Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 25-05-2011, 20:56   #106
CF Adviser
 
Bash's Avatar

Cruisers Forum Supporter

Join Date: Dec 2008
Location: sausalito
Boat: 14 meter sloop
Posts: 7,260
Re: What Heading Am I Really On ?

Quote:
Originally Posted by cwyckham View Post
You guys are just talking semantics. I have only ever seen two winds mentioned in any book or course: true and apparent.
You might want to pick up another book (or take another course.)

Google on "ground wind." It will help you understand Markj's theory about lee-bowing the current.
__________________

__________________
cruising is entirely about showing up--in boat shoes.
Bash is offline   Reply With Quote
Old 26-05-2011, 11:11   #107
Registered User
 
cwyckham's Avatar

Join Date: Apr 2011
Location: Vancouver, BC
Boat: Niagara 35
Posts: 1,870
Re: What Heading Am I Really On ?

Quote:
Originally Posted by Bash View Post
You might want to pick up another book (or take another course.)

Google on "ground wind." It will help you understand Markj's theory about lee-bowing the current.
This confusion in terminology has come up a couple times in the last few weeks and wasted a lot of people's time arguing over semantics instead of reality, so I did a bit of digging to see if I could understand where this mysterious third wind has popped up from (mysterious from my standpoint). I have a theory on how it developed, which I'd like to present for comment. If nothing else, it helped me understand some posts better.

I've only been sailing regularly for the last four years, but in that time I've taken every Canadian Yachting Association cruising course up to and including the offshore skipper course, read voraciously, and spent far too much time on forums. I've also crewed regularly on race boats. In all that time and all those sources, I've only ever seen two winds: True wind is the actual wind over the Earth's surface, and apparent wind is the wind felt on the boat. The difference being because the boat is in motion.

I went through my library last night and can confirm that none of the Annapolis Book of Sailing, Bowditch, Symmetry of Sailing, Walker's Manual of Sail Trim or ASA's Cruising Fundamentals mention ground wind. This guy said he also checked Chapman's and it wasn't there either.

The only place it seems to come up is in manuals for chartplotters, and so I have a theory: For hundreds of years there were two winds: True and apparent. Then we started putting fancy anemometers and paddle wheels on our boats. These measured apparent wind and purported to also tell us true wind. However, when you pushed the "true wind" button it actually told you the wind with respect to the water. Because SOG and COG weren't available, the instrument manufacturers made an approximation by subtracting out boat speed through the water instead.

Then gps started to become common and folks like Raymarine had a problem. They could now tell us what the actual true wind speed was, but all their instruments for the last many years were claiming to tell us true wind speed already. So they made up a new term, "ground wind." This is internally consistent with the boating instrument world, but contradicts common usage since we were using chip logs to measure boat speed. So now suddenly we have many confused people.

Moral of the story is that when we have these discussions, we need to be clear what we're talking about because the electronics guys have done us a real disservice by introducing their own definition of "true wind" that contradicts that which has been in common usage since before Nelson.

I won't be using the term "ground wind" any time soon because it isn't the correct term. Far more people will understand me if I talk about true and apparent wind in their traditional senses. However, now that I understand that there is this issue, I will define true wind in parentheses in my posts so we can avoid semantic confusion.
__________________

__________________
cwyckham is offline   Reply With Quote
Old 26-05-2011, 11:29   #108
Registered User
 
HappySeagull's Avatar

Join Date: Dec 2010
Location: B.C.,Canada
Boat: 29'
Posts: 2,395
Re: What Heading Am I Really On ?

...well,I'd say three.apologies if the geographic being "True" is confusing?
1)True=weather,shore,(geographic?)
2)Seawind a resultant of vectors=Current,+True(+ for the exact maybe include optional "drift" or winddrag subtracted -this' last would be hard and particular to the vessel but a factor for some)
-anyways,this is the "Wind" you begin with on the water,setting sail or engaging a gearbox until headway is gained and so, finally,APPARENTwhere the boat's motion adds it's vector....this is the one you do sail and motor in.
__________________
HappySeagull is offline   Reply With Quote
Old 26-05-2011, 11:46   #109
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,735
Re: What Heading Am I Really On ?

Quote:
Originally Posted by bewitched View Post
My plotter understands far more than ground. It understands my boat because it has learned its polar, it understands the weather forecast because it reads the grib file, it understands tides and currents forecasts because this.information can be loaded too. And of.course it has access to real time.wind and current information from the instruments.

