Polars and Density of Air

[cagney]

Have a look here:
Weight of the Wind - Franks-Weather - The Weather Window

/Thomas

[barnakiel]

I have run the same numbers and came to slightly, but not essentially, different conclusions. IMHO in most mid latitude areas, we may at times face a maximum of 5% variation of the force on the sail.

Now our challenge is that the force on the sail will be proportional to the force on the windo cups and so we cannot separate the values with plain onboard instruments (unless one carries a small met laboratory and a tablet).

In other words: The heavier air will act equally on the windo and on the sails and a boat in 15 knts (as read on the windo) will develop proportionate speed, caeteris paribus.

And so we get stuck. 15kts = 15 kts. Expect same boat speeds UNLESS you wind instrument can deduct the effect of humidity and air density and show you a bare, benchmark, reference wind force.

OP is IMHO right there is a difference. The challenge is that our wind instruments as well as our senses show the cumulative value, not the components. And our sails react to this cumulative value as well. We are stuck in reality.

Bueno. My take. Likely full of loopholes too.

b.

[UNCIVILIZED]

Originally Posted by barnakiel

I have run the same numbers and came to slightly, but not essentially, different conclusions. IMHO in most mid latitude areas, we may at times face a maximum of 5% variation of the force on the sail.

Now our challenge is that the force on the sail will be proportional to the force on the windo cups and so we cannot separate the values with plain onboard instruments (unless one carries a small met laboratory and a tablet).

In other words: The heavier air will act equally on the windo and on the sails and a boat in 15 knts (as read on the windo) will develop proportionate speed, caeteris paribus.

And so we get stuck. 15kts = 15 kts. Expect same boat speeds UNLESS you wind instrument can deduct the effect of humidity and air density and show you a bare, benchmark, reference wind force.

OP is IMHO right there is a difference. The challenge is that our wind instruments as well as our senses show the cumulative value, not the components. And our sails react to this cumulative value as well. We are stuck in reality.

Bueno. My take. Likely full of loopholes too.

b.

If we're on the same page in terms of language/info. Most of this stuff can accurately be measured via density altitude calculators. Assuming you have a barometer, hygrometer, & wind speed meter. And there are add ons for many smart phones that'll allow you to measure all of this data, along with calculation apps for phones. So that you can accurately "weigh" the air in your location, from moment to moment. In addition to better measuring it's force on your sails.
I use a Kestrel hand held weather station to measure such, along with a simple calc program. Yep (semi) weather geek.

Also, for discerning the difference between the masthead wind, & deck level, pick up a 2nd masthead sensor, & mount in on a wand on your transom or arch. It's directional values will have odd directional variances, ditto to some degree for wind speed, given that it's in the lee of sails & rigging, plus their upwash. <-- Crappy air.

[rgleason]

I believe from looking at the diagram below that wind weight is a completely different factor. It deals with "mixing" and the gradient of wind from the suface to the nasthead This is certainly a considerarion which would affect performance.
However the other significant factor is the density of the wind as I said before. They should not be confused.

[rgleason]

Thanks Thomas.
Also barnakiel and uncivil.
I've noticed a bit more difference than 5%. 12 knots vs 17 knots which is more like a 40% difference, between cold spring sailing and summer sailing.
http://weather.mailasail.com/Franks-...Weight-Of-Wind

[rgleason]

Heel could also be a measure of relative force of wind given the same point of sail and the same set of sails, since the righting moment is a component of the wind. So, in the case of my little inaccurate experiment, I would have to have the same point of sail upwind, the same heeling angle, and the same sails.

[did-g]

Originally Posted by rgleason

Thanks Thomas.
Also barnakiel and uncivil.
I've noticed a bit more difference than 5%. 12 knots vs 17 knots which is more like a 40% difference, between cold spring sailing and summer sailing.
Weight of the Wind - Franks-Weather - The Weather Window

More beers in the bilge in summer?

[seandepagnier]

Rick:

You will get more than 5% change in speed from 5% change in weight of the air.


As far as the wind cups go. I don't understand how you think they will measure the same "15 knots" with different wind speeds and weights of wind and thus give the same boat speed. This makes no sense.

The cups in theory have 0 resistance, so they measure only wind speed, not air weight. The fact that they have some resistance will affect their reading to artificially increase their speed slightly with heavier air, but not the same way it will affect the boat speed.



As far as the rain. It isn't because the more humid air is heavier. It is because the rain itself applies a force to the sail. This is my point, that it was a heavier weight, and it isn't just the weight of the air, but whatever is actually putting force on the sail. The same is true for sleet.

