Apparent and true wind speed

[mldpt]

I have autohelm instruments, ST 50 now discontinued, my wind speed & direction will display true wind and apparent, but only if supplied with boat speed, from the log, not from Gps SOG, as would be correct, my problem is that my speed log is in fact Stow instrument, so doesnt talk to my ST50 wind speed, I have a ST50 tridata display, and I suppose I shall have to fit the correct transducer to be able to use the true wind display. But why does it need boat speed to calculate true wind, when it would be more accurate to use SOG, it must use COG for the boats course to calculat the true wind direction but not apparently the true wind speed.
Any ideas.
Mike

 

[philip_stevens]

I've never thought of what you have written, as I have the set-up that uses log speed and not GPS SOG speed. I will be interested in replies to this one.

My instruments are Navman, and the installation instructions (and the way I have connected them) show that the wind gets an input from the log output. That same log output also goes to the GPS to give various data displays from there.

Very interesting!!

regards,
Philip

 

[JeremyF]

Maybe it might be something commercial - Using log data, they can mandate Seatalk, and therefore stick to proprietary instruments. For SOG they would have to conform to NMEA, and Raymarine would loose their stranglehold.

Maybe I'm paranoic, but I suspect that Raymarine give away the basic ST60 tridata to the likes of Bavaria, in the hope of capturing the extra instruments. £500 for a wind instrument is lifting one's leg.

Alternatively ignore all interfacing, and recognise that True is as it sounds and Apparent is the wind strength by the time you're relaying the wind conditions experienced out there when snug in the bar.

Jeremy Flynn

 

[peterb]

Using the GPS speed would give the wind speed relative to the ground. What most people need is the speed relative to the water, i.e. the wind speed that you would feel if you were stopped in the water. Speed relative to the water is the one that has most effect on choice of sails, etc. (if you choose to go by instruments rather than boat behaviour).

 

[ParaHandy]

You would need course as well as speed over ground to correctly resolve the apparent wind vector's strength and direction. Once COG enters the equation, the apparent wind value becomes, almost, meaningless to you on your boat. Hope that's clear although might help if you drew the three vectors on piece of paper....maths gets complicated otherwise (which is why there are few good maths teachers in the country!).

 

[jfkal]

I go with ParaHandy's rationale. However there is a Raytheon issue here as well.
The NMEA output you are probably using does not translate all data but a few. COG and SOG are NOT included. You need a converter box from Autohelm which translates the rest. Got an awakeing trying my new MAXIVIEW: no GPS data displayed. After installing the converter everything is there (except depth keel offset) .

 

[Twister_Ken]

Don\'t worry

True WS is about the least valuable piece of info you can get from a modern set up. The boat sails to the apparent wind, not the true. And in any case the mental arithmatic to go from apparent to true is dead simple (at least to an accuracy of a knot or two). True wind direction is more useful if you're coming up to a turning mark, but again it's fairly easy to visualise it mentally to within 10 or 15 degrees (or look at the boats going the other way).

 

[Chris_Robb]

The most useful reading is Apparent wind as this is the actual force and direction operating against the boat. This is a combination of boat speed, wind direction and Tide flow and dirction - all of which make up the direction and speed of the wind felt on the boat.

On the ST50 true wind is only the factor of taking off boat speed, which of course leaves the tide effect altering the direction of the wind.

I cannot percieve what use a reading of absolute true wind to the ground would be to a yachtsman.

You are currently getting the apparent wind direction. I would not spend any money getting the true wind to work, as it is relatively easy to mentally adjust the apparent wind by deducting the wind effect - ie 20 knots wind less 5 knots boat speed and the true wind is around 16knts (not 15 as you are not going straight into the wind) Its a nice to have - thats all

 

[tome]

Chris

Don't entirely agree that True wind speed isn't useful, especially when running before tide and wind as we were this weekend. Our SOG was close to 10 knots at times in apparent wind of around 25 knots. We were heavily reefed and thankful of it when forced to round to windward. Seeing 35 knots true does have a moderating influence on sail plan.

On the other hand, agree that I wouldn't pay much for this and can easily calculate true to within a couple of knots. You should use SOG (from GPS) rather than boat speed when doing this.

 

[mldpt]

I agree with most of what has been sent in reply to my post, having sailed some 30,000 nm in my boat, without having the true wind speed and direction available, but having got ST50 which offers both true and apparent, I would like to be able to use it, but the point I was making was WHY do they use boat speed and direction through the water to calculate the true readings when this will not in fact give true wind speed and direction. Also I assume that it uses COG for direction, as there is no other way of knowing which way the boat is travelling, so why not use SOG for the boat speed, when both of these are available from the GPS. So I still have to purchase the ST50 speed log as the ST50 wind needs this to calculate the apparent wind speed.
I am aware that if I have 25kts on the nose and speed over the ground is 5 kts the true wind speed is 20kts. and it I am on a dead run at 7kts and the wind speed indicator reads 20kts true wind is 27kts, which means its blowing quite hard, so does anybody know if its possible to use the stow log output to convert to sea talk, I do have a nmea to sea talk interface. regards Mike, www.pagetsladysailing.co.uk

 

[philip_stevens]

Maybe it's just me, but I am finding some of the answers a bit confusing.

