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Go Back   SailNet Community > Skills and Seamanship > Learning to Sail
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  #111  
Old 09-17-2007
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Quote:
Originally Posted by chris_gee View Post
On the various annenometer sites I saw -the ST60 not being accessible due to an idiosyncracity of my computer, the sensitivity was 1 and accuracy 2.5% at low range.
Please post a link as I did (should be easy enough).

Quote:
Originally Posted by chris_gee View Post
That means for zero relative wind at the boom and a mast height above water of 20m the wind should be say 2.25 +-.05. That clearly is reliably above zero or even .3.
Which is in the error range that I listed, meaning I can't accept the numbers you are giving. Should I assume that the link I posted, and the other reasons (boat pitching) are not satisfactory?

Before I can accept your calculations on what apparent wind would be based on mast head apparent wind - you must dismiss my reasons for inaccuracy. Which I would be more then willing to do - but you haven't given me any reason to do so yet.

Until then I am going to have to go with what fits the observed phenomenon and the specifications I have found and shared.
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  #112  
Old 09-17-2007
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Quote:
Originally Posted by Hartley18 View Post
Are you allowing for detected windspeed due to rolling in waves?? Could be considerable on a large yacht downwind..

--Cameron
A very good point I have tried to mention before.
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  #113  
Old 09-17-2007
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Are you allowing for detected windspeed due to rolling in waves?? Could be considerable on a large yacht downwind.

Nah. Nor being towed downwind by a big fish. Twas this big really. Honestly.
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  #114  
Old 09-17-2007
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Quote:
Originally Posted by chris_gee View Post
Are you allowing for detected windspeed due to rolling in waves?? Could be considerable on a large yacht downwind.

Nah. Nor being towed downwind by a big fish. Twas this big really. Honestly.
The boat pitching 1 degree would cause a movement of the top of the mast of over a foot given a 60' mast. One degree is pretty small.

You have ignored my comments regarding the movement of the top of the mast for a while. Sarcasm won't make them suddenly untrue.

Besides, you can only tell stories about big fishes if you say you _caught_ them. Sheesh.
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  #115  
Old 09-17-2007
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Actually "Apart from the fact that conditions are calm, I dont see that pitching etc would effect the accuracy of windspeed, though they could direction, as the sensor is moving in a circle."
To amplify, any effect due to gravity or impetus would be offset as in a circular motion one side and the cups offsets the other. I thought you got it.
The fish is to reflect that anecdotal evidence, particularly when a fairly loose account which doesn't hold up very well to closer examination is of dubious value. One might apply scepticism to fisherman's accounts, and sailors' accounts of wave heights, wind speeds and boat speeds regardless of their general honesty.
The claim that apparent zero wind at deck or boom height evidenced by telltales and masthead annenometer's zero reading, means that boat speed on the log equates with windspeed on the boom and the mast top, other than momentarily, in the absence of current, and significant measurement error other than unreliability of readings under 1 knot, in my view falls into that category of warranting scepticism.
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  #116  
Old 09-17-2007
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Quote:
Originally Posted by chris_gee View Post
Actually "Apart from the fact that conditions are calm, I dont see that pitching etc would effect the accuracy of windspeed, though they could direction, as the sensor is moving in a circle."
Though I already responded to this when you first posted it - I will say this: You mean to tell me that movement of the wind speed meter doesn't affect its reading?

Quote:
Originally Posted by chris_gee View Post
To amplify, any effect due to gravity or impetus would be offset as in a circular motion one side and the cups offsets the other. I thought you got it.
What are you even trying to say here? That the fact that the cups are moving in a circular motion means that movement on the top of the mast doesn't affect the values reported? Because if that is what you are saying - you are nuts.

Think about it. Lets say there is 5kts of apparent wind at the mast top. Lets say the boat pitches forward. The mast top is now accelerating so the apparent wind is lower then 5kts (and circular motion doesn't matter as apparent wind hasn't changed directions yet). Once apparent wind goes to zero at the mast top (assuming it has pitched far enough, fast enough) there will be a "negative" wind speed registered due to the circular motion. Of course there is no "negative" wind speed but it is in the opposite direction. When the boat pitches back, it must go though that whole process in reverse. The "negative" wind speed must decrease and go to zero while the "positive" wind speed must increase back to 5kts.

During that entire time the values are inaccurate and less then 5kts. Do _YOU_ get it now?


Quote:
Originally Posted by chris_gee View Post
The fish is to reflect that anecdotal evidence, particularly when a fairly loose account which doesn't hold up very well to closer examination is of dubious value. One might apply scepticism to fisherman's accounts, and sailors' accounts of wave heights, wind speeds and boat speeds regardless of their general honesty.
You mean like your accounts of anemometer accuracy? Something claimed and never shown?