So it can plan my most efficient route beforehand based on the polar, weather and current information and update that most efficient route in real time as I travel.along it.

The output is a line on the chart, in other words a course over the ground. Obviously this very rarely a straight line. The COG can be sent to a readout on deck.

This is not some high end system only found on race boats, most major chartplotting software will have this functionality.

But this has all drifted far from my original query about how many still use the binacle compass as the basis for steering from one position to.another.
I've never seen a plotter with current information -- can it calculate tidal vectors? I may be behind the times a little.

I don't think we're that far off your original topic -- the conversation about how to steer correctly across waters with cross currents comes exactly back to the question about using the compass.

In the absence of some breakthrough plotter of which I personally am unaware, the way to steer across cross-currents is by calculating a magnetic course based on tidal vectors and leeway, and steering that one course with your compass. Or putting it into your autopilot, as I do. Thousands of sailors crossing the English Channel -- the classical case of crossing a body of water with strong cross currents -- do it just that way.

"Compass" isn't just your binnacle compass. You can also -- as I often do -- use the readout on your autopilot control head, which gives you your magnetic heading based on data from your fluxgate compass (and gyro if you have one).
__________________
Dockhead is online now   Reply With Quote
Old 26-05-2011, 11:55   #110
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,735
Re: What Heading Am I Really On ?

Quote:
Originally Posted by HappySeagull View Post
...well,I'd say three.apologies if the geographic being "True" is confusing?
1)True=weather,shore,(geographic?)
2)Seawind a resultant of vectors=Current,+True(+ for the exact maybe include optional "drift" or winddrag subtracted -this' last would be hard and particular to the vessel but a factor for some)
-anyways,this is the "Wind" you begin with on the water,setting sail or engaging a gearbox until headway is gained and so, finally,APPARENTwhere the boat's motion adds it's vector....this is the one you do sail and motor in.
True Wind -- wind in relation to the water, not to the land, is your most important bit of data:

"There are actually 3 winds.

Apparent wind - which is the wind measured by the wind instruments on board the moving boat.

True wind, which is the wind measured by a boat drifting with the tide.

Ground wind, which is the wind measured by the anchored comittee boat.

All three are of interest, and I agree, it is frustrating that the instrument set ups do not allow for the calculation of ground wind.

However, for the sailor planning their course etc, true wind is the most important."

Calculating True Wind Direction - Yachting and Boating World Forums

True Wind is definitely not wind in relation to land. Your instruments calculate it by using your speed through the water -- they ignore COG in giving you calculated true wind data.

"Geographic Wind" is also sometimes called "Ground Wind"; that's what Raymarine call it, for example. ST60 instruments will give you Ground Wind -- wind in relation to the ground, based on relationship of the wind to your COG. Or True Wind -- based on relationship of the wind to your heading and speed through the water.

Here's how it's explained on Panbo:

Click image for larger version

Name:	WindVectors2d4medium.jpg
Views:	56
Size:	112.9 KB
ID:	27817

http://www.panbo.com/assets_c/2009/1...dium-1135.html
__________________
Dockhead is online now   Reply With Quote
Old 26-05-2011, 12:01   #111
Registered User

Join Date: Feb 2011
Posts: 586
Images: 3
Re: What Heading Am I Really On ?

That's a good point Dockhead. Yesterday, experimented around to see if my software (Sailcruiser) would figure a course optimized for current. It doesn't. I have polars set, etc. but there is no function to calculate best heading through a section where current is a major factor. I think it's because the info. in C-Maps is not sufficient to do anything like this. Would be nice to have this sort of function by inputting your own current data. I wonder if any of the available programs can do this? Sea Clear does not and I don't think Open CPN does either.
__________________
smurphny is offline   Reply With Quote
Old 26-05-2011, 12:08   #112
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,735
Re: What Heading Am I Really On ?

Quote:
Originally Posted by cwyckham View Post
This confusion in terminology has come up a couple times in the last few weeks and wasted a lot of people's time arguing over semantics instead of reality, so I did a bit of digging to see if I could understand where this mysterious third wind has popped up from (mysterious from my standpoint). I have a theory on how it developed, which I'd like to present for comment. If nothing else, it helped me understand some posts better.

