SailNet Community - Reply to Topic

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > On Board > Gear & Maintenance > Calculating true wind
 Not a Member? 


Thread: Calculating true wind Reply to Thread
Title:
  

By choosing to post the reply below you agree to the rules you agreed to when joining Sailnet.
Click Here to view those rules.

Message:
Trackback:
Send Trackbacks to (Separate multiple URLs with spaces) :
Post Icons
You may choose an icon for your message from the following list:
 

Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Click here to view the posting rules you are bound to when clicking the
'Submit Reply' button below


Additional Options
Miscellaneous Options

Click here to view the posting rules you are bound to when clicking the
'Submit Reply' button below


Topic Review (Newest First)
01-24-2013 11:16 AM
johnnyquest37
Re: Calculating true wind

To determine true wind, you need two vectors. The first is the apparent wind speed/direction. You get this by direct measurement on board the boat. The second vector you need is boat speed/heading. Again, you get this from direct measurement. Plot these two vectors to scale on graph paper and and you derive true wind speed/direction relative to the water, which is useful for navigation planning (you sail the apparent wind and you navigate the true wind). Alternatively, you could do some trigonometry, and calculate the true wind.

If you want true wind relative the bottom (to take into account the effect of current), replace the boat speed/direction vector with SOG/COG.

The digital systems onboard these days can do all these calculations, provided they are receiving the correct vector information.
01-24-2013 09:56 AM
Jeff_H
Re: Calculating true wind

Quote:
Originally Posted by Omatako View Post
Sorry, a bit confused.

If the wind instrument is working on its own, then the wind speed you are reading is pure apparent speed, nothing to do with true. The only time the wind in this condition can be considered true is if the boat is guaranteed to be stationary i.e. anchored. The true and apparent become one. As soon as the boat begins to move in any direction, the wind is apparent and not true.

Only if a boat speed indicator is working in the same NMEA system with the wind speed and direction can the readings be vectored to produce true wind speed. And the calculation of true wind speed done by the wind instrument and the boat speed indicator working together has no reference to course or heading or any other such dimension.

How do I know this? I had a protracted argument years ago with Sailing Dog who successfully proved that I was wrong and the correct answer is as outlined above.
I completely agree, at least with the part where you say "a bit confused". Me too.

Here is what I was trying to say, when it comes to calculating true wind direction relative to the course of the boat, there needs to be three pieces of information. The first is the apparent windspeed from the wind instrument, the second is the apparent wind angle to the center line of the boat, and the third is the speed of the boat. Crunching the numbers produces a true wind angle relative to the centerline if the boat.

The source of the speed data typically comes from one of two sources, either the GPS SOG, or from the knotmeter speed through the water. If the data comes from the GPS SOG, then current is automatically factored in and so that would calculate the true wind that you might measure if you were anchored. If the data came from the knotmeter, it has no way of measuring current and so the calculated true wind would approximate the wind that you would measure if you were drifting with the current.

And if you wanted to know the compass course of that wind, the instruments would need data from an electronic compass, in which case, it would then do the arithmentic to add or subtract the angle of the true wind to the boat's course, COG being irrelevant to that calculation.

Jeff
01-24-2013 01:00 AM
Omatako
Re: Calculating true wind

Quote:
Originally Posted by Jeff_H View Post
The reality is that the speed and course over ground will give you a closer to real True wind, in other words the wind that you would feel if you are anchored.

But for most sailing applications, and more specifically oddly enough for most racing applications, the instruments produce a "true wind" which is really a wind velocity that is not corrected for current, in other words, the wind that you would feel if you were drifting with the current.

Both are useful tactically but neither are extremely useful for sail trim or even for sail selection.
Sorry, a bit confused.

If the wind instrument is working on its own, then the wind speed you are reading is pure apparent speed, nothing to do with true. The only time the wind in this condition can be considered true is if the boat is guaranteed to be stationary i.e. anchored. The true and apparent become one. As soon as the boat begins to move in any direction, the wind is apparent and not true.

Only if a boat speed indicator is working in the same NMEA system with the wind speed and direction can the readings be vectored to produce true wind speed. And the calculation of true wind speed done by the wind instrument and the boat speed indicator working together has no reference to course or heading or any other such dimension.

