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Old 08-08-2016, 18:09   #31
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Re: Polars and Density of Air

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Originally Posted by rgleason View Post
Thinwater, . Popcock.
Uncivil, same instrument this spring and this summer.
As I said, its my first take on this, and when I get to look up Marjai and Frank Brathwait oerhaps we'll know more.

Yes, its 12 to 17 is the difference I've noticed, but lets just say it is 13 to 16, its still significant. If it were a difference if ine knot I would not have noticed the effect and we would not be having this discussion.

Has't anyone else noticed the difference between cool/cold dry and warm/humid air?
Just wait for fall, no spring, because the water is not as warm and the air is colder. You'll see what I mean.
How often do you tune your rigging?

I have observed a large change in rig tension between summer and winter due to the difference temperature coefficients of the stainless steel wire and the aluminum mast. This will change the set of your sails and how much rudder drag you experience.
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Old 08-08-2016, 18:18   #32
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Re: Polars and Density of Air

Yes, indeed Viking I know all about that and my Loos is used often.
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Old 08-08-2016, 18:22   #33
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Re: Polars and Density of Air

Another interesting aspect of the problem is that since it involves polar diagrams the flow patterns, and consequent models applied in a mathematical simulation, over the sails will vary considerably. With the vessel hard on the wind with the sails having a very low angle of attack the model may need to be similar to that used in lift/drag calculations for an aircraft wing whereas when the sails are set for a downwind run the model of a flat plate normal to the fluid flow.


Where do polar diagrams come from? Are they derived for mathematical models or empirical from actual vessel sailing tests or derived from wind tunnel data of models?
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Old 08-08-2016, 18:23   #34
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Re: Polars and Density of Air

Indeed, if my rigging were loose and sloppy in the summer, the new jib would bag and lose drive and lessen the effect I have noticed, because we'd be heeling over unnecessarily due to the jib shape, and would have to reef. I adjust my navtek & backstay often.
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Old 08-08-2016, 18:24   #35
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Re: Polars and Density of Air

Causing us to have to reef earlier.
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Old 08-08-2016, 18:34   #36
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Re: Polars and Density of Air

Raymond, Polars are derived from two sources.
1. Sailing with calibrated instrumentation and recording the nmea0183 data from. Real world data, best taken in consistent conditions, preferably with no current, same set of sails, clean bottom, or condition noted. All points of sail, across a variety of wind speeds. The polars are derived from the data using polauto (windows) or polar_pi.
2. Performance data calculated from boat design data. Often done by manuf designer.
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Old 08-08-2016, 18:35   #37
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Re: Polars and Density of Air

I am ashamed to admit I have noticed nothing.

My experience varies from competitive dinghy racing (N Europe - May to Oct) to sailing about 50k sea miles (anywhere between Norway and the Equator).

I sail mostly by the windex, the woolies and by the 'feel'. We do have a basic wind instrument here.

I can't say I noticed anything.

I did read about the fact before.

The closest thing to what you describe is watching Rodrigues fishermen wet the canvas to catch more wind. I know this was done in racing dinghies when their sails were cotton. So I assume some sails may still draw better when wet. And they are more likely to be wet in the spring/fall and in the mid and high- latitudes.

Just a though. Maybe your sails have different properties in changing weather conditions. Not sure how likely this is with very modern sails.

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Old 08-08-2016, 18:44   #38
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Re: Polars and Density of Air

Thanks Barnakiel. You have some great racing dinghies. You'd probably notice the difference between a warm summer and spring wind. So why not try an experiment this summer and then one this fall or spring?
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Old 08-08-2016, 18:46   #39
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Re: Polars and Density of Air

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Indeed, if my rigging were loose and sloppy in the summer, the new jib would bag and lose drive and lessen the effect I have noticed, because we'd be heeling over unnecessarily due to the jib shape, and would have to reef. I adjust my navtek & backstay often.
In summer the mast expands - tightening the rigging.

