True Wind Speed and Direction - SailNet Community
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post #1 of 35 Old 11-27-2008 Thread Starter
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True Wind Speed and Direction

The Admiral and I once thought we wanted a Raymarine ST60 wind speed and direction instrument. To me, though, such an instrument would be less useful than it could be without the ability to also yield true wind speed and direction. Talking to a fellow club-member a few days ago, he asserted the ST60 wind speed and direction instrument could do that. I just looked at Raymarine's site and, sure enough, it claims the instrument can do it. Curious: I downloaded the PDF of the user's manual. Yup: It says it does it. But here's the thing I don't get: Perhaps my mad math skillz are rusty, but ISTM that for the instrument to yield true wind speed and direction, it would have to know boat speed and heading. But the instructions for the device don't indicate having to hook up Seatalk connections to devices that will provide that. So how the heck can it calculate true wind speed and direction without knowing boat speed and heading?

Jim
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post #2 of 35 Old 11-28-2008
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I believe that they are assuming that you have the speed log and the flux-gate compass installed already. One of my bones of contention with these companies, of particular concern in internet sales, is that it like pulling hen's teeth to get the details you need pre-purchase. Whether this stems from a history of customers purchasing primarily through a knowledgeable dealer and not direct, I do not know. But it would be hardly the first time one opened the box to find the "installation" manual only to discover facts that no one seemed to know pre-purchase.

The West Marine catalog usually has more information than the manufacturers provide within their technical articles and this case is no exception; you need velocity information, either from GPS or speed log and electronic compass.

Now as to what is compatible......

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post #3 of 35 Old 11-28-2008
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SEMjim :

It can't. You are right.
The central processor must know true boat speed and true heading before it can make that correction. If there is a tide running, even boat water speed will not do it, it has to be groundspeed, so a GPS feedback is needed.
Happily, relative wind speed and direction are more important.
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post #4 of 35 Old 11-28-2008
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The boat speed I understand, and that I agree if you are using speed over water you won't get an accurate reading, although if you're not in an extremely strong current you should be able to get a pretty good reading of the true wind speed.

The compass connection is not necessary if you are just looking to get a true wind direction that is in relation to your current boat heading. And if you really want to know what the compass direction of the wind is, just look at your compass and the true wind direction in relation to your boat and you should be able to get a pretty accurate reading of the compass heading of the wind.
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post #5 of 35 Old 11-28-2008
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Jim


the ST60, when connected to the ST60 Tridata or ST60 speed log WILL indicate TRUE WIND speed and direction, as well as APPARENT wind and direction.

If you disconnect the seatalk cable it will only indicate apparent wind speed and direction.

Your speed log must also be calibrated correctly to allow the true wind to be calculated accurately.

The ST60 mainly measures apparent wind only and calculates true by compting boat speed..IT DOES NOT NEED a compass connected to it..

Now..if ypu are into or down a current, boat speed over ground changes, and as a consequence, the true wind direction and sopeed will change accordingly...

A current smaller than say 2 or 3 knots is not going to make a big difference.

but it does do all that..needs a speed log and a connecting cable...NO COMPASS needed
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post #6 of 35 Old 11-28-2008
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So THAT'S why mine reads the same on both true and aparent!
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post #7 of 35 Old 11-28-2008 Thread Starter
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Quote:
Originally Posted by sailaway21 View Post
Now as to what is compatible......
Garmin 498C, in our case. Probably too much to hope for.

Thanks, Sway.

Jim
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So THAT'S why mine reads the same on both true and aparent!
If you actually tried sailing...it would work...at anchor they are equal...
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post #9 of 35 Old 11-28-2008
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My ST60 reads both apparent and true wind speed. I also have the ST60 knotmeter too. There is definably a difference in the reading, direction etc when on apparent vs true.

I do not have it hooked to a GPS, which might yield something slightly different when I am in a tide running here in puget sound, but there is a difference in what it reads as is.
Marty

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post #10 of 35 Old 11-28-2008 Thread Starter
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Quote:
Originally Posted by Giulietta View Post
Jim


the ST60, when connected to the ST60 Tridata or ST60 speed log WILL indicate TRUE WIND speed and direction, as well as APPARENT wind and direction.
Happily, we have an ST60 speed log.

Quote:
Originally Posted by Giulietta View Post
The ST60 mainly measures apparent wind only and calculates true by compting boat speed..IT DOES NOT NEED a compass connected to it..
I guess my mad math skillz are rusty, because you and CalypsoP35 are correct--something I could've determined from a trivial bit of research. For example: True and Apparent Wind Speed Calculator (sailingusa.info).

Cool

However: By "true wind direction," I meant as a compass heading. Wind compass heading is another thing, as CalypsoP35 noted. The ST60 device doesn't provide for that, in any event, and the necessary addition/substraction even I should be able to handle .

Thanks, Alex, everybody else!

Jim
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