True Wind vs Crazy Speed Transducer - SailNet Community
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post #1 of 17 Old 10-12-2009 Thread Starter
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Question True Wind vs Crazy Speed Transducer

I've noticed our Raymarine speed transducer gives very erratic readings. I don't expect perfection out of it, but is it normal to see it bounce around in an almost 2 kt range? We're in an inland lake, so I don't expect current effects. I pulled and cleaned the transducer, but it's still erratic.

This would not bother me, as I tend to pay attention to the gps SoG value instead. Trouble is, my wind instruments(RM ST60) seem to use the transducer speed for true wind calculations. Does anyone know if it can be configured to look at SoG instead? It seems like that would be the preferred choice, as otherwise current would affect the true wind calculations, wouldn't it?

I'm using a RM ST60 tri-data, st60 wind instruments, and a C80 chartplotter with a gps receiver.

s/v Dancing Days
Beneteau 31
Lewisville Lake, TX
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post #2 of 17 Old 10-12-2009
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I'm not familiar with the Raymarine instruments, but you shouldn't be seeing fluctuations like that. Not sure if that would be a problem in the transducer or the "Brain", sounds like call to Raymarine is in order.
I doubt that you would be able to easily use the SOG from the GPS to determine TW, or even if that would give accurate readings in all wind/current conditions.

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post #3 of 17 Old 10-12-2009
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It's a curious topic, as I am working on precisely this functionality for PolarCOM right now.

According to NMEA "true" wind is defined as a wind that would be felt by the same vessel if it was in the same place in the water, not moving relative to the water.

I.e. "true" wind by that definition is "true" in relation to water but NOT in relation to the ground. This does make sense, since effect of wind on any vessel is only important relative to the vessel movement in the water. Unless it is stuck on the rocks or something.

In any case, this is what the standard says and so any compliant device has to use only heading and speed through water. Although SOG may be a logical choice (and a preferable for some), compliant devices won't do that.

Incidentally, PolarCOM will give an option to use SOG/COG for "true wind on land" kinda thing precisely for the reasons you describe.
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post #4 of 17 Old 10-12-2009
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This is one of greatest mistakes Raymarine does to its customers. Bad conceptual design and wrong, absolutely wrong wind information. SOG is the only way to go, but they insist in speed through water to calculate true wind !! And there is no way to fix this, until Raymarine engineering responsible go fired !! This still piss me off , take a look at :
I fully disagree to Raymarine .....

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post #5 of 17 Old 10-12-2009
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Originally Posted by negrini View Post
This is one of greatest mistakes Raymarine does to its customers. Bad conceptual design and wrong, absolutely wrong wind information. SOG is the only way to go, but they insist in speed through water to calculate true wind !! And there is no way to fix this, until Raymarine engineering responsible go fired !! This still piss me off , take a look at :
I fully disagree to Raymarine .....

You can't blame them. This is what standard says (describing what a "true" wind is in a VWT sentence):
Quote:
VWT - True Wind Speed and Angle
True wind angle in relation to the vessel's heading and true wind speed referenced to the water. True wind is the vector
sum of the Relative (Apparent) wind vector and the vessel's velocity vector relative to the water along the heading line of
the vessel. It represents the wind at the vessel if it were stationary relative to the water and heading in the same direction.
They just do what they are told to do, I suppose. Plus - the idea is to return "true" wind even if you don't have a GPS connected.

That said, an option to use SOG/COG is certainly useful and would give the user control of which type of a "true" wind he'd can get.
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post #6 of 17 Old 10-12-2009 Thread Starter
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Ah, well I guess I'm stuck with transducer speed. As annoying is it is that I can't configure the ST60 to use SoG. If my transducer was more stable, it would make no difference since I'm on a lake.

How much variance do other people see on their paddle-wheel transducers? Does the ST60 apply any damping to the boat speed input? There must be damping somewhere, since the wind gauge doesn't jump around near as much as speed-through-water. I assume the average of the values coming out of the transducer is close to correct.

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post #7 of 17 Old 10-14-2009
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Brak, NMEA has discontinued VWT for almost 2 years now, since its conception was from non-gps era. I still believe Raymarine should be smarter implementing more accurate information as, regardless bacampbe reading errors, speed over water is affected by current, what if strong enough will generate erratic (dangerous in some cases) information. SOG for 2010 !!

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post #8 of 17 Old 10-14-2009
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SOG still requires COG *and* HDG to be used to get true wind out of relative apparent wind (the only kind you can actually measure ). I.e. even if you have a GPS, you still need an electronic compass to back out SOG-based true wind.

On the other hand, to back out STW (speed through water) based true wind you need nothing at all except that speed and the actual apparent wind value and angle relative to boat heading (i.e. what VWR provides).
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post #9 of 17 Old 10-14-2009 Thread Starter
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Ah, I was about to say I don't follow--but when I think about it you're accounting for the fact you may not actually be pointed in the direction of travel, right? That's actually the best reason I've seen for using STW.

OTOH, I _do_ have an electronic compass. But with all the other electronics on board to mess with it, I suspect it's no more accurate than my STW.

s/v Dancing Days
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Last edited by bacampbe; 10-14-2009 at 09:43 PM.
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Brak, this is incorrect. All those sentences are native to any GPS nowadays. But note, there is no need for HDG, nor a compass at all, as useful true wind is relative to boat, so when you calculate the vector, wind direction relative to Earth is no needed. Just like STW, there is no need for COG, just the apparent angle. What I want is a safe true wind speed indicator to decide weather or not raise my spy.

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