I've only been sailing regularly for the last four years, but in that time I've taken every Canadian Yachting Association cruising course up to and including the offshore skipper course, read voraciously, and spent far too much time on forums. I've also crewed regularly on race boats. In all that time and all those sources, I've only ever seen two winds: True wind is the actual wind over the Earth's surface, and apparent wind is the wind felt on the boat. The difference being because the boat is in motion.

I went through my library last night and can confirm that none of the Annapolis Book of Sailing, Bowditch, Symmetry of Sailing, Walker's Manual of Sail Trim or ASA's Cruising Fundamentals mention ground wind. This guy said he also checked Chapman's and it wasn't there either.

The only place it seems to come up is in manuals for chartplotters, and so I have a theory: For hundreds of years there were two winds: True and apparent. Then we started putting fancy anemometers and paddle wheels on our boats. These measured apparent wind and purported to also tell us true wind. However, when you pushed the "true wind" button it actually told you the wind with respect to the water. Because SOG and COG weren't available, the instrument manufacturers made an approximation by subtracting out boat speed through the water instead.

Then gps started to become common and folks like Raymarine had a problem. They could now tell us what the actual true wind speed was, but all their instruments for the last many years were claiming to tell us true wind speed already. So they made up a new term, "ground wind." This is internally consistent with the boating instrument world, but contradicts common usage since we were using chip logs to measure boat speed. So now suddenly we have many confused people.

Moral of the story is that when we have these discussions, we need to be clear what we're talking about because the electronics guys have done us a real disservice by introducing their own definition of "true wind" that contradicts that which has been in common usage since before Nelson.

I won't be using the term "ground wind" any time soon because it isn't the correct term. Far more people will understand me if I talk about true and apparent wind in their traditional senses. However, now that I understand that there is this issue, I will define true wind in parentheses in my posts so we can avoid semantic confusion.
Well, I was taught about Ground Wind 35 years ago when I learned to sail -- long before the chart plotter was a glimmer in the eye of its inventor.

I have a different theory: Sailing is complicated enough, and most of the world's sailors do not sail in strongly tidal waters. Where you don't have strong currents, True Wind and Geographic Wind are practically the same. So no one screws your brains with the added layer of complication.

Whereas, in a place like the UK where the tides run faster than some boats can sail, the difference between True Wind and Ground Wind is crucial to understand, and virtually everyone knows what it is.
__________________
Dockhead is online now   Reply With Quote
Old 26-05-2011, 12:21   #113
Registered User
 
cwyckham's Avatar

Join Date: Apr 2011
Location: Vancouver, BC
Boat: Niagara 35
Posts: 1,870
Quote:
Originally Posted by Dockhead

Well, I was taught about Ground Wind 35 years ago when I learned to sail -- long before the chart plotter was a glimmer in the eye of its inventor.

I have a different theory: Sailing is complicated enough, and most of the world's sailors do not sail in strongly tidal waters. Where you don't have strong currents, True Wind and Geographic Wind are practically the same. So no one screws your brains with the added layer of complication.

Whereas, in a place like the UK where the tides run faster than some boats can sail, the difference between True Wind and Ground Wind is crucial to understand, and virtually everyone knows what it is.
Well that single data point certainly puts a dent in my little theory. All of my source material is north American, so I don't know if that makes a difference. Do British reference books talk about the three different winds? What is the British equivalent to Bowditch?

We have some of the largest tidal currents in the world in the PNW, so I don't know if that's the reason for the difference. (google skookumchuck for some scary pictures).
__________________
Chris
SailMentor.com - Become the Confident Skipper of Your Own Sailboat
cwyckham is offline   Reply With Quote
Old 26-05-2011, 12:34   #114
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,735
Re: What Heading Am I Really On ?

Quote:
Originally Posted by cwyckham View Post
Well that single data point certainly puts a dent in my little theory. All of my source material is north American, so I don't know if that makes a difference. Do British reference books talk about the three different winds? What is the British equivalent to Bowditch?

We have some of the largest tidal currents in the world in the PNW, so I don't know if that's the reason for the difference. (google skookumchuck for some scary pictures).
If you think it about, it's logical.

