Judging true wind direction

Raymarine (ex autohelm) use the log.

For interest, on Raymarine autopilot instrument, you can call up COG and SOG on the DISP button using the handbook programming information.

Very useful to have those numbers in front of you.
 
Going back to the original question, I was interested in finding out how many people can read wind direction from the appearance of the sea. I have the impression that quite a few experienced sailors can't.
 
snowleopard said:
..... I have the impression that quite a few experienced sailors can't.
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It probably depends on the way a sailor has obtained their experience.

As a kid I sailed dinghies on inland lochs. These lochs were quite gusty places. One soon develops an eye for the tell tale approach of a williwaw type wind. On a relatively flat water loch, its easy to see the dark wavelets rushing towards the dinghy. Similarly in near calms watching for cats paws.

Later, I sailed with a very knowledgeable skipper for many years on offshore boats and he taught matters like this, particularly the wavelets. In a cross swell, or multiple cross swells it is only the wavelets that matter. A simple stare at the sea soon determines the direction without having to follow any particular patch of water. I would hazard a guess that my dinghy experience greatly enhanced my ability to read the wavelets.

Of course, many experienced sailors may not have considered calculating the true wind direction because they have mastered sailing to the apparent wind where knowing true wind direction is not particularly relevant. In a normal speed yacht its not so important but in a fast yacht it is, for example if conducting a MOB under sail - it could make the difference to stopping the yacht, or overshooting.

I discovered my own limitations when sailing Hoby Cats a few years ago and my assumptions when picking a tack point were all wrong being based on slow boat experience.
 
snowleopard said:
Going back to the original question, I was interested in finding out how many people can read wind direction from the appearance of the sea. I have the impression that quite a few experienced sailors can't.
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As yet, no one has actually given any answer (apart from electrickery) that will give the "true" wind direction.

Ripples are in fact a product of the wind relative to the water movement, and while very useful for sailing, it is not actually the "true" wind direction. Smoke, flags on land etc, might be the true direction at the land location, but might well not be that locally.

Beyond ripples, I can't, in answer to the OPs question.
 
To work out True Wind from an Apparent Wind Angle and Speed you will need to construct a Vector triangle.

Draw a line equating to your Apparent Wind Angle and Speed, from the down wind end of this line draw a line that equates to your COG and SOG, Now draw a line to connect the two ends together and you have your True Wind Angle and Speed.

Not difficult just not often practised.

Simes
 
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.


- W
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I know what you're saying but I bet most of the people on this forum have been enjoying a cracking broad reach, rounded a point or a mark, hardened up onto the wind and ended up with the lee-rail under....
 
pyrojames said:
Ripples are in fact a product of the wind relative to the water movement, and while very useful for sailing, it is not actually the "true" wind direction. Smoke, flags on land etc, might be the true direction at the land location, but might well not be that locally.
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It doesn't matter which way the water is moving - the ripples or wavelets will always be perpendicular to the wind. And if you're close enough to make out the direction of a flag or smoke onshore, then you are in the same locality.
 
MoodyNick said:
I'd be interested to know what systems do that. Autohelm/Raymarine certainly don't unless it is a recent innovation.
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When I was last looking a changing boats I sailed on one with a very posh Raymarine set up, it offered three different options for wind speed and direction: -

Apparent (as described)
True (zero boat speed but drifting with any tide present)
Ground (as if anchored or moored)
 
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Without getting into calculations wave direction and ripples on the water in case of confused seas give you a clue. I the worst cases, just look at the spray over the crest of a breaking wave.
 
I'm not sure why you would want to know the true wind direction it has no real relevance to a cruising yacht. We never even think about it. Having said that I think racing sailors use it to calculate whether they are on the best wind angle for their performance stats.
 
pyrojames said:
Ripples are in fact a product of the wind relative to the water movement,
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That is the definition of "true" wind.

Wind in relation to the land is "ground" wind.

