Logic?

[Albanach]

It eludes me why the Raymarine Smartpilot is not smart enough to calculate true wind speed from SOG, once the latter is fed thru by the GPS via NMEA.


My query to Raymarine produced this reply:
“For True wind data you need to have Speed through the water, not SOG. Using SOG and Apparent wind is a completely different calculation called Ground Wind.”


Surely, once the SOG is available, it should be relatively simple to for the computer to switch and calculate from that data, especially if STW is not available due to a fouled paddlewheel. I would think that the formula would be the same and the result would certainly be more accurate.


Is there a more intelligent answer than Raymarine’s on the subject? I noticed on the search facility that Witzend had a similar query some time ago.

 

[Rick486]

I don't understand their answer. SOG must enter the calculation of true wind from apparent in any case, since SOG influences boat speed relative to the true wind. For example, a boat making zero speed through the water with zero true wind speed still sees an apparent wind. I will say that when my paddle wheel is stuck, true wind and apparent wind are the same so I guess Ray is neglecting the current in their true wind calc. Probably an ok assumption in most cases, come to think of it. That is, in most cases, SOG is pretty close to speed through the water.

 

[Rick486]

I don't understand their answer. SOG must enter the calculation of true wind from apparent in any case, since SOG influences boat speed relative to the true wind. For example, a boat making zero speed through the water with zero true wind speed still sees an apparent wind. I will say that when my paddle wheel is stuck, true wind and apparent wind are the same so I guess Ray is neglecting the current in their true wind calc. Probably an ok assumption in most cases, come to think of it. That is, in most cases, SOG is pretty close to speed through the water.

Come to think of it here at 0500, one would need an outside reference to current at the point of measurement as the snart pilot has no way to separate out speed through the water from SOG without a current reference, which would be prone to errors as well. So their answer is spot on.

 

[shemandr]

I'm not an engineer but I think Raymarine is right, too. It's speed through the water that counts to calculate true wind. If you used SOG, and let's say you were slowed by 2 knots of current. So SOG would be 2 knots less the STW. But your anerometer is seeing 2 more knots of apparent wind than SOG. That's a hugh difference for a boat traveling 5 knots.

 

[kenn]

I'm backing the consensus. Calculation using SOG would yield something which would be the vector sum of wind plus current. Call it "true net force"

 

[Bill Roosa]

Ray is correct. consider traveling backward going 4 knots through the water in a 5 knot opposing current (-1 knot SOG) What is your apparent wind then?
The rest is just vector math but you have to start with the right vectors!

 

[Don12364]

It all Depends!

I think it depends on your definition of "True Wind".
For the man on the shore (i.e. the met man's forecast) the on board 'aparrent' wind needs correcting for both boat speed and direction AND tidal stream speed and direction. Thus one reduces the boat's movement to zero which is the same as the GPS could do.
However, for the man on the boat, he will think the true wind is what is left after the boat's speed and direction has been allowed for because he is unaware of the current.

So IMHO Albanch is correct for comparison with the weather forecast but not for the helmsman in a tideway..............................I may be wrong though as Raymarine do not say what their measurement is!

Many years ago we were racing off Cowes in an ultra light wind. It was from astern and everyone had spinnakers up. Ours kept on collapsing until I noticed that it was actually blowing in. The current under us from astern, even in open sea, was greater than the windspeed so we were being thrust into the air faster than it was blowing.
We took down our kite, set the No.1 genoa and beat to windwards with a 3 knot weather going tide and only a 1 knot breeze. i.e. We had 2 knots on the nose.
We tacked through the entire fleet who still had their kites up and we won.
Afterwards everyone wanted to know about our 'special wind'.
I guess they are still wondering.

 

[Stu Jackson]

Don, isnt' that a perfect example of why true wind SHOULD take currents into account, and thus use SOG?

Here's another example: Boat beam reaching on starboard tack, boat heading south, true wind from the west, AND a strong current running directly (TO the) west. The boat is being pushed by the current into the wind. In this example, boat speed and apparent wind are meaningless and could be anything.

 

[Witzend]

Albanch, I have always had a problem with this as well.
If you are land sailing with a gps that is giving you speed and direction and you also have an on-board anemometer that is providing apparent wind speed and direction. And these two instruments are sharing data, you mean to tell me true wind speed and direction can not be calculated? So why should it matter what the water currents are doing? That is only offsetting your SOG which it already knows.

 

[Todd Smith]

Don has it right.

"True wind" in sailing instruments if defined as the movement of the air over the water. You need apparent wind and speed through the water to do this calculation.

"Ground wind" in sailing instruments if defined as the movement of the air over the land. You need apparent wind and speed over the ground to do this calculation.

It is the same calculation, but with different inputs and different outputs.

BUT, because a sailboat moves through the water, it is the "true wind" (as defined above, relative to the water) that matters most for sail trim. A land sailor would care about the "ground wind" instead.

 

[FourPoints]

Don has it right.

"True wind" in sailing instruments if defined as the movement of the air over the water. You need apparent wind and speed through the water to do this calculation.

"Ground wind" in sailing instruments if defined as the movement of the air over the land. You need apparent wind and speed over the ground to do this calculation.

It is the same calculation, but with different inputs and different outputs.

BUT, because a sailboat moves through the water, it is the "true wind" (as defined above, relative to the water) that matters most for sail trim. A land sailor would care about the "ground wind" instead.

I reject this on the grounds that the definition of the word true indicates what the real wind speed and direction are relative to earth, and any consideration of current into this calculation reverts it back to an apparent wind calculation. There can only be one way to determine what the wind is truly doing, and that is based on SOG/COG.