Your boat -- your sails, your rig -- doesn't care about wind in relation to ground. It's an entirely irrelevant datum for sailing. You sail in the interface between the two fluids -- Air and Water. So it's the speed and direction of the Air in relation to the Water which is what you care about -- for purposes of sailing. So it's True Wind -- in relation to water -- which determines whether you're at risk of a gybe or not, for example. And it's only True Wind which tells you what the wind is going to be like when you head up or head off -- and only True Wind. And it is ONLY in relation to True Wind -- in relation to water -- that you can know where Upwind is.

True Wind in relation to Land -- Ground Wind or Geographic Wind -- is of extremely limited usefulness to you. It is ONLY relevant in case you need to calculate how the True Wind (in relation to water) will CHANGE when the tide changes.

When you're standing on deck and your boat is underway, you feel the wind on your face and it's Apparent Wind. If you peel off the vector created by your boat's motion -- next, you get True Wind. You've got to peel off still another vector -- the current -- in order to get to Ground Wind, and by that time you are very, very far away from what you feel standing on deck. Once you get into the ground -- then you're into navigation, a whole different field from sailing. It's there that you need to add in the vector for current. But you don't care much about wind there -- because you don't sail in Ground Wind. You sail in True Wind, in the interface between the water and the air. You're not sailing on land.


That's why NMEA defines True Wind as above, and no other way, so all your electronics systems know it ONLY in that sense -- in relation to the water. Which is what you care about.
__________________
Dockhead is online now   Reply With Quote
Old 26-05-2011, 12:46   #115
Registered User
 
cwyckham's Avatar

Join Date: Apr 2011
Location: Vancouver, BC
Boat: Niagara 35
Posts: 1,870
Quote:
Originally Posted by Dockhead

If you think it about, it's logical.

Your boat -- your sails, your rig -- doesn't care about wind in relation to ground. It's an entirely irrelevant datum for sailing. You sail in the interface between the two fluids -- Air and Water. So it's the speed and direction of the Air in relation to the Water which is what you care about -- for purposes of sailing. So it's True Wind -- in relation to water -- which determines whether you're at risk of a gybe or not, for example. And it's only True Wind which tells you what the wind is going to be like when you head up or head off -- and only True Wind. And it is ONLY in relation to True Wind -- in relation to water -- that you can know where Upwind is.

True Wind in relation to Land -- Ground Wind or Geographic Wind -- is of extremely limited usefulness to you. It is ONLY relevant in case you need to calculate how the True Wind (in relation to water) will CHANGE when the tide changes.

When you're standing on deck and your boat is underway, you feel the wind on your face and it's Apparent Wind. If you peel off the vector created by your boat's motion -- next, you get True Wind. You've got to peel off still another vector -- the current -- in order to get to Ground Wind, and by that time you are very, very far away from what you feel standing on deck. Once you get into the ground -- then you're into navigation, a whole different field from sailing. It's there that you need to add in the vector for current.

That's why NMEA defines True Wind as above, and no other way, so all your electronics systems know it ONLY in that sense -- in relation to the water. Which is what you care about.
That is far and away the best explanation I've read so far of why there is utility to defining three winds. Thank you.

I think the wind with respect to the land is very useful to know, actually since that is what the forecast, ocean buoy reports, etc. Are reporting. This allows me to compare what I see to what I'm supposed to be seeing and to learn what 30 knots really looks like!

I now see the utility of having three defined, I just wish there wasn't this contradiction in the definitions. Every major reference used in north America seems to call true wind with respect to land!

I c
__________________
Chris
SailMentor.com - Become the Confident Skipper of Your Own Sailboat
cwyckham is offline   Reply With Quote
Old 26-05-2011, 13:07   #116
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,735
Re: What Heading Am I Really On ?

Quote:
Originally Posted by cwyckham View Post
That is far and away the best explanation I've read so far of why there is utility to defining three winds. Thank you.

I think the wind with respect to the land is very useful to know, actually since that is what the forecast, ocean buoy reports, etc. Are reporting. This allows me to compare what I see to what I'm supposed to be seeing and to learn what 30 knots really looks like!

I now see the utility of having three defined, I just wish there wasn't this contradiction in the definitions. Every major reference used in north America seems to call true wind with respect to land!

I c
Thanks!

Yes, I agree with you, that Ground Wind is extremely useful for purposes of meteorology for the purpose you mention -- I forget and left that out. And land people, including meteorologists, define True Wind like that (they don't have boats!).