Both useful to know if you understand the relation between one and the other, and how it will change with a change of tide, but not the same thing.
 
On that basis the "true wind" is different if you are in a plane, or on the land, or in different parts of the tidal stream. Any attempt to get "true wind" from GPS COG and apparent wind will be wrong?

I consider that true wind is the wind over the earth's surface and other influences such as boat/aircraft/tide/current speed modify your perception of it. On your reasoning, the true wind at anchor is different to that as soon as you up the hook?!
 
pyrojames said:
On that basis the "true wind" is different if you are in a plane, or on the land, or in different parts of the tidal stream. Any attempt to get "true wind" from GPS COG and apparent wind will be wrong?

I consider that true wind is the wind over the earth's surface and other influences such as boat/aircraft/tide/current speed modify your perception of it. On your reasoning, the true wind at anchor is different to that as soon as you up the hook?!
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Planes wouldn't ever see true wind, as they are not (or shouldn't be) in the water. So they see apparent, and can measure ground wind.

The definition of true wind is the difference between the water speed and direction and the wind speed and direction.
So when you're sat at anchor your instruments are directly measuring ground wind as you are not moving.
When you up anchor and drift with the tide you are now measuring true wind.
When you set sail you are now measuring apparent wind. Your instruments can calculate true wind using your water speed and ground wind using gps and heading information.
 
Warning Over Analysis Risk, Warning Over Analysis Risk

:rolleyes:

For those with nothing better to do and a penchant for wind: -

http://en.wikipedia.org/wiki/Apparent_wind (there is even an algorithm for the true enthusiast of wind)

http://coaps.fsu.edu/woce/truewind/paper/ (fill yer boots with this, Lots of pages in small text, formulas, graphs, tables and in black in white, so its must be important)

http://coaps.fsu.edu/woce/truewind/ (Holy Moally it has Fortran codes but states that true wind is referenced to true north - that should get you zephyr heads excited)

........ ad nauseam
 
What a load of bolloocks. Planes only ever see their own speed through the air once up.

On you arguement, the race officer can only judge the "true wind" by looking at the ripples, or by using electrickery in the committee boat? And he can't know the true wind on other areas of the course. He cant know it if he is sitting on a shore end of a line because he will only have the "ground" wind. Pile of cods wallop. By you reasoning, the "true wind" speed changes as you alter course too?
 
BlowingOldBoots said:
:rolleyes:

For those with nothing better to do and a penchant for wind: -

...
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Thank you. I really must go to work!

"True winds (defined herein as a wind with a direction referenced to true north and a speed referenced to the fixed earth)" from the World Ocean Circulation Experiment.

and from Wiki, if you believe it..
"In sailing, the apparent wind is the actual flow of air acting upon a sail. It is the wind as it appears to the sailor on a moving vessel. It differs in speed and direction from the true wind that is experienced by a stationary observer."

Odd that neither consider that the true wind is modified by current or tide. :P
 
pyrojames said:
On you arguement, the race officer can only judge the "true wind" by looking at the ripples, or by using electrickery in the committee boat? And he can't know the true wind on other areas of the course. He cant know it if he is sitting on a shore end of a line because he will only have the "ground" wind.
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Absolutely correct.

Good race officers recognise the effect that tide will have on the true wind, and set their courses accordingly.

Those that set courses in highly tidal areas without considering it often wonder why the boats are able to lay a mark that their windex (on their anchored committee boat) is pointing at.

By you reasoning, the "true wind" speed changes as you alter course too?
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NO - true wind is different from ground wind because it is the component of the ground wind and the wind that is created by the boat being pushed through the air by the current.

So if I am in a Northerly ground wind of 10 knots in an anchored boat, but then up anchor and drift east at 3 knots on the tide I have added an easterly component to the wind. So the wind will not be North anymore, it will be east of North. And it won't be 10 knots anymore, it will have increased.
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