy. So I visited the boat this afternoon and changed it from 1.0 to 0.8. If I ever get a truly windless day (today it was gusting to over 20 at the nearby weather station, Key Bridge), I'll run the experiment again, and hopefully get the GPS speed when I'm going in either direction.

 

[dlochner]

As a mathematician you know n=1 studies are pretty weak. Multiple trials under different conditions will yield a more robust data set.

 

[jaysweet]

As a mathematician you know n=1 studies are pretty weak. Multiple trials under different conditions will yield a more robust data set.

Right. I'm assuming my observations had a high degree of error. In fact, I didn't even write them down: this is just from memory. I really doubt the right calibration factor is 0.69, but it might be 0.75 or 0.8. Anyway, as I said, if and when we get a day when the Chesapeake is glassy smooth with no wind (and probably 95 degrees F to boot), I should be able to motor in any direction and have the apparent wind equal to the GPS speed, and then I'll know the calibration is correct. Today, I just guessed at 0.8.

Still kind of p%^&*d off that there isn't a Raymarine table somewhere that says "if you use model X wind transducer with an old ST60, you need a calibration factor of 0.75" (or whatever).

 

[jaysweet]

I dont think thats right. Say you were going into 10 kts of current and 10 kts of wind, both right on the nose. And if you could measure it in this test, you had 10 kts of speed through water. You would have 10 kts apparent wind and zero GPS speed. If you were to drift perfectly with the current in this test, you would have zero apparent and 10 kts ground speed. So, GPS and apparent cannot always be relied on alone for this type calibration. Thats why I was saying earlier to calibrate with as close to no wind as possible. You remove one major unknown. Maybe early in the morning, and if you have significant current in your area do two runs 180 deg apart. That should be very close. I dont have much current in my area, so I wasnt thinking of that earlier.

I have to still assert that my statement was correct. GPS speed is speed over ground. True wind is wind speed over ground. Of course, you'd really want to talk about velocity (which is a vector, not a scalar), but if we're moving directly into or away from the wind, we can simplify and just talk speed (which is a scalar). So apparent wind is the (vector) sum of true wind and your motion over the ground. The water movement has absolutely nothing to do with it. If your ST60 is set to measure apparent wind, which is just exactly what you sense at the (moving) anemometer, then the current can be doing anything it wants to be doing. If I'm moving at 5.5 knots due North over the ground, the current could be running at 10 knots from the north, or 10 knots from the south, or 1000 knots from the east, and it won't affect what my anemometer reads at the masthead, and I'm still moving at 5.5 knots over ground. If you're out doing your test, and a giant sucks up all the water with a straw, leaving your boat moving over the bottom on wheels (which you were smart enough to attach to your sailboat ahead of time), the lack of water won't affect the apparent wind your wind instrument senses at the masthead one bit.

How your wind instrument calculates (or tries to calculate) the TRUE wind is another matter altogether.

 

[Davidasailor26]

There is a difference between “true wind speed” and “ground wind speed”. True wind speed is generally defined, at least by the instrument manufacturers, as the wind speed relative to the surface of the water. Ground wind speed is the wind relative to the surface of the earth. The reason instruments usually show true wind speed is because if you’re a racer trying to sail to your polars, or measuring your performance, the boat’s speed through the water in a certain wind speed also over the water is what matters.

Having said that, if you’re definitely reading apparent wind then there are no calculations built in. You’re just reading wind speed relative to the boat, and if your GPS course and speed are inverted you’re right that the apparent wind should match.

 

[jaysweet]

There is a difference between “true wind speed” and “ground wind speed”. True wind speed is generally defined, at least by the instrument manufacturers, as the wind speed relative to the surface of the water. Ground wind speed is the wind relative to the surface of the earth. The reason instruments usually show true wind speed is because if you’re a racer trying to sail to your polars, or measuring your performance, the boat’s speed through the water in a certain wind speed also over the water is what matters.

Having said that, if you’re definitely reading apparent wind then there are no calculations built in. You’re just reading wind speed relative to the boat, and if your GPS course and speed are inverted you’re right that the apparent wind should match.

I didn't know that. And looking it up, the definitions I see on the web are pretty ambiguous, which is annoying. For example, the predict wind definition is "True Wind Speed (TWS) is a critical concept in sailing and maritime navigation, referring to the speed of the wind relative to a stationary observer on the water.", but it's a bit ambiguous about what "a stationary observer on the water" means. I'll take your word for it that they mean stationary with respect to the surface of the water, but they could be more careful with the statement. Thanks for the correction!

Actually, looking around, I'm not sure what the standard definition is. Here's a link from Blue Water Sailing: True Wind from Apparent Wind – Blue Water Sailing , which says

"There are several ways to define the wind. For weather work at sea we care only about the true wind. This true wind is the speed and the direction of the wind relative to the fixed earth under the ocean."

Anyway, as you say, this isn't relevant when we're talking about apparent wind, which is always just what an obeserver on a moving boat feels as he/she holds up a wind instrument.

 

[jssailem]

When I am comparing tools like the Wind transducer, I find a separate tool helpful in the process of calibration.
Something like this is helpful

Cost around $13 USD.
You must account for the variance in wind speed at 10 meters versus the surface, but this can get you close for calibration purposes.

 

[shemandr]

Call me a heretic but I think as someone sailing apparent WS is what I care about. What else matters?
I understand to rate your performance against polars you have to work in TWS. But analogous to true North vs Magnetic North, one is actually what you are seeing. I don't know why there is such an effort to determine TWS when at the helm you are sailing in AWS.
In the sailing area I'm used to current is paramount. So it's really helpful to know STW vs SOG. The difference between SOG and STW is current and a few other phenomenon. Nevertheless we are sailing in the conditions that exist at the moment. That is what is great about it.

 

[jaysweet]

Call me a heretic but I think as someone sailing apparent WS is what I care about. What else matters?
I understand to rate your performance against polars you have to work in TWS. But analogous to true North vs Magnetic North, one is actually what you are seeing. I don't know why there is such an effort to determine TWS when at the helm you are sailing in AWS.
In the sailing area I'm used to current is paramount. So it's really helpful to know STW vs SOG. The difference between SOG and STW is current and a few other phenomenon. Nevertheless we are sailing in the conditions that exist at the moment. That is what is great about it.

Well, sure. For purposes of sailing, I agree that apparent wind is what I care about. As I said, I don't even have my paddlewheel sensor hooked up, so I never set the ST60 to read out TWS (which it has to calculate using apparent wind plus direction plus STW). But you can't even talk about how to figure out if your anemometer is reading correctly without at least considering the concept of true wind, which was what this post was originally about. It just got sidetracked a bit at around #16-17.