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Old 16-09-2012, 18:58   #31
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Re: Wind Speed/Direction Indicators

Kind of off topic, but a related question.

Do any units compensate for heel?

To my mind, when you heel, say 30 deg, the wind speed should drop for the same reason that the wind pressure on the sail drops.

So, if I'm heeled 30 deg, and reading 26 knots broad reach, then I really have something like 52.

I have a raymarine unit and I think it's drunk half the time, but maybe it's just me.
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Old 17-09-2012, 07:35   #32
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Re: Wind Speed/Direction Indicators

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So, if I'm heeled 30 deg, and reading 26 knots broad reach
That is a tender boat if you are healing 30* on a broad reach.

Wind speed doesn't drop just because the boat is heeled. Apparent wind speed changes with the speed of the boat in relation to the direction of the true wind. This is independent of heel.

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Old 17-09-2012, 12:13   #33
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Re: Wind Speed/Direction Indicators

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So, if I'm heeled 30 deg, and reading 26 knots broad reach, then I really have something like 52.
YIKES!
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Old 17-09-2012, 12:45   #34
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Re: Wind Speed/Direction Indicators

The wind measured by masthead transducer will indeed drop when the boat heels. For small angles of heel the difference won't be much, but with an apparent wind angle of 90 degrees (on the beam) the measured value is effectively the cosine of the heel angle times the apparent windspeed. At 30 degrees heel, this would cause a 30 kt wind on the beam to measure 26 kts.

If the wind is forward or aft of the beam the heel angle has less of an effect.

Another factor, which can be equally important is upwash from the sails. Still another is the fact that the windspeed is not constant: it varies with height above the water. For that matter, the wind angle also varies with height (the apparent wind angle for sure, and possible also the true wind angle).

The high-end "performance processor" measurement systems do sense the boat's heel, pitch, yaw, etc, and use these to adjust the measurements. They also account for masthead motion, which in any kind of sea, can significantly impact the measured wind speed and angle.

This gets really complicated. My system is fairly simple, and I don't really trust the numbers. At best they give me a ballpark value. I like the wind in my hair, bits of yarn, etc, but the instruments can be useful.
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Old 17-09-2012, 17:43   #35
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Re: Wind Speed/Direction Indicators

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Originally Posted by Paul Elliott View Post
The wind measured by masthead transducer will indeed drop when the boat heels. For small angles of heel the difference won't be much, but with an apparent wind angle of 90 degrees (on the beam) the measured value is effectively the cosine of the heel angle times the apparent windspeed. At 30 degrees heel, this would cause a 30 kt wind on the beam to measure 26 kts.
Yes, this is very true. My response was more of a practical one because I can't imagine why one would be heeled 30* on a beam reach. The sails should be adjusted way before that point. There should be little heel on a beam reach so, in practice, little change in wind speed.

And as you noted, upwind, there is almost no effect, and downwind a 30* heel should be alarming in itself.

But I'm not the most qualified here, since a 10* heel for us causes enough concern that we aren't even looking at the wind...

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Old 17-09-2012, 17:53   #36
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Re: Wind Speed/Direction Indicators

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There should be little heel on a beam reach so, in practice, little change in wind speed.
Simply not true
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Old 17-09-2012, 18:11   #37
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Re: Wind Speed/Direction Indicators

G'Day all,

Honestly, worrying about such minor errors in windspeed readout is silly unless one has an instrument that compensates for boat motion... specifically rolling. The effects of heeling (ie changes in angle of incidence to the anemometer cups) or changes in true wind speed due to changes in height above the water are trivial compared to rolling or turbulence.

IMO, the windspeed data vary too much due to natural causes to be viewed as anything but a general trend. You can perhaps prove this to yourself by watching the readout while at anchor or tied to a dock... goes up and down all the time.

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Old 17-09-2012, 18:17   #38
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Re: Wind Speed/Direction Indicators

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Simply not true
On a cat I think it would be.
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Old 17-09-2012, 19:05   #39
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Re: Wind Speed/Direction Indicators

Yes, I should have said beam reach. in fact it was 2 am and I was not trimmed as well as possible. I was trying to beat but that was about as good as it got. Not exactly mr. Knox Johnston here.

And faulty memory said cosine 30 was .5. Cosine 60 is .5. So not as big an error as I thought.
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Old 17-09-2012, 23:53   #40
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Re: Wind Speed/Direction Indicators

For what it's worth, we occasionally sail on a beam reach and heeling 30 degrees (VALIS is a monohull). 15 or 20 degrees is about average heel angle for a good breeze, but if we are regularly hitting 30 degrees I really should have already reefed. Sometimes I'm just having fun and don't bother, but heeling that much isn't the fastest way to sail.
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Old 18-09-2012, 04:35   #41
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Re: Wind Speed/Direction Indicators

Charlie,

A cat is stiffer but they do heel if the wind force is great enough. But every monohull WILL heel on a reach... close, beam or broad.. and anyone who says they don't is wrong.

30 degrees of heel is excessive and will lead to excessive leeway as the hull form and keel is able to slip to lee more and resist less.... which is why some use swing keels. They wouldn't be required if the boat was sailing flat.

The reason why sailboats heel is that the lift (upwind) is distributed over the entire length of the sail and is perpendicular to the sail surface. A boat without a keel will tip right over if it tries to sail a beam reach.

On heading with the wind aft of the beam with the sails on the lee side the force is pushing but on the lee side and again the keel resists but adding a heat lever to right the boat.
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Old 18-09-2012, 06:20   #42
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Re: Wind Speed/Direction Indicators

I bought the Tacktick wind indicator about 3 years ago. It performed perfectly until this past summer. Initially it would suddenly stop working, then come back on. Later, it simply would not work at all. Now, it APPEARS to be working ok but I took it down anyway to send it in and have it checked out. Its been maddening to say the least.
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Old 18-09-2012, 10:38   #43
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Re: Wind Speed/Direction Indicators

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Originally Posted by bgallinger View Post
I bought the Tacktick wind indicator about 3 years ago. It performed perfectly until this past summer. Initially it would suddenly stop working, then come back on. Later, it simply would not work at all. Now, it APPEARS to be working ok but I took it down anyway to send it in and have it checked out. Its been maddening to say the least.
Madding, yes!
A couple days ago I had my tacktick on all day, in the marina, w/o a glitch. Maybe it's sun spots or some frequency disturbances on certain days. With all the cell, wifi, vhf, ssb, cb, tv, am/fm, garage door openers and other frequencies I'm surprised there isn't a lot of bleed over into each other.
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Old 18-09-2012, 11:20   #44
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Re: Wind Speed/Direction Indicators

Interesting point about the heel angle when reaching. On a beam reach, it would seem that 20 degrees of heel is more realistic. Cos(20) is 0.94, so I don't think this is a huge effect.

You don't get a 25% error until you get to a 40 degree heel angle. You're in a world of hurt already at a 40 degree heel angle on a beam reach.
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