SailNet Community banner

1 - 17 of 17 Posts

·
Registered
Joined
·
249 Posts
Discussion Starter #1
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.
 

·
Courtney the Dancer
Joined
·
3,971 Posts
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.
 

·
Sea Slacker
Joined
·
1,789 Posts
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.
 

·
Registered
Joined
·
476 Posts

·
Sea Slacker
Joined
·
1,789 Posts
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 :mad: :mad: :mad: , take a look at :
http://www.sailnet.com/forums/gear-maintenance/41946-i-fully-disagree-raymarine.html#post294906

You can't blame them. This is what standard says (describing what a "true" wind is in a VWT sentence):
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.
 

·
Registered
Joined
·
249 Posts
Discussion Starter #6
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.
 

·
Registered
Joined
·
476 Posts
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 !!
 

·
Sea Slacker
Joined
·
1,789 Posts
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).
 

·
Registered
Joined
·
249 Posts
Discussion Starter #9 (Edited)
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.
 

·
Registered
Joined
·
476 Posts
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.
 

·
Registered
Joined
·
476 Posts
PS: when I called out Raymarine, my suggestion was searching the net for SOG first, then STW after. But remember, calculating True wind from STW is a big mathematical mistake, accepted only during pre-GPS era.
 

·
Sea Slacker
Joined
·
1,789 Posts
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.
Ah, but you do need HDG to get a correct result. Here is why:
Let's say you have the following -
1) relative apparent wind (VWR) as delivered by windvane
2) GPS information including COG/SOG.
3) nothing else
Now - think about this - your vessel is heading in some direction which is most likely NOT aligned with COG. Relative wind is a vector that you can calculate based on a system that has boat heading as one axis (and another axis perpendicular).

On the other hand your SOG (and SOG induced "wind") is a vector that by itself can only be considered in a system with one axis along the COG (and another perpendicular).

Without heading there is nothing to reconcile these two systems and no way to do any reasonable operations on these vectors. You need heading to convert one of these vectors to the coordinate system of the other.

I could bring an example but I think if you try the math you will see what the issue is. Now, granted in most situations heading and course over ground are "quite close" and the results are not too far off, but by the same token in most situations STW and SOG are quite close too. And so the error would be about the same in both situations.
 

·
Sea Slacker
Joined
·
1,789 Posts
Here is a practical example. The first set of dials shows COG and heading that differ by 20 degrees. The true wind is calculated here based on SOG (and using both COG and HDG to adjust vectors). You can see true wind both relative to the boat (second dial from the left) and as an absolute direction (last dial on right). There is no STW (as the heading dial shows).

On the second image I adjusted heading to match COG. This is what would effectively happen if you used SOG for true wind calculation but did not have an adjustment angle (and assumed direction of travel matching heading).

It is quite clear that true wind resulting from such calculation is not the same as the one that results from properly accounting for COG/HDG difference. And the difference is not insignificant - 30 degrees and about 40% stronger.

Incidentally, third picture shows calculations using STW and heading. I set STW to be equal to SOG, which is not necessarily true but it helps make an example simpler. As you can see it is also incorrect, but the result is essentially the same as when using SOG alone without proper heading :) I.e. there is no gain in calculation precision.





 

·
Registered
Joined
·
476 Posts
Yep !! I must correct my statement, you're right indeed. I was refering to a particular case where SOG is aligned to HDG, but your approach is rather more complete. There are 2 factors affecting a correct true wind speed and angle: 1- By using SOG instead STW you remove the stream current factor, and 2- by using COG and HDG you remove the apparent angle incertainty. In fact, NMEA 2000 has solved this (PGN 130306) by adding new True Wind difinition, allowing user to choose from SOG or STW, but both account for COG and HDG for a more precise true calculation. Brak, thanks for it, I enjoyed the discussion a lot ...
 

·
Sea Slacker
Joined
·
1,789 Posts
In fact, NMEA 2000 has solved this (PGN 130306) by adding new True Wind difinition, allowing user to choose from SOG or STW, but both account for COG and HDG for a more precise true calculation.

Woo-hoo, so PolarCOM is NMEA2000-"compliant" now :) :)

If only there was a hardware adapter that provided NMEA2000 frames to be read directly by a PC (rather than all those monstrosities that "convert" NMEA2000 to 0183). That's one drawback of NMEA2000 - no simple direct interface to computers, unlike its predecessor. I think that helps hardware manufacturers sell more stuff, but certainly not software.
 

·
Registered
Joined
·
476 Posts
Although NMEA is a .org association, it's managed by people from marine industry and naturally defend their interests. There is no other explanation to not using standard transport protocol like TCP/IP or even UDP, or using standard connections like RJ-45 on dry areas, etc ... That would simplify the entire system and save a lot to us.
 

·
Sea Slacker
Joined
·
1,789 Posts
Although NMEA is a .org association, it's managed by people from marine industry and naturally defend their interests. There is no other explanation to not using standard transport protocol like TCP/IP or even UDP, or using standard connections like RJ-45 on dry areas, etc ... That would simplify the entire system and save a lot to us.

The easy solution they could have taken would be to simply encapsulate 0183 in ethernet frames (or any higher level protocol from the standard stack at their choice), add a few routing tags and keep everything and everyone compatible while overcoming performance and connectivity issues. But that's neither here nor there, too late.

I wonder if some of the bigger software makers are represented there. what would they do? go back to incompatible proprietary interfaces? that seems counter-productive. Stick with NMEA0183 - sure, but that means manufacturers will have to continue supporting it. I know no one gives a rat's behind about my product, but what about all those MaxSeas and Navionicses etc? They also need to get data somehow, and I just don't see any provisions for PC connectivity in NMEA 2000.
 
1 - 17 of 17 Posts
Top