sail profile - SailNet Community

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > General Interest > General Discussion (sailing related)
 Not a Member? 


Reply
 
LinkBack Thread Tools
  #1  
Old 12-11-2001
Junior Member
 
Join Date: Dec 2001
Posts: 1
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0
bazcarr is on a distinguished road
sail profile

I need some sail theory explained for me. Every main sail I have seen so far seems to curve away from the wind. Whenever the boom is sheeted in tight, the sail curves away from the wind as it goes higher up the mast. Is this actually a desirable effect? If it is, what good does it do? If not, what harm is it doing, and why hasn''t someone already invented something to fix the problem?
Reply With Quote Share with Facebook
  #2  
Old 12-12-2001
Jeff_H's Avatar
Moderator
 
Join Date: Feb 2000
Location: Annapolis, Md
Posts: 6,579
Thanks: 5
Thanked 95 Times in 71 Posts
Rep Power: 10
Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about
sail profile

The phenomina that you are describing is called "twist". Controling twist is a critical part of optomising performance. Some degree of twist in light to moderate winds is almost always a good thing.

To begin with, if you look at a sail in section (cut horizontally through the sail) it is a wing. Even very efficient wings have an "incident angle" and "slip angle". In other words a wing (or a sail)needs to be placed at an angle to the wind to work. For a given wind and any given sail and any given boat in a given condition, at any given spot on the sail, there is an optimum angle of attack in order to achieve the best performance.

What further complicates all of this is that the wind at the top of the sail is actually different than the wind at the bottom of the sail. Called ''gradiant effect'', in light to moderate winds the wind speed typically increases the higher you get above the surface of the water.

Visualize this effect this way, there is friction between air and water and between air and air. Because of this friction at the surface of the water there is a (barrier)layer of air that does not move at all relative to the water. Next to this layer of air is another layer of air that moves slowly over this stationary barrier layer. That layer feels the friction of the barrier layer and the friction of the layer above it that is motavated by the ambient wind. Each higher layer moves a bit more quickly comapred to the layer below until at some point up in the air there is a point at which the air moves at the speed of the ambient winds and does not feel the affect of the barrier.

In light air, this affect can be tens of feet deep. At very much higher wind speeds this whole gradiant effect, barrier to free flowing wind, is only a couple inches deep. We usually sail where the effect is somewhere in between but typically taller than the average mast height even in moderate winds.

In a sailboat, this means that the boat feels more true wind at the masthead than it does at the deck. Because of the way that apparent wind works, the higher wind speed at the masthead produces higher apparent winds at the masthead that are also more abeam to the boat than the apparent winds that are felt lower in the sail. Getting back to your question, twist allows the sail to have differing attack angles as you move up the sail, each at a proper angle of attack relative to the apparent wind that it is passing through.

If you eliminated twist in light to moderate conditions, some of the sail will be over trimmed or some of the sail will be under trimmed for the conditions. Of course as windspeeds increase, gradient wind effect decreases and so as wind increases in speed, twist should be reduced. This is done by lowering the traveller and increasing halyard, outhaul and sheet tension. In a really strong breeze the sail needs a comparatively flat camber and angle of attack and so the sail should be bladed out. This means maximum halyard tension, outhaul tension, backstay tension, mainsheet tension. To further reduce the angle of attack the traveller is dropped as well. This will decrease weather helm and heeling.

Jibs have twist as well. Twist in jibs is controlled by the jib sheet lead angles. Moving the jibsheet car aft tightens the lower sail and increases twist in the sail, moving the track forward pulls down on the leech and so increases twist. On jibs you increase twist in really light air to open the slot and in really heavy air to reduce heeling.

The clues to proper amount of twist comes from the teltales. On mainsails the leech teletales at the battens provide the best information. All of the teletales up the leech should be flying when the sail is set properly. When there is inadequate twist the teletale at the head will be stalled and sucked back into the sail.

On jibs, the luff teletales should all be flying and all of the teletales hould ''break'' evenly. On small jibs (with battens), leech teletales are very helpful with sail trim as well.

One of the problems with battenless sails is that it is much harder to control twist without developing leech flutter. That problem as much as the smaller sail area is what kills performance in in-mast furling sails in lighter conditions.

Regards
Jeff
Reply With Quote Share with Facebook
  #3  
Old 12-13-2001
Senior Member
 
Join Date: Oct 2000
Posts: 629
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 14
kimberlite is on a distinguished road
sail profile

jeff,
that was extremely well written
eric
Reply With Quote Share with Facebook
  #4  
Old 12-14-2001
Jeff_H's Avatar
Moderator
 
Join Date: Feb 2000
Location: Annapolis, Md
Posts: 6,579
Thanks: 5
Thanked 95 Times in 71 Posts
Rep Power: 10
Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about
sail profile

Thanks!
Reply With Quote Share with Facebook
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Windward performance deseely General Discussion (sailing related) 21 04-01-2012 02:42 PM
Sloop, Cutter or Ketch jsgsail Boat Review and Purchase Forum 17 12-26-2008 02:47 PM
New to Sailing, please Help =) xyris Learning to Sail 19 11-17-2008 08:30 AM
Ketch/Yawl Handling svsymphony Seamanship & Navigation 19 07-05-2008 01:19 AM
Traveler usage? Humpwalker Seamanship & Navigation 22 02-17-2007 10:54 AM


All times are GMT -4. The time now is 03:01 AM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012

The SailNet.com store is owned and operated by a company independent of the SailNet.com forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.