[TeddyDiver]

Molecular weight of air is about 29g/mol, water vapor 18g/mol..
Venturi type wind speed meter would be better..

[Viking Sailor]

The goal weather routing is to calculate an ETA via a route based upon a performance model of a vessel, while relying on old and/or predicted data. Errors in any of the above, or the skill and attention of the crew, will overwhelm any variation in the ETA induced by calculation of air flow and mass.

Weather routing is not intended to be perfect. It is intended to provide a best guess based upon available data. Unfortunately, the real world rarely repeats exactly and forecasts can be wrong. So, the average and predicted data will from time-to-time give misleading results. Fortunately, it repeats close enough, and forecasts are near enough, to make weather routing a useful tool.

Considering all the possible sources of variations in the models and data it does not seem worthwhile to put a lot of effort into what most likely is a non-problem.

[rgleason]

Sean actually I have not measured the boat speed difference between a cold spring wind and a warm humid wind, at this point. All I have done is sailed with full 135% jib and full main (no reefs) and noticed what the wind speed (using the same 15 yr old Raymarine masthead unit) is when I could no longer hold the full jib and had to reef the jib. The heel of the boat at the point we had to reef in both spring and summer was approximately the same, the weight in the boat was approximately the same, (perhaps 50 lbs more food in the summer due to longer cruises), no change due to beer in the bilge, for did-g, etc. The difference noticed is that we have to furl at 12 knots (you can call it 13 if you like) in the spring and in the summer with warm moist (humid as in high relative humidity, as in psycochromatic chart humidity) we do not have to furl until 17 5 or 18 knots!

Thats all I know at this point.
I'd like to be able to calculate why and perhaps be able to translate a spring polar into a summer polar or visa versa. Or perhaps I'll just record a sprin polar and a summer polar and use them appropriately.

But it seems to me that this effect is more significant than 5% and worth considering even though "crew error and sail trim" can result in significantly more of an error.

Its like anything in physics, heisenberg uncertainty principle, act of measuring changes what you are neasuring, etc, If you ignore too many small factors pretty soon you are just guessing and hoping the uncalculated stuff you know about evens out. When it won't.
Take the case of sailing a northern route where it is colder and dryer versis a warmer route where the air is more humid and less dense. Over a long trip this factor might make a significant difference. Or perhaps you wanted to do a routing which really did account for the real effect of the wind, including density of air? Particularly if we find out there is a significant difference say of greater than 5% of what?

We are still learning about it. I can tell you it is not a small rounding error.

[thinwater]

Originally Posted by rgleason

Thanks Thomas.
Also barnakiel and uncivil.
I've noticed a bit more difference than 5%. 12 knots vs 17 knots which is more like a 40% difference, between cold spring sailing and summer sailing.
Weight of the Wind - Franks-Weather - The Weather Window

12 kt vs 17 kt is double the wind energy and force.

This is why difference in density are really small beans. I honestly believe it is simply the colder temperatures making us more conservative in the winter. Who wants to get wet and knocked on their ear when it's 30 F?

[UNCIVILIZED]

Originally Posted by rgleason

I believe there is a huge difference between the density of dry, cool air and warm humid air, with the result that polars taken without accounting for these factors so that they can be "normalized" to some set standard for pressure, air temp and humidity, will be off significantly.

Why do I say this? Because in the spring we can barely keep out 135% jib fully deployed at 12 knots, yet midsummer we can sail with it at 17 knots. There is a big difference and I believe it is due to the density of air, and amount of moisture (makes air less dense) and also the pressure.

You are aware that winds of 17kts are TWICE as powerful as 12kt winds, right? Wind strength being a squared equation & all.
So the numbers seem a bit of a stretch. Even if you summer in the tropics, & winter in Tierro del Fuego. Instrument calibrations are done how often, & via what methods?

[rgleason]

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.

[UNCIVILIZED]

There IS a difference, & many sailors of note have written of it. Though the difference isn't anywhere near what you're stating. As even at 13kts vs. 16kts, you're talking about 50%.

But say those numbers are correct. Then a vessel under sail in cold climates would lose the ability to go to windward in proportionally less wind. As the hull form, & parasitic drag would cancel out the lift of the sails much, much sooner.
And cargo vessels would need to make 50% more HP to make the same speeds in winter climates. Which would be a lot of fuel, AKA $.

Or birds in a warm weather cell would suddenly have to work 50% harder just to stay aloft when they flew into a cold front. Ditto on aircraft. At least those flying at under say 5,000m.