Can someone explain roach to me? - 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? 


Like Tree7Likes
Reply
 
LinkBack Thread Tools
  #1  
Old 01-28-2013
MedSailor's Avatar
Closet Powerboater
 
Join Date: Mar 2008
Location: Anacortes PNW
Posts: 2,948
Thanks: 162
Thanked 74 Times in 65 Posts
Rep Power: 7
MedSailor is on a distinguished road
Can someone explain roach to me?

I have a nagging question that just hasn't yet been answered to my satisfaction. Why is roach in a mainsail a good idea?

I feel satisfied with the arguments of battens (full and partial) vs battenless, but the physics and benefits of a lot of mainsail roach elude me. I had thought that the roach was probably for rule beating purposes and would allow you to get a better PHRF rating by having a low measured luff but more sail area.

The problem with that theory is that pretty much all open class boats have lots of roach, especially at the head of the sail. There must be some specific benefit of roach besides "more sail area". I've also noticed that the wings of airplanes don't go to a point like a triangle. They have "roach" too.

So what does it do? Lots of roach seems to be at odds with the ultra high aspect modern rigs that dominate the races. If high aspect and all luff and no foot is the ideal, then why the roach?


MedSailor
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


I have a sauna on my boat, therefore I win.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Quick reply to this message Share with Facebook
  #2  
Old 01-28-2013
Brewgyver's Avatar
Senior Member
 
Join Date: Dec 2011
Location: So Cal
Posts: 376
Thanks: 6
Thanked 12 Times in 12 Posts
Rep Power: 3
Brewgyver is on a distinguished road
Re: Can someone explain roach to me?

The roach allows the sail to form a curved surface, creating an airfoil, and threrefor generating "lift", just like the wing of an airplane, or the rotor blades of a helicopter. The physics involve Bernouli's Principle, best illustrated with... an illustration


The Luff and Loach of the sail are equivalent to the leading and trailing edges of an airlplane wing, and the straight lind distance between those is the "chord". (also, the aeronautic equivalent of "roach" is "camber")

The air flowing past the sail has to flow faster on the convex side of the airfoil, which results in lower pressure, which is called "lift". The"lift" is more or less perpendicular to the chord, so of course in the case of the sail as airfoil, the "lift" pulls the sail and boat forward, rather than up.

If you think about sailing on any point higher than a beam reach, you can see that it is the lift generated by airflow over the curved shape of the sail, and not increased sail area that is propelling the boat forward. It could be argued that increased sail area alone is only really advantagous when running downwind, wing and wing.

Does that help?
__________________
s/v My Sweet Girl!
Catalina 30 (Mark I)
Atomic 4

Last edited by Brewgyver; 01-28-2013 at 05:56 PM.
Reply With Quote Quick reply to this message Share with Facebook
  #3  
Old 01-28-2013
jsaronson's Avatar
Senior Member
 
Join Date: Dec 2011
Posts: 887
Thanks: 4
Thanked 25 Times in 25 Posts
Rep Power: 4
jsaronson is on a distinguished road
Re: Can someone explain roach to me?

Roach allows more sail area. More sail should generate more speed.
Reply With Quote Quick reply to this message Share with Facebook
  #4  
Old 01-28-2013
blt2ski's Avatar
Senior Member
 
Join Date: May 2005
Posts: 6,762
Thanks: 0
Thanked 25 Times in 24 Posts
Rep Power: 10
blt2ski will become famous soon enough
Re: Can someone explain roach to me?

Roach DOES make more sail area. My maxed out per phrf main is 195#, vs 180 for the basic supplied main, actual triangle measure is 161 sq ft.

So orach to a degree, and maybe at one time was a rule beater, but now, you have to measure the foot, luff, pluse the measure from luff to the back edge of the main af the half, 3/4 and 7/8 of the way to the top IIRC, so that the total roach amount gets included. If you go over a certain % of the base foretriangle, you get penalized, ie need to go faster to finish. If my main was around 100#, I would get a 3 sec penalty per mile, so instead of a 192, I would be a 189 in phrf speak. I could go bigger yet, and get more deducts too......some classes this extra is built into the base ratings. If one wer to go smaller, say 180 as I was, I get a 3 sec credit, and was a 195 with the smaller main.