Quote:
Originally Posted by chris_gee View Post
The claim that apparent zero wind at deck or boom height evidenced by telltales and masthead annenometer's zero reading, means that boat speed on the log equates with windspeed on the boom and the mast top, other than momentarily, in the absence of current, and significant measurement error other than unreliability of readings under 1 knot, in my view falls into that category of warranting scepticism.
I really couldn't care less about your scepticism. The only real thing you seem to have an issue with in all of this is the accuracy of an anemometer and I have posted a website with product specifications identifying the speeds _you_ calculated for the mast top to be within the range of low speed error.

Compound that with the valid argument that pitching affects the anemometer and the values are to be expected.

You have said nothing that would invalidate either of these things. Other then your own fish story.

I honestly never expected I would get so much crap by trying to explain something that someone saw.
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  #117  
Old 09-17-2007
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Rockter will become famous soon enough
Byron....

Let it ride there man.

5 kt is damn all anyway.... my ship will be barely moving anyway at 5 kt, on any course.

Sail when there is more wind behind you and it won't happen, then it won't matter.
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  #118  
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1. True I did not allow for the change in relative wind due to pitch. However you had conceded that the to and fro cancelled out. On a yacht that size even were the waves a foot they are about a metre apart. That means the yacht is on about say 10 waves depending on LWL. Any pitch would be minimal. Further at 5 knots there would be about 4 changes per second. The ST60 is averaged to smooth these fluctuations. Although this can be user set I doubt that anything more rapid than every 3 seconds or so would be useful. Therefore your point is irrelevant.
2. The website you refer to variously has an anemometer threshold of 4 and claims a tested accuracy at that range of a few percent. However it also mentions the display works only in .5 increments.
3. Were that sensitivity typical then yes the 0-4 range could include the required windspeed, plus or minus 2.
4. However I don't think it is typical nor that Raymarine or Alex would buy it. One site giving less than a knot sensitivity is http://www.globalw.com/products/we550.html.
5. I don't intend to repeat a search on various models to prove what I suggested is typical of a good quality sensor.

You seem to be obsessed with proving an observation was possible. Some have taken this as far as saying in effect that an observation is better than theory.
The observation may be possible but more important is its reliability, accuracy, interpretation, and useful relevance.
When push comes to shove whatever the accuracy of the readings a boat of that size in light winds will not consistently match a consistent windspeed even if measured accurately at the mast. In fact I pointed to current and anemometer inaccuracy as possible explanations for it's appearing to do so.
That is it for me.

Last edited by chris_gee; 09-17-2007 at 07:06 PM.
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  #119  
Old 09-17-2007
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Quote:
Originally Posted by zz4gta View Post
When heading downwind at different angles what should the tell tails look like? I was out a bit ago and they were swept forward, pointing toward the mast on a broad run.

Should I let main out more to be more perpendicular to the wind? Should I even pay attention to the tell tails on a broad or dead run?
If you are on a calm body of water with no swells and some wind.
If you are on a dead down wind run and your tell tails are pointing foward you are going slower than the true wind speed.
If the tell tails are hanging nearly straight down you are aparently going the about the same as the true wind speed.

If however the tell tails suddenly start flogging aft and you find youself standing on the companionway door you have most likely sailed over the falls.
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  #120  
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Quote:
Originally Posted by chris_gee View Post
1. True I did not allow for the change in relative wind due to pitch. However you had conceded that the to and fro cancelled out.
I said that they could cancel out - and I also explained how they wouldn't always cancel out.

Quote:
Originally Posted by chris_gee View Post
2. The website you refer to variously has an anemometer threshold of 4 and claims a tested accuracy at that range of a few percent. However it also mentions the display works only in .5 increments.
3. Were that sensitivity typical then yes the 0-4 range could include the required windspeed, plus or minus 2.
4. However I don't think it is typical nor that Raymarine or Alex would buy it. One site giving less than a knot sensitivity is http://www.globalw.com/products/we550.html.
5. I don't intend to repeat a search on various models to prove what I suggested is typical of a good quality sensor.
To quote the website you lined: "Accuracy: .2 MPH over the range 11 to 55 MPH" Threshold only means it starts registering values. How accurate is it? Not as accurate as you want it to be.

Quote:
Originally Posted by chris_gee View Post
You seem to be obsessed with proving an observation was possible. Some have taken this as far as saying in effect that an observation is better than theory.
No not obsessed. Having fun. This has been my entertainment for a while. And I was the one who said observation trumps theory. Why? Because if you can't align your theory to your observation - your theory, or your use of it, is broken. In this case it is because of the assumptions people have made - not the theory itself.

Quote:
Originally Posted by chris_gee View Post
The observation may be possible but more important is its reliability, accuracy, interpretation, and useful relevance.
And all I have been pushing for this entire time is a reasonable interpretation of what happened instead of throwing it in the "impossible" category.

Quote:
Originally Posted by chris_gee View Post
When push comes to shove whatever the accuracy of the readings a boat of that size in light winds will not consistently match a consistent windspeed even if measured accurately at the mast. In fact I pointed to current and anemometer inaccuracy as possible explanations for it's appearing to do so.
Fair enough - can't disagree with that.
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