True and Apparent Wind

And I suspect that heading goes on the SeaTalk bus at the damped heading rate (where damping is user selectable) from the compass instrument too, not the sensor rate (if fluxgate, don't know about their "quasi" gyro). Suspect this as best I remember all other heading indications on the bus (eg multi) and the NMEA from the pilot computer or NMEA box respond at similar damped rate as the compass instrument does.

John
 
That's probably correct. I've looked at the Seatalk bus and there can be a big delay between the compass sending a heading and it appearing on the bus so it's never going to track TW accurately. Even worse if the compass has to go through an NMEA bridge.

I think the only one which comes close is the B&G system, and even this doesn't look too clever in a turn.

I could put up some graphs showing results of an experiment we did a couple of years ago. We motored the boat around a fixed calibrated wind instrument and recorded the ST60 true wind whilst also recording the calibrated instrument. True wind speed errors were up to 15% and direction errors were up to 12 degrees, hardly impressive! The apparent wind readings from the ST60 were fine.
 
If you do compare one instrument against another make sure the display damping factors are set to similar values or the readings will differ... unless the wind speed/direction is steady of course. Otherwise you will be comparing the two instrument filter responses.
 
My thinking is that the number 26 bus linked with an inverted ovaron flux capacitor can give you true wind. I believe you are confusing this with the lessor Limpex 'tm' capacitor linked to the number 43 bus which is inferior in every way although this is not allways apparent to less experienced skippers. That is why we allways tell our cadets to start out with the well licked inverted two finger, co-incedental with the teltale, aproach.
 
I think this highly technical approach responding to the simple original question has distanced the originator (pisces) from contributing further - or do you have any 'follow up' queries
 
All I was trying to say was that the Limpex capacitor has been superceded by the Ovaron which delivers a higher warp speed. These are for the numbnuts who cant tell from which direction the wind is coming from.
 
Florida Uni .. ?

Rev Ian Paisley got his divinity doctorate there? they'll no all aboot wind, then !!??
 
[ QUOTE ]
used both fingers on the same hand - and I got some very rude retorts from passing yachts /forums/images/graemlins/confused.gif

[/ QUOTE ]

You used the "stinkie" method. Good grief, no wonder the "raggies" were upset.
 
Re: Apparent is apparently OK - true!

Wrong, wrong wrong......

In order to get the true wind you need to crrect for tidal drift,heel angle, up draft etc as well
 
Re: Apparent is apparently OK - true!

Not if you use the Ovaron. It is only the inferior Limpex that would need all of the parameters you mention inputed to the output bus to achieve true speed apparantly.
 
I am sorry that I still have to disagree with you. The learned reference is absoluttely correct for what it is discussing, however, that has little real relevance to sea going wind instruments. For a start, very few people have a sufficiently accurate heading sensor to achieve the accuracy they are discussing. Secondly, due to the size of the transponders, I would be very surprised if your instruments are not damped and worked on some form of average velocity/bearing - thus making accurate readings during a turn impossible. However, I will say again that the bearing of true wind relevant to ship's head can be calculated or plotted, its not instantaneous, but its as accurate as any other method. Get a plotting sheet and prove it to yourself
 
well my B&G lot seem to do the job. Fluxgate, (pointing sort of the right way now, thanks Tome) log, wind angle transducer, wind speed indicator.

Takes the guesswork out of your finger pointing

Ian
 
I recall the original query was about obtaining the true wind direction ..... to get this one needs a heading reference (compass - mag or gyro) and a velocity metre (anamometer) if one is stationary, just read it off the guage ... however - if moving one needs to know ones true speed and direction - then one can use trig to get the resultant - simple. as to the accuracy, one would need a vertical reference sensor to detect the movement of the instrument on the support (mast) intagrate this with the original readings and hey prestow .... true wind direction.
 
No not scared off - just had to do some work!

I think that the discussion has made my mind up to get a cheap wind
instrument, and use the saved money to get a mast head light and a new
antenna. All this big boat stuff in new to me and I find it very useful to ask
advice before I splash out my hard earned cash!

I am proud that I finally managed to start a discussion that has spread
onto two pages.
 
[ QUOTE ]
The learned reference is absoluttely correct for what it is discussing, however, that has little real relevance to sea going wind instruments

[/ QUOTE ]

I believe it was written for sea going wind instruments on ships and yachts. A decent calibrated fluxgate will provide the required heading reference in place of a gyro, and the damping in the wind instruments on yachts is well within COAPS recommendations.

Am I missing something, or are you just being stubborn?/forums/images/graemlins/smile.gif
 
IMHO you are missing something.

please re-read my previous post (dont see the point in repeating it) The plotting can be done using apparent only. I dont intend to draw it out for you and post it, but if it really interests you It could be done. just use a plotting sheet and try it yourself.
 
Tried it, plotting using apparent bearing doesn't work unless you have heading. How do you relate SOG and COG otherwise?

You mentioned earlier that you could do this by trig. Can you explain how, as it's easier for me to understand?
 
easy - use the formulae supplied by yourself, but use 000 as the ship's course. The formulae will give you an answer of 000 +/-X degrees.

apply that +/-X to the ship's head and you then have the true wind direction (assuming of course that there is no leeway or tidal current!)


go on give it a try /forums/images/graemlins/wink.gif
 
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