How do I know this? I had a protracted argument years ago with Sailing Dog who successfully proved that I was wrong and the correct answer is as outlined above.
01-24-2013 12:43 AM
Omatako
Re: Calculating true wind

Quote:
Originally Posted by MarkSF View Post
The GPS does not provide heading, it only provides course. It's dumb, in that it thinks the boat is pointing in the direction it's going....
Sorry I don't agree and go as far as saying that reality is diametrically opposite to your description

If the boat is effectively stationary and drifting on a tide, the GPS will give an accurate reading of the direction and speed of the boat's movement. GPS is far from dumb.

Equally if you are sailing a compass course of say, 165 degrees and there is a 090 degree current setting, the steering compass will read 165 (assuming it is accurate) and the GPS will read a vectored course including the current drift.

In other words, the compass will tell you where the boat is pointing and the GPS will tell you where you're going.

And I agree with Tommays - what you're trying to do is beyond the realm of a pocket calculator.
01-22-2013 11:00 PM
Hudsonian
Re: Calculating true wind

Although there is not complete agreement is regarding usage, I believe that "true wind" is the wind that you would feel if drifting with the current. "Ground referenced wind' is the wind that you would feel if at anchor. Therefore, if you set aside leeway, the instruments can compute "true wind " velocity and direction with merely with velocity of the boat through the water, apparent wind speed, and apparent wind direction. Of course, the apparent wind and true wind are relative to the boat.
01-22-2013 07:53 PM
Jeff_H
Re: Calculating true wind

The reality is that the speed and course over ground will give you a closer to real True wind, in other words the wind that you would feel if you are anchored.

But for most sailing applications, and more specifically oddly enough for most racing applications, the instruments produce a "true wind" which is really a wind velocity that is not corrected for current, in other words, the wind that you would feel if you were drifting with the current.

Both are useful tactically but neither are extremely useful for sail trim or even for sail selection.
01-22-2013 07:36 PM
tommays
Re: Calculating true wind

You are trying to do something far more complex than it sounds

Which is why systems use the through hull knot meter as the boat speed when doing the math on the speed and direction the masthead sensor supply's

The above INFO is whats happing to the sails at that moment in time which is the purpose of the system

Unless somethings really changed my Chartplotter has current stations and can tell me to react as the current changes the course cross

BUT lacks the ability to pick the correct course from (and compensate for current ahead of time ) Northport TO Mamaroneck which is a fairly easy thing for and experienced navigator to do
01-22-2013 06:37 PM
kellysails
Re: Calculating true wind

Hmmm, so wouldn't the NMEA ($GPVTG - Track made good and ground speed) serve as the current heading? It seems as though the GPS is reporting TMG via Magnetic heading. Jeeze I could be all wet here. But I remember even my ancient old GPS's would provide the current Magnetic heading.



$GPVTG

Track Made Good and Ground Speed.

eg1. $GPVTG,360.0,T,348.7,M,000.0,N,000.0,K*43
eg2. $GPVTG,054.7,T,034.4,M,005.5,N,010.2,K


054.7,T True track made good
034.4,M Magnetic track made good
005.5,N Ground speed, knots
010.2,K Ground speed, Kilometers per hour


eg3. $GPVTG,t,T,,,s.ss,N,s.ss,K*hh
1 = Track made good
2 = Fixed text 'T' indicates that track made good is relative to true north
3 = not used
4 = not used
5 = Speed over ground in knots
6 = Fixed text 'N' indicates that speed over ground in in knots
7 = Speed over ground in kilometers/hour
8 = Fixed text 'K' indicates that speed over ground is in kilometers/hour
9 = Checksum

The actual track made good and speed relative to the ground.

$--VTG,x.x,T,x.x,M,x.x,N,x.x,K
x.x,T = Track, degrees True
x.x,M = Track, degrees Magnetic
x.x,N = Speed, knots
x.x,K = Speed, Km/hr
01-22-2013 06:21 PM
MarkSF
Re: Calculating true wind

The GPS does not provide heading, it only provides course. It's dumb, in that it thinks the boat is pointing in the direction it's going....
01-22-2013 05:56 PM
kellysails
Re: Calculating true wind

OTOH, the heading provided by the GPS on the chart plotter should be good enough to calculate true. Although it likely does not update as quickly.
This thread has more than 10 replies. Click here to review the whole thread.

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


All times are GMT -4. The time now is 09:07 PM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012

The SailNet.com store is owned and operated by a company independent of the SailNet.com forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.