What's in a name.

[Buck Turgidson]

Whilst reading the excellent confessions thread I noticed that there is often some ambiguity in the definitions of terms used when sailing. I'm a relative newcomer to sailing and hungry to learn as much as I can. These Forums are an excellent source of inspiration and there are clearly some very knowledgeable and experienced contributors so perhaps I could call on them to clarify a few terms which appear to mean different things to different men (and women).

Up to now I have understood the term "True wind" to mean the actual wind vector relative to a stationary point on the ground. I thought the correct term for the wind relative to the water i.e. corrected for tide was "Sailing wind" and the wind relative to the boat to be "apparent wind".

It would appear that I'm misinformed and have seen the term "Ground wind" used to represent what I believed to be "True wind".

Different instrument suppliers have the same problem as I see One manufacturer using SOG to calculate True wind while another uses boat speed to calculate the same thing. I've just read that another uses boat speed to calculate True wind and SOG to calculate Ground wind.

Is there a definitive standards document which defines these terms? I understand that for the recreational sailor the difference between True/Ground/sailing wind is not very important as apparent wind is what we use to set sails and will suffice for our needs but I would have thought for the racing sailor a standard definition should be vital in calculating optimum tack with reference to wind or mark.

As a professional aviator of some years I'm not unfamiliar with different definitions, you only have to look at the number of ways we express speed in aircraft to see that. But every pilot is taught at a very early stage the difference between Indicated, Calibrated, Equivalent airspeed and Mach and their relationship with Groundspeed. And nobody gets to redefine those terms to suit their own needs.

Help a poor lad out and point me to a definitive source of reference please!

Buck.

 

[prv]

Is there a definitive standards document which defines these terms?

Seems unlikely, and if there were few people would follow it anyway.

For what it's worth I've never heard the term "sailing wind" used in this sort of context.

True wind was first defined relative to the water because before GPS we had no way of relating it to the ground. So any instrument that uses SOG in preference to STW and labels the result true wind is different to what went before and therefore (in so far as is possible in such an unstandardised field) wrong. Note that falling back to SOG if STW isn't available, I wouldn't consider a fault - and rather wish my Raymarine ST60s did it as my log is often clogged up.

Pete

 

[JumbleDuck]

Up to now I have understood the term "True wind" to mean the actual wind vector relative to a stationary point on the ground.

Me too. You added tidal and boat speed effects to true wind to get apparent wind. This whole "ground wind" business sees to be a recent invention, presumably driven by the need to believe that anything a GPS can do must be useful.

 

[johnalison]

My Raytheon meter will give a "true" wind reading with the GPS switched off but will only display the direction and Beaufort strength if the GPS is on. I haven't taken the trouble to see if the displayed speed in a tideway is different with GPS on or off, having only owned the boat for 14 yrs.

 

[JumbleDuck]

My Raytheon meter will give a "true" wind reading with the GPS switched off but will only display the direction and Beaufort strength if the GPS is on. I haven't taken the trouble to see if the displayed speed in a tideway is different with GPS on or off, having only owned the boat for 14 yrs.

There are hardly any places where tidal effects will make any significant difference to the apparent wind, and where they do there is probably more to worry about ...

 

[markhomer]

There are hardly any places where tidal effects will make any significant difference to the apparent wind, and where they do there is probably more to worry about ...

on west coast of Scotland there are MANY places where tidal effects make significant differences , also I imagine in many other places , though as you say the stronger the effect , the more to concern yourself about in other respects .

 

[JumbleDuck]

on west coast of Scotland there are MANY places where tidal effects make significant differences , also I imagine in many other places , though as you say the stronger the effect , the more to concern yourself about in other respects .

I'd be interested in some examples. For me it would have to be over 5kts to be significant and the only extended places I've encountered with that sort of tide are the sounds of Islay and Luing. Coirebhreacan as well, I suppose, but that definitely come under the category of "more things to worry about than the wind".

 

[Lucky Duck]

Perhaps over simplified but with a 6 knot ground wind;

Wind with tide (say 2knots) - true wind 4 knots
Wind against tide - true wind 8 knots

For me that is the difference between motoring and sailing.

 

[trapezeartist]

OK, so I don't think we have any disagreement on what Apparent wind is.

As prv says, True wind is calculated from the log and windicator because it is a concept that pre-dates GPS. Effectively it is the speed of the wind relative to the water.

Raymarine use the term Ground wind for the wind that is calculated from sog and cog. If you're going to get interested in anything other than Apparent, this is the meaningful one. You can compare it with the weather forecast and write it in your log.