Therefore, if raymarine and the likes are using STW and a fluxgate heading to determine true wind, they are making a false statement if they call it true wind.

I understand that the apparent wind of a "stopped" vessel (drifting in the current) is generally more important than the true wind over ground as that is actually what the vessel will experience (except for when it isn't, see below), however to call this true wind is a blatant falsehood. It could be called relative wind since it is the wind relative to the water you are currently in, but it is not true wind.

This distinction matters when you are transiting the mouth of a harbor that has a 4kt current and want / need to get a reading on the true wind speed and direction to prepare for what you will experience after you leave the harbor entrance minutes later and the current starts to subside. Using the STW method you would calculate a significantly different wind reading for what the current conditions really represent, particularly if the true wind was parallel to your direction of travel and the current in the harbor entrance.

 

[Ron McCoy]

My understanding of Raymarine (from their user manuals) is that SOG is a GPS imput calculation measuring distance traveled over a specific time period. A wind factor is not even in the discussion.

 

[Witzend]

Therefore, if raymarine and the likes are using STW and a fluxgate heading to determine true wind, they are making a false statement if they call it true wind.
[/quote]

I couldn't agree more. Lets use the English language as intended. Relative is not true.

 

[Todd Smith]

FourPoints and Witzend, if you object to the word "true" that's fine by me. Call it any word you want, but that's what the instrument makers use. There is no falsehood, because the instrument makers define their terms in the user manuals.

To me it's the "wind relative to the water" and that's what I want to know for sailing.

Todd

 

[FourPoints]

I do object to the use of the word true if they are basing the calculation on any speed input relative to the vessel such as speed through the water. Using those inputs you are calculating the "wind relative to the water", which by the very definition of the word relative, procludes it from being a valid calculation for true wind. Given the choice between having my instruments display "true wind" and "wind relative to the water", I would want true wind anytime I am not racing (which is never on my boat).

Imagine if you were planning a passage from Charleston SC to the Bahamas, but your weather forecasts were relative to the gulf stream?

 

[kenn]

Albanch, I have always had a problem with this as well.
If you are land sailing with a gps that is giving you speed and direction and you also have an on-board anemometer that is providing apparent wind speed and direction. And these two instruments are sharing data, you mean to tell me true wind speed and direction can not be calculated? So why should it matter what the water currents are doing? That is only offsetting your SOG which it already knows.

Of course the current on land is a whole lot less

Ya got me thinking.... ok, try this on:

When your boat is not powered by sails or motor or oars, the only force acting on the boat would be current. So as far as the sailor is concerned, the "true" wind experienced by a floating object is actually the vector sum of the wind plus the current.

Example: if the actual wind blows north to south and the current flows from west to east, then from the boat's perspective the wind would appear to come from somewhere between east and north... the vector sum of the wind plus the current.

So, if the boat instrumentation actually showed REAL (over ground) wind, your lay lines would be skewed from what the boat would actually experience.

So that's why, as far as the boat is concerned, "true" wind is actually the vector sum of wind and current, and so, together with apparent wind direction, speed through water is what's needed to calculate "true" wind, because this measurement includes current.

 

[Ross]

If you don't supply ALL of trhe variables the problem can't be solved.

 

[Witzend]

So that's why, as far as the boat is concerned, "true" wind is actually the vector sum of wind and current, and so, together with apparent wind direction, speed through water is what's needed to calculate "true" wind, because this measurement includes current.

I get your drift about the relavence of needing the current effect into the calculation for course, but for me it is an argument of the definition of the word "true". I believe it would be more accurately termed "relative" wind. Everything is relative to something else, remember Einstein.

 

[Albanach]

Thanks, folks, for the many varied and interesting responses.

From these, it appears to me that both a calculation from STW or SOG, exclusively of the other, will produce a flawed result, as they do not take into account the current flow.

Linking the STW and SOG observations may possibly enable this to be done and give more accurate values. As Ross stated : “If you don't supply ALL of trhe variables the problem can't be solved.” Maybe Raymarine can get one of today’s computer whizzkids to design such a soft ware – not sure how it would work with Stu’s example of a crosscurrent.

For my purposes, I would be happy to receive a SOG based imperfect ‘True Wind’ reading, rather than nothing. It would provide me ( if the paddlewheel stalls) with a comparison with my own rough estimate of ‘true’ windspeed and particularly wind direction, to be expected initially when eg. reversing course from running to close hauled or similarly to FourPoints’ requirements, as I am not racing, when entering different environs.

 

[Todd Smith]

Thanks, folks, for the many varied and interesting responses.

From these, it appears to me that both a calculation from STW or SOG, exclusively of the other, will produce a flawed result, as they do not take into account the current flow.

Linking the STW and SOG observations may possibly enable this to be done and give more accurate values. As Ross stated : “If you don't supply ALL of trhe variables the problem can't be solved.” Maybe Raymarine can get one of today’s computer whizzkids to design such a soft ware – not sure how it would work with Stu’s example of a crosscurrent.

For my purposes, I would be happy to receive a SOG based imperfect ‘True Wind’ reading, rather than nothing. It would provide me ( if the paddlewheel stalls) with a comparison with my own rough estimate of ‘true’ windspeed and particularly wind direction, to be expected initially when eg. reversing course from running to close hauled or similarly to FourPoints’ requirements, as I am not racing, when entering different environs.

I think that you have misunderstood some of the back and forth. (or I have ) You can get a perfectly accurate "true wind relative to the ground" from the apparent wind and gps SOG. Raymarine calls this "ground wind" according to your first post. If you can display this, then you are all set.

Todd