Buck Turgidson
Well-Known Member
Whats the difference between zero wind and 10kts current and zero current and 10kts wind?
Lee bowing
- Thread starter ghostlymoron
- Start date
RupertW
Well-Known Member
Be very careful what you start here. If lee bowing is contentious then the myths around wind against tide are a very special brew of experience and old sea lore beating any knowledge of vectors and physics.
lw395
Well-Known Member
One is a canoeing opportunity, one is a sailing opportunity!
lw395
Well-Known Member
1) Meterologists take weather info from things which are not anchored to the ground
2) If your plane is in the tide, you are not going to be worried about a small error vector in your true wind
3) At altitude, there's usually enough wind that the tide vector would be lost in statistical noise anyway
lw395
Well-Known Member
A marine instrument system calculates True Wind, (or call it 'Sailing Wind' if you prefer) by measuring:
Apparent Wind speed
Apparent Wind Angle
Boat Speed
This produces a 'true wind' vector relative to the boat. The first boat I had with instruments didn't have a compass sensor, it stopped at 'true wind' relative to the boat's heading! I think that was quite common in earlier decades? Electronic compass sensors were a lot of beer tokens in the 70s. There are still plenty of modestly equipped boats sailing like this even now?
Add in a compass sensor and you get 'True Wind' in compass degrees Magnetic.
Add in a bunch of deviation and variation data, you might get your true wind in degT.
Each of the sensors is prone to errors
Each of the sensed signal is 'noisy' needing averaging to get useable results
The real value of the inputs is varying with time anyway.
This means that your instrument 'true wind' is delayed, averaged and full of compounded errors.
To get true ground wind, the inputs need to be:
Apparent wind speed and angle
Boat speed
Compass
COG and SOG
So there is a further layer of data munging which further undermines the integrity of the measurement.
So, what is the point of all this?
The objective in measuring the wind should be to inform a decision.
When I'm running with the kite up, the instruments indicate to me a windspeed which helps me decide what sails I need on the next leg. An indication of the direction tells me if it's a beat or not. This is all about the 'sailing wind'.
Personally I never feel the need to measure the 'ground wind' as indicated by my instruments, it's enough to know the indicated sailing wind and be aware that the tide is influencing that (and in which direction).
I don't see knowledge of the fine detail of the ground wind as being anything like as powerful in informing decisions as knowledge of the 'sailing wind'.
What I'm not trying to do here is tell others what to do, I am more asking people to explain their thought processes and what they actually want and get from their 'B'n'G' wind. So I've tried to explain my lines of thought.
TBH I'm happiest racing with a burgee, some telltales and a TackTick microcompass these days. Windspeed is indicated by how much my muscles ache.
Daydream believer
Well-Known Member
It's kind of true, but you'd need NASA instruments to measure it and I mean the Cape Canaveral variety!
In practice, people who refer to Coriolis lifting on one tack are talking through their hats
Wind gradient is by comparison a very real and important concept.
No doubt someone will be along soon to strongly disagree :ambivalence:
No you do not. (although you are right about someone disagreeing)You just need a pole with a few wind vanes on it at varying heights.
You will soon see that for opposite tacks that you need to adjust the twist of your sail for the different angle of the wind as it changes in height
If this link works my comment about the coriolis effect can be seen quite clearly. And not actually that high either.
If you scan through the forum post they state that the pole is at the end of the bridge in San Francisco but my attempts a google do not seem to be able to find it. Perhaps others would have better luck. I have seen photos of other similar poles & they show the effect more clearly.
All without "Hi tech" gear!!!
https://www.myhanse.com/wind-direction-tower_topic11275.html
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Daydream believer
Well-Known Member
This link also discussed wind shear
http://www.ockam.com/2013/06/02/wind-shear/
http://www.ockam.com/2013/06/02/wind-shear/
TLouth7
Well-Known Member
For a boat in open water, nothing*.
Does this mean you are coming round to our side of the argument that windspeed relative to the water is a real and (to a sailor) highly relevant quantity?
*By which I mean that the experience of sailing will be precisely the same. Clearly on a given heading your ground track will be rather different.
ghostlymoron
Well-Known Member
I'm regretting starting this thread now.
Dockhead
Well-Known Member
Peace!
"True wind" is not a single, harmonised term. It is used differently on land, than on sea. Meterologist's "true wind" is not the sailor's "true wind".
See here: http://forum.raymarine.com/showthread.php?tid=987
http://www.raymarine.fi/knowledgebase/index.cfm?view=7595
http://www.cruisersforum.com/forums...d-apparent-and-true-wind-direction-73563.html
"Don't be confused by the fact that meteorologists use True Wind to describe the difference between wind and land. Of course they do -- they are sitting on land, not on water. That in no way contradicts our usage of True Wind as being the difference between wind and water." -- Raymarine
When sailing, we deal with three different kinds of wind -- apparent wind, wind referenced to ground, and wind referenced to water. If you use meterologist's "true wind" as true wind, then what do you call water-referenced wind? You need a term for that.