 

[William_H]

I understand OP predicament about different terms. When teaching sailing I point to the book we use with 3 pages of sailing terms and explanation. I say you don't have to use the correct term but it is good if you recognise the often used terms. With no real concern for tide flow around here I have never heard of thiose terms related to the water movement. We have either true wind ie related to a person standing still (on earth) and apparent wind as seen by a mast head anenometer and related to the boats motion (over ground).
good luck olewill

 

[Hydrozoan]

Perhaps over simplified but with a 6 knot ground wind;

Wind with tide (say 2knots) - true wind 4 knots
Wind against tide - true wind 8 knots

For me that is the difference between motoring and sailing. QUOTE]

Seems adequately complex to convey the essence - or are we missing something?

 

[flaming]

As prv says, True wind is calculated from the log and windicator because it is a concept that pre-dates GPS. Effectively it is the speed of the wind relative to the water.

This is the most important one in my view.

When sailing your boat does not derive its power from the amount of wind over the ground, it derives its power from the difference in velocity between the water and the air. For example if it was a flat calm ashore but you had a fast running river of 6 knots you would be able to use a sailing boat to get to the other side. If however it wasn't a flat clam, but a wind of 6 knots was blowing in the same direction as the river you would not be able to reach the other side.

Ground wind is interesting only when the tide/current is going to change as knowing what it is will enable you to calculate what the new true wind will be after the change.

If however you were a racing sailor, and you were creating a useful set of polars to know how fast you should go for a certain wind strength, you MUST do this with true wind. If you did that with ground wind your numbers would be way out depending if you were going with the tide or against it. Similarly if you found that you needed to reef, and looked at your windex and it said 20kts Ground wind, you might think "ah, so when it's 20 knots I need to reef". But suppose you were actually going with a 2 knot tide that time, and next time you're out you're going against it and your dial also reads 15 knots. You Will have 4 knots less true wind. And you probably wouldn't need to reef.

 

[Buck Turgidson]

This is the most important one in my view.

When sailing your boat does not derive its power from the amount of wind over the ground, it derives its power from the difference in velocity between the water and the air. For example if it was a flat calm ashore but you had a fast running river of 6 knots you would be able to use a sailing boat to get to the other side. If however it wasn't a flat clam, but a wind of 6 knots was blowing in the same direction as the river you would not be able to reach the other side.

Ground wind is interesting only when the tide/current is going to change as knowing what it is will enable you to calculate what the new true wind will be after the change.

If however you were a racing sailor, and you were creating a useful set of polars to know how fast you should go for a certain wind strength, you MUST do this with true wind. If you did that with ground wind your numbers would be way out depending if you were going with the tide or against it. Similarly if you found that you needed to reef, and looked at your windex and it said 20kts Ground wind, you might think "ah, so when it's 20 knots I need to reef". But suppose you were actually going with a 2 knot tide that time, and next time you're out you're going against it and your dial also reads 15 knots. You Will have 4 knots less true wind. And you probably wouldn't need to reef.

So in the racing world True wind is definitely with reference to the water then?
My next question then is; Are top spec logs calibrated to take into account local velocity at different heel angles and leeways?

I assume that paddle wheel logs have no place on racing boats as they will only ever give an accurate reading with no heel or leeway and even then they will read local hull speed which will be higher (or lower) than free stream speed.

It's all good stuff chaps, keep it coming

Buck

 

[MoodySabre]

So in the racing world True wind is definitely with reference to the water then?
My next question then is; Are top spec logs calibrated to take into account local velocity at different heel angles and leeways?

I assume that paddle wheel logs have no place on racing boats as they will only ever give an accurate reading with no heel or leeway and even then they will read local hull speed which will be higher (or lower) than free stream speed.

It's all good stuff chaps, keep it coming

Buck

A log gives the speed through the water - a GPS gives the speed over the ground. The difference is the speed of the tide.

I thought paddle wheel logs were essential to racers. I'm not and mine doesn't work. SOG gets me home before the tide disappears.

 

[Buck Turgidson]

A log gives the speed through the water - a GPS gives the speed over the ground. The difference is the speed of the tide.

I thought paddle wheel logs were essential to racers. I'm not and mine doesn't work. SOG gets me home before the tide disappears.

Sure, but what I mean is that sonic or electromagnetic logs would be less prone to error due to slip/leeway.

 

[haydude]

I have never heard of sailing wind and ground wind. Instead I hear and use apparent wind and true wind.
The wind instrument of a boat in motion will always measure the apparent wind, which is the wind speed and direction as they appear to the instrument. True wind speed and direction are then calculated adding/subtracting the GPS ground speed and direction.

 

[Lucky Duck]

That is at odds with my experience of Raymarine and B&G instruments which all have used the log for true wind rather than the GPS.

 

[flaming]

Sure, but what I mean is that sonic or electromagnetic logs would be less prone to error due to slip/leeway.

I've never actually come across an electromagnetic log.

 

[Buck Turgidson]

I've never actually come across an electromagnetic log.

http://www.nke-marine-electronics.com/nke-products/speed-depth-sensors/electromagnetic-speed-sensor.html

 

[Searush]

Does anyone really care? The wind I feel on my cheek tells me if I can sail or need to motor.