The same happens with jib/genoa and spin sizes too, while you can go bigger per say, you may take a hit faster from the base if you go too large, or go smaller, you get credits. Many times the credits frankly are not in the end worth taking. As you go double or triple or more the miles per second credit you get. Fixed props are this way, the 9 or 12 sec credit vs a folder is not worth it if you race frequently! you gain 20-30 secs a mile with a folder vs a fixed.

Marty
msmith10 likes this.
__________________
She drives me boat,
I drives me dinghy!
Reply With Quote Quick reply to this message Share with Facebook
  #5  
Old 01-28-2013
flyingwelshman's Avatar
Senior Member
 
Join Date: Aug 2007
Location: Ontario, Canada
Posts: 1,749
Thanks: 18
Thanked 25 Times in 18 Posts
Rep Power: 8
flyingwelshman will become famous soon enough
Re: Can someone explain roach to me?

My understanding is that the roach provides more sail area higher up the sail, where the wind is not as affected by friction from the water.
__________________
1989 Hunter 30'
Southern Georgian Bay

The sea, once it casts its spell, holds one in its net of wonder forever. - Jacques Yves Cousteau
Reply With Quote Quick reply to this message Share with Facebook
  #6  
Old 01-28-2013
RichH's Avatar
Senior Member
 
Join Date: Jul 2000
Location: Pennsylvania
Posts: 2,961
Thanks: 10
Thanked 88 Times in 80 Posts
Rep Power: 15
RichH will become famous soon enough
Re: Can someone explain roach to me?

Aerodynamic theory explains that the more perpendicularly the airflow crosses or leaves the trailing edge the more efficient the total aerodynamic effect.
A roach extends the sail’s area in the upper section so that this area more approximates or approaches the above. (and because it more rapidly transitions the ‘tip’ geometry toward that desired 'perpendicular to exit flow' geometry)

Iceboats carry radically raked masts to approximate the same - leech almost dead perpendicular to the horizontal.

The latest modern racing rigs have extended 'flat head' mainsails for the same reason --- more perpendicular flow ‘off’ of the leech; for more maximized efficiency and more optimized lift. The more efficient the mainsail, …. the jib becomes more aero-efficient, too … when sailing ‘aerodynamically’.

The absolute master of aerodynamic design for similar wind speeds, does the same ---- the albatross’ wing.
(So don’t be at all surprised when someday in the future you might even see mainsails with extended ‘primary feathers’ along the top of that big ‘square-ish’ & ‘flat top’ mainsail.)


-----------
BTW ... fabric sails dont have 'thickness', and therefore much of what you learned from sailing books about what causes ‘lift’ doesnt happen in that manner, and yet the Wright Brothers original wings were essentially sails with essentially NO thickness …… hmmmmmm.
More confusing to sailors, and some pilots, some of the airflow circulates 'around' a wing/sail (even ‘thick’ wings) ... there is a flow 'component' on the 'windward side' that is going --- FORWARD, due to the circulation flow ‘around’ the wind/sail!
Aerodynamics is definitely NOT an intuitive science, and your (US) high school 'science' teacher's explanation of 'lift' was almost DEAD WRONG
Gary M likes this.

Last edited by RichH; 01-28-2013 at 07:27 PM.
Reply With Quote Quick reply to this message Share with Facebook
  #7  
Old 01-28-2013
Senior Member
 
Join Date: Jun 2001
Posts: 578
Thanks: 0
Thanked 3 Times in 3 Posts
Rep Power: 14
sailordave is on a distinguished road
Re: Can someone explain roach to me?

Quote:
Originally Posted by Brewgyver View Post
The roach allows the sail to form a curved surface, creating an airfoil, and threrefor generating "lift", just like the wing of an airplane, or the rotor blades of a helicopter.
The Luff and Loach of the sail are equivalent to the leading and trailing edges of an airlplane wing, and the straight lind distance between those is the "chord". (also, the aeronautic equivalent of "roach" is "camber")

The air flowing past the sail has to flow faster on the convex side of the airfoil, which results in lower pressure, which is called "lift". The"lift" is more or less perpendicular to the chord, so of course in the case of the sail as airfoil, the "lift" pulls the sail and boat forward, rather than up.

