Judging true wind direction

Woodlouse said:
True wind will always be further aft than indicated by a tell tale or mast windex. Just how far forward the apparent wind comes forward from the true depends on how fast your going.
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Not quite the whole story . . . the amount the apparent wind is forward of the true wind is a factor of the true wind speed, true wind direction and the boat speed. In practice apparent wind is all that usually matters as this is what the boat is sailing to.

If you are fortunate enough to have an integrated set of instruments then you will have the ability to set the wind instrument to true wind rather than apparent. The only time this is useful IMO is when you are motoring and wondering if it is worth switching the donk off and attempting to sail. In this case true wind speed/direction can be useful.

- W
 
webcraft said:
If you are fortunate enough to have an integrated set of instruments then you will have the ability to set the wind instrument to true wind rather than apparent. The only time this is useful IMO is when you are motoring and wondering if it is worth switching the donk off and attempting to sail. In this case true wind speed/direction can be useful.

- W
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Also useful when you're going downwind and see the TWS and think, "I didn't realise the wind had come up that much. Probably time to reef."
 
webcraft said:
Not quite the whole story . . . the amount the apparent wind is forward of the true wind is a factor of the true wind speed, true wind direction and the boat speed. In practice apparent wind is all that usually matters as this is what the boat is sailing to.

If you are fortunate enough to have an integrated set of instruments then you will have the ability to set the wind instrument to true wind rather than apparent. The only time this is useful IMO is when you are motoring and wondering if it is worth switching the donk off and attempting to sail. In this case true wind speed/direction can be useful.

- W
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I think you've hit the nail on the head.


The rule of thumb is that true wind is ALWAYS aft of apparent wind - despite the exceptions that some people claim. (This is assuming you are actually sailing properly and not setting up some bizarre situation in very light airs where you are sailing backwards over the ground in a very fast tidal stream or something.)

To answer Snowleopards OP - you can either make an educated guess based on your boats speed and the wind speed, or you have to look at the ripples on the water. Some people are much more skilled at this than others.

The only time that true and apparent are from identical directions is when dead down wind or as you go head to wind through the tack. These are also the two times when the true wind strength is never the same as the apparent wind speed - again unless you set up some bizarre situation when you happen to stall the boat into a stationary position as you are exactly head to wind or stern to wind...

All this is academic as it apparent wind we all trim the sails to.
 
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snowleopard said:
So if you don't have instruments to tell you the true wind direction, can you judge it and how do you do it?

NB I'm not talking about apparent wind so wet fingers, burgees, telltales etc don't count.

I too have been surprised at the number of experienced sailors who can't
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Never managed to sail the boat in true wind - only in apparent wind. True wind is what I felt when I got up that morning before we left.
 
snowleopard said:
????????????????????????

Run that by me again slowly please.
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I believe that in this quote:

The rule of thumb is that true wind is ALWAYS f'wd of apparent wind
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the letters "f'wd" are an abbreviation for the word "aft".

Or possibly he mixed up "true" and "apparent".

In fact, the way I do it is to switch the display on the instruments to TWD or TWA.

A lot of people say that the sail to the apparent wind. As an aside, you can't do that in my boat, at least not with the spinnaker up in breeze. If you put the autopilot on and had it steer to the apparent wind, the boat would gybe in minutes if not sooner.
 
True DOES matter

I dont agree with all these people saying that only apparent wind matters. Obviously that is how the boat is trimmed and sailed on the current tack .... but what about the next one? True wind is the one which is consistent, apparent changes as the boat changes direction

Will you need to reef to head up? Could you let out a reef to go down wind? Is it worth getting the kite out, or not?

Planning ahead is essential, and knowing what conditions to expect is a major factor in that.
 
A quick sat morning spreadsheet comes up up these numbers...

Boat speed true wind speed TW angle to bow A W angle to bow AW speed diff between T & A
5 9.10 67.0 45.0 12 22
5 9.50 73.0 50.0 12 23
5 10.0 79.0 55.0 12 24
5 10.4 84.0 60.0 12 24
5 10.8 90.0 65.0 12 25
5 11.3 95.0 70.0 12 25
5 11.7 99.0 75.0 12 24
5 12.1 104 80.0 12 24
5 12.6 108 85.0 12 23
5 13.0 113 90.0 12 23
5 13.4 117 95.0 12 22
5 13.8 121 100 12 21
5 14.0 125 105 12 20
5 14.5 129 110 12 19
5 14.8 133 115 12 18
5 15.1 137 120 12 17
5 15.4 140 125 12 15
5 15.7 144 130 12 14


Cut & paste not great, collumns are ..
boatspeed , truewind speed, true wind angle to bow, AW angle, AW speed, how far aft true wind is.

From formulas here feel free to check, i'm quite often wrong ;)

Which goes against my memory of many miles with an aries wind vane, I remember it always seemed to gave a much bigger course change per click just behind the beam. But the numbers suggest it's actually round about 70deg. Must have been wrong again. :)
 
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