Seatalk 1 question(s)

[Iain B]

Hello, folks!
Old boat, old system, but it still works. If it ain't broke...
However...instruments are kinda new to me. Walker log and a compass did me for 20 years and 20 trips from Edinburgh to Bergen...on a motorboat.
Now I've got a sailboat. 40yr old ferrocement Samson C-Quinn. It has toys. Raymarine ST7000 autopilot, ST60 instruments. NEW windvane transducer. All Seatalk 1. (Yes, I know the windspeed indication is 30% high because of this!)
I'm assuming that the windspeed indication, let's say when motoring directly into the wind, will calculate true wind speed by deducting boat speed from wind speed. Doesn't seem to, true and apparent are only 1m/s different at 6 knots. I was running dead downwind today, same 1m/s... Surely the true and apparent should be wind speed plus, minus, boat speed? Am I wrong? I've (almost) got the calibration on the boat speed to read the same as the GPS (not on the Seatalk bus, though there is one on it). Presumably, true and apparent wind angle will be affected similarly.
From what I've read, it doesn't matter which instrument is connected to which, the Seatalk bus doesn't seem to care.
Any insights, clues, or £10 notes gratefully received
Thanks,
Iain

 

[lustyd]

Do you have a ST1 fluxgate compass?

 

[AngusMcDoon]

For your wind instrument to calculate and display true wind from the measured apparent wind data it needs to receive boat speed via Seatalk. Nothing else is needed. I can't remember whether ST60's will calculate true wind only from speed through the water or whether they'll do it from GPS supplied speed over ground. Someone else will know.

Here's the geeky explanation. It's not a simple addition or subtraction of boat speed to measured wind speed (unless the wind is directly inline with boat heading) - it's a bit of vector arithmetic. Here's the vector diagram for the boat going into a wind at 45 degrees from the bow on the starboard side (arrow length is strength of the wind, arrow angle is relative to the boat)...



TWS true wind speed
TWA true wind angle
AWS Apparent wind speed
AWA Apparent wind angle
STW Wind made by you speed through water

You know AWS, AWA and STW. That's enough to calculate length TWS and angle TWA in that triangle of vectors using a bit of trigonometry. As you can see AWS (the wind you feel on your face on the moving boat) is stronger than TWS (the wind you'd feel if you were not moving), and the direction is shifted closer to the bow.

It doesn't matter which Seatalk 1 instrument is connected to which, but they all need to be daisy chained together. You need as a minimum your log display head connected to you wind display head (or GPS receiver if it will use SOG).

 

[laika]

I can't remember whether ST60's will calculate true wind only from speed through the water or whether they'll do it from GPS supplied speed over ground. Someone else will know.

What has been cited in previous discussions on this is raymarine's statement that what they call "True Wind" is always relative to water surface and therefore calculated using STW rather than what they call "Ground Wind" which more modern devices will calculate (relative to the ground) using SOG:
Apparent Wind, True Wind and Ground Wind, and data required to calculate

 

[Iain B]

Lystid, yes, original fluxgate. Also the original paddle wheel speed sensor. I'll pull the paddle wheel and clean it, as it seems to be a wee bit slow/inaccurate anyway. Can't hurt!
Angus McDoon, I kind of figured there would be vectors, just used the 'simple' up/downwind to explain my puzzlement...
Laika, handy link, I think perhaps I have enough info. to dig into it a bit deeper, shame the Raymarine forum and Chuck are gone...
Any more input is always welcome, thanks so far!
Iain