If you think about sailing on any point higher than a beam reach, you can see that it is the lift generated by airflow over the curved shape of the sail, and not increased sail area that is propelling the boat forward. It could be argued that increased sail area alone is only really advantagous when running downwind, wing and wing.

Does that help?

Sorry to debunk that tired old theory, but sails, like airplanes wings do not work because of our friend Bernoulli. Instead it is the Coanda effect (
) that supplies the "lift" on our boats (and airplanes).

Basically, a fluid attaches to a curved surface and is redirected off that surface providing drive. (simple explanation) Think of what happens when you turn on your kitchen faucet. Now, take a simple tablespoon and hold it vertically w/ the underside of the spoon towards the running water. Slowly move the spoon towards the water. When you actually engage the water what happens? (rhetorical question) The water attaches to the back of the spoon and is redirected out of the flow and sprays out into the sink. That's what happens w/ a foil, be it a wing, a sail or a keel. Subtle difference from Bernoulli. And why a jet fighter that has almost no curvature to its wing can fly upside down. It just has a higher angle of attack and thus forces the air downward. Sorta like what happens when you stick your arm out the window of a moving car w/ you hand held flat. If you tilt your arm so your hand is acting like a wing, it wants to rise.

But, back to the OP question. Roach, as has been pointed out adds more sail area providing more drive. And yes, the further up the mast you can add sail area the better because of surface friction. That's why all the hot race boats have the square top mains; puts more drive at the top of the mast where there is better wind speed and apparent wind angle.
__________________
CS 36M DIANTHUS

Last edited by sailordave; 01-28-2013 at 08:01 PM.
Reply With Quote Quick reply to this message Share with Facebook
  #8  
Old 01-28-2013
MedSailor's Avatar
Closet Powerboater
 
Join Date: Mar 2008
Location: Anacortes PNW
Posts: 2,948
Thanks: 162
Thanked 74 Times in 65 Posts
Rep Power: 7
MedSailor is on a distinguished road
Re: Can someone explain roach to me?

Thanks RichH, I was really hoping you'd stop by.

The "more sail area" argument just wasn't holding water with me. Many new boats that have no rule limitations on design are building roachy heads into their mains. If it was truely just sail area, then they'd make the mast taller right?

Your explanation may also explain something I read in an article by Carol Hasse on small jibs whereby she was saying that if you have an extremely long luff with very little LP you end up with a lot of narrow sail at the top of the luff that does little to create lift but does a lot to induce heel.

On my boat I have a VERY low aspect mainsail (P=42, E=17), which would mean that I have a larger angle at the head of my sail than I high aspect rig. Would I benefit (in theory) from battens at the top, or is my rig low enough aspect that I already have enough sail area at the head? Is there a painful formula that I can use to figure out how much sail area should be at the head or how much roach would be idea for my sail?

Albatross wings:


MedSailor
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


I have a sauna on my boat, therefore I win.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Last edited by MedSailor; 01-28-2013 at 08:12 PM.
Reply With Quote Quick reply to this message Share with Facebook
  #9  
Old 01-28-2013
zz4gta's Avatar
I don't discuss my member
 
Join Date: Aug 2007
Location: Chesapeake Bay
Posts: 2,462
Thanks: 0
Thanked 22 Times in 22 Posts
Rep Power: 8
zz4gta is on a distinguished road
Re: Can someone explain roach to me?

Square top mains provide a lot more roach, to the extreme. Sail area up high is good, b/c that's where there is more breeze. Also, triangles have some tip vortexes that are slow. Square top mains reduce the drag from these.
sailordave, Sabreman and SloopJonB like this.
__________________
Merit 25 # 764 "Audrey"
Reply With Quote Quick reply to this message Share with Facebook
  #10  
Old 01-28-2013
RichH's Avatar
Senior Member
 
Join Date: Jul 2000
Location: Pennsylvania
Posts: 2,961
Thanks: 10
Thanked 88 Times in 80 Posts
Rep Power: 15
RichH will become famous soon enough
Re: Can someone explain roach to me?