One other thing -- I googled a bit to see what others say about this, and came to the conclusion that the whole thing is not as well established or consistently understood as I was taught, and what I'm saying might be widely accepted only in the UK. There are similar discussions on other boards going back for years, with similar confusion and argument.

So I have a third theory about it --

Until we had precise ways to measure wind speed and direction, none of this really mattered. Certainly Bowditch couldn't care less -- the Beaufort scale itself had not even been invented when Bowditch wrote his magnum opus. Much less any way to measure wind angles and velocity.

So it must have been racers who started to think about this stuff and probably only after the war, when the first wind instruments were invented. So it was probably really an archaic theoretical matter until probably about the time I learned how to sail, and probably was not widely discussed or thought about until NMEA wrote the first protocols for networked instruments on board.
__________________
Dockhead is online now   Reply With Quote
Old 26-05-2011, 13:55   #117
Registered User

Join Date: Jun 2007
Location: SW Florida
Boat: FP Belize, 43' - Dot Dun
Posts: 3,424
Re: What Heading Am I Really On ?

FWIW, I believe that Raymarine started using the term 'Ground Wind' when they started offering the choice of using SOG (from a GPS) or STW (from the paddlewheel) to calculate True Wind. It's their way of distinguishing between 'Speed over Ground' or 'Speed thru Water'. I wouldn't bet on there being any other meaning.
__________________
DotDun is offline   Reply With Quote
Old 26-05-2011, 14:01   #118
Long Range Cruiser
 
MarkJ's Avatar

Cruisers Forum Supporter

Join Date: Dec 2007
Location: Australian living on "Sea Life" currently in England.
Boat: Beneteau 393 "Sea Life"
Posts: 12,828
Images: 25
Re: What Heading Am I Really On ?

I liked Happy Vector example. Vectors is certainly the way to do it for those that did vectors at school and university. However it was a bit new so some folks may not have learned it.


Mark
__________________
Notes on a Circumnavigation.
OurLifeAtSea.com

Somalia Pirates and our Convoy
MarkJ is offline   Reply With Quote
Old 26-05-2011, 14:38   #119
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,735
Re: What Heading Am I Really On ?

Quote:
Originally Posted by DotDun View Post
FWIW, I believe that Raymarine started using the term 'Ground Wind' when they started offering the choice of using SOG (from a GPS) or STW (from the paddlewheel) to calculate True Wind. It's their way of distinguishing between 'Speed over Ground' or 'Speed thru Water'. I wouldn't bet on there being any other meaning.
Since it's part of the NMEA specification, I doubt Raymarine invented it. When I learned about it 35 years ago, it was called "geographic wind", a term which I guess is out of fashion.
__________________
Dockhead is online now   Reply With Quote
Old 26-05-2011, 14:44   #120
Registered User
 
wolfaroo's Avatar

Join Date: Mar 2010
Location: UK South Coast
Boat: Unknown MFV 60ft
Posts: 111
What Heading is this Thread Really On ?

Dockhead, I hate to throw a spanner in the works but I personally don't agree with the term 'ground wind' being used as true wind minus tide induced wind. It not only contradicts almost all writings on the subject, it may also be the cause of confusion for you in relation to lee-bowing the tide as you mentioned earlier.


It is much easier to just consider 2; TRUE and APPARENT, with the tide-induced wind being a component of APPARENT wind:


TRUE wind is the wind experienced when not underway (i.e. at anchor etc).


APPARENT wind is the wind experienced when underway, and depends on the following 6 factors:


boat heading
boat speed
tidal stream set
tidal stream rate
true wind strength
true wind direction




As for the use of the term 'ground wind' - I am more than happy for you to correct me on this, so please don't think I am trying to prove you wrong, but in all RYA, MCA & UK sailing theory I have studied TRUE wind doesn't include tide-induced wind and there is no mention of ground wind in that context?
__________________

__________________
"It is good to have an end to journey towards; but it is the journey that matters in the end."
Ursula Le Guin
wolfaroo is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
What's Your Heading ? Windseeker Marine Electronics 17 12-02-2011 04:51
Crew Available: Anybody Heading to the Bahamas ? run aground Crew Archives 1 04-01-2010 10:40
Heading to Hawaii seancrowne Pacific & South China Sea 29 17-08-2009 21:30



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 19:51.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.