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 MarkSF 01-22-2013 03:12 PM

Calculating true wind

I have a Simrad is20 wind transducer and display. The display is on the NMEA 2000 network, and is using the SOG and COG from the chartplotter, to calculate true wind.

Am I right in thinking that I really need a heading sensor to calculate true wind accurately?

I think that the calculation is assuming that the boat's heading is the same as the course, which is erroneous if there is a current at any angle to the heading.

So I think I need to use heading, and GPS SOG. Is this right?

 kellysails 01-22-2013 05:29 PM

Re: Calculating true wind

I believe you are correct. This site details it.

True Wind Speed And Direction Calculator

 kellysails 01-22-2013 05:56 PM

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.

 MarkSF 01-22-2013 06:21 PM

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....

 kellysails 01-22-2013 06:37 PM

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

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
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

 tommays 01-22-2013 07:36 PM

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

 Jeff_H 01-22-2013 07:53 PM

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.

 Hudsonian 01-22-2013 11:00 PM

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.

 Omatako 01-24-2013 12:43 AM

Re: Calculating true wind

Quote:
 Originally Posted by MarkSF (Post 979635) 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.

 Omatako 01-24-2013 01:00 AM

Re: Calculating true wind

Quote:
 Originally Posted by Jeff_H (Post 979673) 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.

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