The "more sail area" argument just wasn't holding water with me. Many new boats that have no rule limitations on design are building roachy heads into their mains. If it was truely just sail area, then they'd make the mast taller right? ...
******* yes, probably thats whats going on in the high class venues. A taller mast would increase the heeling moment -- its like supply and demand ... needs 'balance'. I strongly feel the roachy heads are for sail efficiency increase .... and the more SA thats aloft the better the upwash is forming waaay out in front of the boat, more SA up top the more 'upwash' aloft. With strictly triangular tops, the upwash directly in front of the top is less, simply because there's less sail area 'coming forward' to create that upwash. all this for 'upwind' sailing.

Your explanation may also explain something I read in an article by Carol Hasse on small jibs whereby she was saying that if you have an extremely long luff with very little LP you end up with a lot of narrow sail at the top of the luff that does little to create lift but does a lot to induce heel.
******* the preponderance of the 'lift' comes from the luff section, the 'trailing sections' through to the leech make the luff section more efficient (simplistically but NOT in direct proportion to 'length' ... and you can see the scarcity of large overlapping jibs on high end boats these days, too); making the luff longer increases the heeling moment. see sketch below.

On my boat I have a VERY low aspect mainsail (P=42, E=17), which would mean that I have a larger angle at the head of my sail than I high aspect rig. Would I benefit (in theory) from battens at the top, or is my rig low enough aspect that I already have enough sail area at the head? Is there a painful formula that I can use to figure out how much sail area should be at the head or how much roach would be idea for my sail? Sail what you have, or at least 'fill in' all the space from the backstay towards forward with roach.
****** But your boat has a staysail and if flown 'correctly' when upwind (and usually above 7kts) the staysail 'can' become a forward 'extension' of the mainsail system. Here's how: http://www.arvelgentry.com/magaz/The...e_Head_Rig.pdf. If your boat isn't tender after it initially 'hardens up' on a beat --- easily heels over to a quite constant angle and no more; then a proper 'matching' of a new mainsail with a full 'shoulder' (top panels with extra draft) would probably be the best way to go, will reduce your light wind performance a bit but could add a lot more 'power' (not speed) to the top portion of the mainsail. This is an individual sail design that you're not going to get from a 'stock sail' loft AND you're going to need a sailmaker who actually 'deals' with such design changes. A sail with lots of 'shoulder' will develop lots of extra power and the boat needs to be somewhat stiff or in racing a crew that can hike waaaaay out. ... or a 'blend' between plain vanilla and full shouldered.
An alternative to full-shoulder, is to 'hook up' the leech by over tensioning the mainsheet until the second batten 'just' points above the boats center line, then 'very' slightly ease the traveller until that #2 batten is parallel to the CL., such can create a lot of increased 'draft' up top (and the mid section and foot, so dont overdo it) To do this the sail has to be correctly/perfectly raised (if dacron) and the luff boltrope cant be 'shrunken' (an age problem).


http://i1086.photobucket.com/albums/...Closehaued.jpg

Last edited by RichH; 01-29-2013 at 12:02 AM.
Reply With Quote Quick reply to this message Share with Facebook
Reply

Quick Reply
Message:
Options

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.
Click Here to view those rules.

Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




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
Roach and How Many Battens? Sabreman General Discussion (sailing related) 24 07-24-2011 08:36 PM
Extra large roach mainsail klubko Gear & Maintenance 32 11-14-2010 08:24 AM
adding roach AllThumbs Sailboat Design and Construction 28 10-10-2008 04:49 PM
Can a sail with too much roach be cut down without ruining It's shape? Sabre66 General Discussion (sailing related) 2 05-29-2007 08:43 PM
Roach or not mainemandean Gear & Maintenance 26 01-05-2007 02:19 PM


All times are GMT -4. The time now is 04:22 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.