The instrument makers call ground-referenced wind (meterologist's true wind) "Ground Wind".
So it would be more correct and logical (and useful) to use the terms the way they, and racers, use them, and the way your instruments will call them:
Apparent Wind
True Wind (water referenced)
Ground Wind (ground referenced)
NormanS
Well-Known Member
RupertW
Well-Known Member
Yes it’s relevant, no it’s not True wind, obviously
dom
Well-Known Member
It's nothing do do with international agreements. You mention Garmin which annually sell around $400m and $600m of marine and aviation equipment respectively. Being a tech company it can easily do the vector calcs, so nothing to do with "can't be bothered".
TAS (true airspeed) is indeed ground referenced for airplanes and it is used for navigation and other calcs. But TAS is temperature and pressure compensated, so one wound use IAS (indicated airspeed) for taxiing, takeoff, landings, etc as it provides a better measure of lift.
This distinction is of little interest to a sailors, just as tides aren't of much interest to pilots. So Garmin design each around its intended use. It's really not hard.
dom
Well-Known Member
Wind shear is an important concept, but unless one has a VERY tall mast it has nothing to to with Coriolis.
The relevance to sail twist is this: the wind 15m or so above the sea will be faster than the wind blowing at sea level due to friction effects. By comparison the boat's velocity at the gooseneck and top of the mast is exactly the same.
As we all know the boat's forward motion swings the AWA (what the sail feels) forward in comparison with the TWA howsoever defined. As the wind speed is faster at the top of the mast, the difference between AWA and TWA will be a bit less up there. A suitable sail twist will accurately accommodate the somewhat freer wind angle aloft.
People do indeed attribute some slight port/stbd preference to Coriolis, which does after all cause a toilet/sink to empty with an anti-clockwise swirl in the Northern Hemisphere and a clockwise one in the Southern. Except that's another old wives tale!
What a thread
Roberto
Well-Known Member
+1
in the surface layer, the portion of the boundary layer closest to the ground, say a few tens of meters, there is no Coriolis influence; earth rotation/Coriolis begins to come into action in the layer above (Ekman), so Coriolis might influence a mast 100m tall, not a 10-20m one.
There certainly is speed gradient, and there may be direction shear in the surface layer, but not from Coriolis: usually when a different air mass layers above the other, an example is the onset of breeze in a calm day, it will be felt starting from the top of the mast, it s when one has a lot of difficulty in finding the correct twist for the sail one tack from the other
lots of academic references, first ones from google
http://www.curry.eas.gatech.edu/Courses/6140/ency/Chapter9/Ency_Atmos/BL_Surface_Layer.pdf
http://publications.lib.chalmers.se/records/fulltext/219608/219608.pdf
https://books.google.fr/books?id=tR...#v=onepage&q=surface layer wind shear&f=false
etc etc
lw395
Well-Known Member
AIUI, the 'coriolis effect', which is really just a bit of conservation of angular momentum, is significant in the wind 'at altitude'. It causes wind to spiral into a low instead of blowing from A to B.
Low down, friction from the earth's surface (land or sea) slows the spiraling and reduces the effect of the coriolis. So the back or veer as a result of altitude is present at low levels, along with the speed variations.
All this in the absense of other influences.
My simple view of things is that near the coasts, we are awash with 'other influences'.
Any of which can negate the effect of the coriolis force.
There are so many things varying all the time, the optimum amount of twist will change yard by yard sometimes, never mind when you tack.
It often changes after a tack anyway, as you build speed and then look for optimum pointing.
I have never come off the water thinking 'today I always wanted more twist on Port than Starboard' or v/v, but I'm still learning or trying to!
Low down, friction from the earth's surface (land or sea) slows the spiraling and reduces the effect of the coriolis. So the back or veer as a result of altitude is present at low levels, along with the speed variations.
All this in the absense of other influences.
My simple view of things is that near the coasts, we are awash with 'other influences'.
Any of which can negate the effect of the coriolis force.
There are so many things varying all the time, the optimum amount of twist will change yard by yard sometimes, never mind when you tack.
It often changes after a tack anyway, as you build speed and then look for optimum pointing.
I have never come off the water thinking 'today I always wanted more twist on Port than Starboard' or v/v, but I'm still learning or trying to!
ghostlymoron
Well-Known Member
I've often wondered how true wind is calculated by a wind instrument from speed and direction of wind and boat speed (through water) from log. There doesn't seem to be enough data.
lw395
Well-Known Member
It's enough to give true wind speed and direction relative to the bow of the boat.
ghostlymoron
Well-Known Member
Doesn't it also need direction of travel of the boat?
dom
Well-Known Member
I guess one could also have big-style direction shear with an approaching mesocyclone. And if it touched down that Garmin avionics kit might come in handy for the landing
Pointless Coriolis fact: a tornado forming within a massive mesocyclone (15-20,000m high) will naturally come under the Coriolis influence encouraging it to rotate counter-clockwise in Northern Hemisphere and vice versa. Yet even with tornadoes, about 5% end up rotating the wrong way.
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