If I use the log to give me speed, and this is fed to the wind instrument to give true wind speed and direction, OK. But if the boat is doing NO SOG (as it could in the Alderney Race) the True wind would think the boat is travelling at say, 6 knots, when it is in fact either standing still over the ground or even going astern - in spring ebb tide.

So in this instance (and lets not be too clever with the maths) the true wind would be the actual (relative) wind, and the SOG would be more accurate than the Log.

Is my thinking correct, or am I totally boggled.

regards,
Philip

 

[tome]

Spot on. In your example SOG = 0 and true wind = apparent wind.

Log speed is only useful for true wind determination when tidal stream is slack, at all other times you should use SOG

 

[mldpt]

Exactly that is my point, we all know that true wind isnt important, but the fact is that its not possible to calculate true wind useing log speed through the water, so why does autohelm do it, when sog and cog are already available from the GPS.
Regards Mike.

 

[tome]

Can't answer this except to speculate. If this gets long-winded please skip, if anyone wants to PM please do so.

NMEA is the standard for marine instrumentation and provides a wide range of sentences for distributing information amongst systems and instruments, eg GPS radar, autopilot. It has some inherent structural limitations (example is GPS positions which do not indicate whether in WGS-84 or local datum), is slow at 4800 baud, contains much redundant information and requires at least 4 separate power and data wires. This led to many manufacturers defining their own standards eg Raytheon Seatalk or Simrads Roblink (only 2 wires).

Fallout is that instrument manufacturers no longer provide true NMEA capability but favour their own in-house standards with NMEA as an afterthought, eg buy the optional NMEA converter box, Onassis.

Raytheon actually produce two separate GPS receivers, one for their own Seatalk or an alternative for NMEA (ie if you must buy other instruments). Probably the Seatalk GPS works fine, but they don't expect you to buy the NMEA version when using their Seatalk instruments. Ok, but most of us buy our favourite GPS which are all NMEA.

I'm speculating that Seatalk instruments can't read an NMEA RMC sentence which contains SOG and COG without the 'box'.

True wind is the wind experienced by an earth-stationary instrument. Provided a vessel SOG and COG are known, it's easy to calculate from apparent wind.

If only Seatalk log speed (and god-make-it-be-true course steered) are available, then 'Tide Relative Wind' is the sort of useless number a trigonometric computer nerd without sailing experience may be tempted to calculate.

Having done so, he'd want to tell the world.

 

[Guest]

Dear Oh Dear. Do people now need this sort of information just to sail their boats? Feel the wind my boy!

 

[Chris_Robb]

Mislead you - meant to say that the SOG element in true wind was not that important. And yes - one can calculate it fairly easily.

SOG is actually relevant - re anticipating a change in a tide, especially at springs, when a change for east going to west going giving perhaps 6 knots change may cause you to Reef. - Actually if you have done your passage plan properly, you wont need the SOG to tell you that this will happen - all it will do is confirm it!

So a nice to have - but not essential

My standard ST50 wind gear at the top of the mast is blanketed on a dead run by the Top light which is a Triclour over a anchor light. This means that in a gale dead down wind - it will only whos 6 7 knots!! I feel its a design fault in that my mast head fittings are pretty standard, and yet the wind vane is below the level of the toplight. Does any one else suffer this????

 

[mldpt]

Of course Ratheon and Autohelm read NMEA and convert to their own seatalk. I know there is no need to know the true wind or for that matter the apparent wind, or even the depth, but the fact is we ormost of us splash out on instruments we dont need, the point I was making was that, if you should happen to be lucky enough to have Ratheon windspeed ST50 and also ST50 log, you still dont get true wind speed, because any **** knows you cannot calculate the true wind speed and direction from speed through the water as this does not take into account tidal flow, I assume it uses COG otherwise how would it know in what direction the boat was tranelling, so why not use SOG as well both of these are supplied by the GPS.
I wish I hadnt asked.
Regards MIke

 

[philip_stevens]

I started this thread going, and very interesting reading it has made.

Going from what you have just written, I agree that SOG would give a true wind speed. Having read your reply, I have had a look at the installation instructions for my Navman instruments, and it only shows a log NMEA connection to and between the log and wind instruments. No information is sent to the wind instrument concerning COG - which it needs with SOG to give a really true wind speed reading.

So if I could give my wind instrument a SOG connection to the GPS, I would be getting a better true wind speed.

As you say, RNR (really not required) but when it is there, why not have it on display.

All very strange that the instrument makers should only have a log input.

Now, what would the PBO "experts" have to say?? I'll watch with interest in the Sept or Oct issues.

regards,
Philip

 

[mldpt]

who started this thread

 

[philip_stevens]

Apologies, I stand corrected. I came in on the second act.

But it has made good reading.

regards,
Philip