Also, sorry but I have been sailing for over 60 years and have never notices this effect. That is way I am looking for alternative explanations.
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Old 08-08-2016, 18:54   #40
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Re: Polars and Density of Air

Viking, I too have sailed longer than that. My mast is deck stepped and itdoes not tighten the rigging by elongation. The rigging gets looser. In the spring you will also notice the rig get tighter with a cold night.
I guess we have different boats.
I think the thing for me to do is to calibrate instruments and record spring and fall polars. Which I want to do anyway.
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Old 08-08-2016, 19:28   #41
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Re: Polars and Density of Air

Viking,
PS: I think I should be check my rig to make sure I haven't got this reversed! As AL
coeff of expansion is more. But quite honestly, I had to retighten the shrouds this July as they were too slack. You've got me wondering now about this.

I really believe my rig is tensioned ok. Just this week I was going to put a marking stick (using a paint stick) on the backstay adjuster and use my Loos Gauge to write the values. But now I will check all the rigging.
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Old 08-08-2016, 19:35   #42
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Re: Polars and Density of Air

I probably should have posted this somewhere else, but I did not think there would be so many doubters, and I thought I would be learning more about how to account for it to improve polar recording and weather_routing, rather than discussing whether the difference even existed. Now I am sorry I ever brought it up.
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Old 08-08-2016, 19:35   #43
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Re: Polars and Density of Air

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Thanks Barnakiel. You have some great racing dinghies. You'd probably notice the difference between a warm summer and spring wind. So why not try an experiment this summer and then one this fall or spring?
I am not in the North anymore. We live in Las Palmas now (this is Canary Islands, Africa offshore) where the air is always moist and the temperatures vary only between maybe 19 degs cent on a winter night and say 22 degs cent on a summer day.

I will talk to local racers ask them without making clear what I am after.

I think you cannot tell the difference (much as you could measure, or calculate it) because the wind instrument, your body senses and the sail are all registering the force, not the speed of the wind. Hence 15kts is 15kts.

You can't get the pure wind speed data without calculating back and allowing for temperature and humidity. And you do not have to, as the wind sensor data (normal, cupped or mill sensor) already contains the cumulative of the speed, the humidity and the temperature factor. So to say the instrument is already showing you what you may be trying to back-calculate - the actual force of the wind.

I have the picture clear and I find the discussion valid but I do not see this is immediately perceivable, without somewhat specialised instrumentation and then maybe some calculations too.

Imagine the following though: you can see a rainy squall and so you sail on and under the cloud as you know rain plus wind drive the boat harder than the same wind alone. So now you are sailing faster for a time. But the rain gets blown away and now your wind drops (it is quite common to have a lull just behind a showery cloud).

Have you gained or lost towards the guy sailing away from the cloud, in clear air? Etc.

Too many unpredictable variables, too much volatility, etc. Not all variables are discrete too. I can see some stochastic processes approaching with that black cloud.

What I am driving at is that much as the facts speak for themselves, we might have practical challenges employing this knowledge on the racing course.

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Old 08-08-2016, 19:50   #44
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Re: Polars and Density of Air

Use the ideal gas law to estimate the difference in air density of air at different temperatures.

Humidity decreases the density of air due to differences in the molar mass of water and air.

The effects of density are small and not first order. The OP is likely experiencing significant variations in the dynamic pressure due to wind speed due to gusts, turbulence and local weather conditions.

Air has a nominal density of 1.2kg / m2. Take the area of your sail and you'll realize how much mass of air is acting on your sails.

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Old 08-08-2016, 20:08   #45
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Re: Polars and Density of Air

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Originally Posted by rgleason View Post
Viking,
PS: I think I should be check my rig to make sure I haven't got this reversed! As AL
coeff of expansion is more. But quite honestly, I had to retighten the shrouds this July as they were too slack. You've got me wondering now about this.

I really believe my rig is tensioned ok. Just this week I was going to put a marking stick (using a paint stick) on the backstay adjuster and use my Loos Gauge to write the values. But now I will check all the rigging.
The first thing I would check is if it looks like the mast is pushing down on the cabin top.

The second thing I would check is to see if the stay look like they are being stretched. (i.e. check the turnbuckles)
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