Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 07-08-2016, 16:13   #1
Registered User
 
rgleason's Avatar

Join Date: Mar 2012
Location: Boston, MA
Boat: 1981 Bristol 32 Sloop
Posts: 8,077
Polars and Density of Air

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 is offline   Reply With Quote
Old 07-08-2016, 16:36   #2
Registered User
 
rgleason's Avatar

Join Date: Mar 2012
Location: Boston, MA
Boat: 1981 Bristol 32 Sloop
Posts: 8,077
Re: Polars and Density of Air

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 is offline   Reply With Quote
Old 07-08-2016, 16:39   #3
Registered User
 
rgleason's Avatar

Join Date: Mar 2012
Location: Boston, MA
Boat: 1981 Bristol 32 Sloop
Posts: 8,077
Re: Polars and Density of Air

https://wahiduddin.net/calc/calc_da.htm
__________________
rgleason is offline   Reply With Quote
Old 07-08-2016, 17:02   #4
Registered User
 
rgleason's Avatar

Join Date: Mar 2012
Location: Boston, MA
Boat: 1981 Bristol 32 Sloop
Posts: 8,077
Re: Polars and Density of Air

The small difference in lb/ft3 times sail area x wind speed would definitely start to add yp.
__________________
rgleason is offline   Reply With Quote
Old 07-08-2016, 17:23   #5
Senior Cruiser

Cruisers Forum Supporter

Join Date: Aug 2009
Location: between the devil and the deep blue sea
Boat: a sailing boat
Posts: 16,562
Re: Polars and Density of Air

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

b.
__________________
barnakiel is offline   Reply With Quote
Old 07-08-2016, 17:34   #6
Registered User
 
rgleason's Avatar

Join Date: Mar 2012
Location: Boston, MA
Boat: 1981 Bristol 32 Sloop
Posts: 8,077
Re: Polars and Density of Air

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.
__________________
rgleason is offline   Reply With Quote
Old 07-08-2016, 19:32   #7
Registered User
 
boat_alexandra's Avatar

Join Date: Aug 2009
Location: BVI
Boat: bristol 27
Posts: 2,673
Re: Polars and Density of Air

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?
__________________
boat_alexandra is offline   Reply With Quote
Old 07-08-2016, 19:50   #8
Registered User
 
rgleason's Avatar

Join Date: Mar 2012
Location: Boston, MA
Boat: 1981 Bristol 32 Sloop
Posts: 8,077
Re: Polars and Density of Air

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.
__________________
rgleason is offline   Reply With Quote
Old 07-08-2016, 21:04   #9
Registered User

Join Date: Apr 2007
Location: Australia
Boat: Island Packet 40
Posts: 1,324
Images: 7
Re: Polars and Density of Air

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.
__________________
RaymondR is offline   Reply With Quote
Old 07-08-2016, 22:46   #10
Sponsoring Vendor
 
Neptune's Gear's Avatar

Community Sponsor

Join Date: Oct 2012
Location: Gulf Harbour, New Zealand
Boat: Farr Phase 4, 12.8m
Posts: 959
Re: Polars and Density of Air

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
__________________
Matt Paulin
Neptunes Gear Ltd
www.neptunes-gear.com
Neptune's Gear is offline   Reply With Quote
Old 08-08-2016, 04:12   #11
Registered User
 
rgleason's Avatar

Join Date: Mar 2012
Location: Boston, MA
Boat: 1981 Bristol 32 Sloop
Posts: 8,077
Re: Polars and Density of Air

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 is offline   Reply With Quote
Old 08-08-2016, 04:13   #12
Registered User
 
rgleason's Avatar

Join Date: Mar 2012
Location: Boston, MA
Boat: 1981 Bristol 32 Sloop
Posts: 8,077
Re: Polars and Density of Air

By the way, humid air is less dense! Not really an intuitive fact.
__________________
rgleason is offline   Reply With Quote
Old 08-08-2016, 04:22   #13
Registered User
 
UNCIVILIZED's Avatar

Join Date: Jun 2014
Location: Up the mast, looking for clean wind.
Boat: Currently Shopping, & Heavily in LUST!
Posts: 4,408
Re: Polars and Density of Air

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
__________________

The Uncommon Thing, The Hard Thing, The Important Thing (in Life): Making Promises to Yourself, And Keeping Them.
UNCIVILIZED is online now   Reply With Quote
Old 08-08-2016, 04:39   #14
Registered User
 
Alan Mighty's Avatar

Join Date: Mar 2014
Location: Scarborough Marina, Moreton Bay
Posts: 549
Re: Polars and Density of Air

Neptune's Gear is on the money: so-called 'wind weight', a term to capture wind gradient as it affects sails.
Attached Thumbnails
Click image for larger version

Name:	Wind weight.jpg
Views:	81
Size:	220.7 KB
ID:	129127  
__________________
“Only a fool learns from his own mistakes. The wise man learns from the mistakes of others.” - Otto von Bismarck
Alan Mighty is offline   Reply With Quote
Old 08-08-2016, 05:20   #15
Registered User
 
Alan Mighty's Avatar

Join Date: Mar 2014
Location: Scarborough Marina, Moreton Bay
Posts: 549
Re: Polars and Density of Air

See also:

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

__________________
“Only a fool learns from his own mistakes. The wise man learns from the mistakes of others.” - Otto von Bismarck
Alan Mighty is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Density of search & rescue incidents in the region of Nova Scotia Skip JayR Health, Safety & Related Gear 0 08-10-2015 18:57
Marine Traffic density hoppy Navigation 4 23-08-2015 06:46
High Density Foam weephee Construction, Maintenance & Refit 26 07-11-2014 06:16
Nuclear Energy Density Trim50 Electrical: Batteries, Generators & Solar 87 13-03-2011 09:55


Our Communities

Our communities encompass many different hobbies and interests, but each one is built on friendly, intelligent membership.

» More about our Communities

Automotive Communities

Our Automotive communities encompass many different makes and models. From U.S. domestics to European Saloons.

» More about our Automotive Communities

Marine Communities

Our Marine websites focus on Cruising and Sailing Vessels, including forums and the largest cruising Wiki project on the web today.

» More about our Marine Communities


Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 07:42.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.