Polars and Density of Air

[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.

Density of Dry Air, Water Vapor and Moist Humid Air

Perhaps polar_pi and weather_routing should have built in calculators for these factors. Such that all recorded polars are "normalized" to some stanard, and then when they are used they could be adjusted to present conditions air density, etc.

Just an idea at this point, but it surely is a factor to consider...

I suppose the next step is to actually calculate the difference between "spring" air and average "summer" air to see if it really is significant.

[rgleason]

Here is another Calculator that is easier.

--next post

I am not sure my observations are supported by the actual difference in calculated density betwewn typical spring and summer conditions, because the density numbers seem small, but when applied to a sail, it may add up.

[rgleason]

https://wahiduddin.net/calc/calc_da.htm

[rgleason]

The small difference in lb/ft3 times sail area x wind speed would definitely start to add yp.

[barnakiel]

Agreed. But your take on 'huge' is specific.

b.

[rgleason]

Yes b. I notice a difference using the same 135% jib and the main between 12 knots max in the spring and 17 in the summer. I think that is ahuge difference.

I am trying to account for it
Spring Air temp : 65 degf
Spring Relative Humidity: 30-40%
Summer Air temp: 80 degf
Summer Relative Humidity: 90-100%

The other variable is air pressure.

[seandepagnier]

This is a true point, and grib files do provide both temperature, pressure and humidity.

We could make a correction provided a reasonable formula. Say the given polar is intended for 25C and 1015mbar.

I don't think in practice it is so simple, because the wind gradient changes, and so for this reason, the resulting formula is likely non-linear and specific to each particular boat.


Another thing I noticed is if it is raining very hard, there is also a lot more force on the sail.

Also, colder water tends to have steeper waves. I think it's because of viscosity?

[rgleason]

Sean, I assumed it would be linear with respect to change in density. Are you saying, not so?
I trying to picture the wind gradient.

Very interesting observation about hard rain and more force on sails. Maybe air gets colder and denser?
I'll work on getting a formula.

[RaymondR]

The momentum equation used in fluid hydraulics contains both mass and velocity factors. Consequently, assuming a constant wind velocity, the potential energy contained in a volume of air acting on the sail and being transferred to it is dependant upon the mass of air acting on it. Since the mass is the density by the volume anything which changes the density, such as the inclusion of water vapour, must effect the energy contained in a moving volume of air. However I don't think relative humidity is a measure of the absolute vapour content of a volume of air, I think it's more a comfort factor.

[Neptune's Gear]

Iirc, programs like Expedition allow a correction factor, 3 levels I seem to recall, for wind called 'weight'. Might pay to check the expedition manual? I think it was compensation for temp and pressure....


Sent from my iPad using Cruisers Sailing Forum

[rgleason]

Good description and ideas, Matt and Raymond. I was thinking Marjai or Frank Braithwaite would have info on this effect and was going to check their books.

[rgleason]

By the way, humid air is less dense! Not really an intuitive fact.

[UNCIVILIZED]

You may wish to look up & study Density Altitude. Here's one link, but any online search will pull up a number of explanations & calculators for it.
https://en.wikipedia.org/wiki/Density_altitude

PS: Thanks for the link to the Tool Box

[Alan Mighty]

Neptune's Gear is on the money: so-called 'wind weight', a term to capture wind gradient as it affects sails.

[Alan Mighty]

See also:

http://www.ockam.com/OldSite/gradient1.png