# Can someone explain roach to me?



## MedSailor (Mar 30, 2008)

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


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## Brewgyver (Dec 31, 2011)

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?


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## jsaronson (Dec 13, 2011)

Roach allows more sail area. More sail should generate more speed.


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## blt2ski (May 5, 2005)

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


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## flyingwelshman (Aug 5, 2007)

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.


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## RichH (Jul 10, 2000)

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, &#8230;. the jib becomes more aero-efficient, too &#8230; 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.)

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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 &#8230;&#8230; 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


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## sailordave (Jun 26, 2001)

Brewgyver said:


> 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.
> ...


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 (Coandă effect - Wikipedia, the free encyclopedia) 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.


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## MedSailor (Mar 30, 2008)

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


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## zz4gta (Aug 15, 2007)

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.


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## RichH (Jul 10, 2000)

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_Double_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/j449/svAquila/BeamvsClosehaued.jpg


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## Stumble (Feb 2, 2012)

While increasing roach on an existing boat will increase sail are, that is not why modern designs are moving to high aspect, large roach designes. And really wind shear (wind further above the water is faster) has only a minimal effect. 

The real answer is that the higher aspect a sail is the more efficient it is because it reduces tip vortexes, and maximized the lift generated by entire sail. This is why a number of classes that have maximum sail area restrictions on fixed height masts are also switching to a high aspect sail. This is the same reason that gliders have extremely long but narrow wings, you get a lot more lift out of the same surface area (read drag) with a long skinny foil than you do with a short stubby one.


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## Jeff_H (Feb 26, 2000)

This is not completely about rules or strict aerodynamics. It is to compensate for a variety of shortfalls of a Bermuda rig. One of the short falls of a Bermuda sail on a mast (versus a wing sail), is that a very large percentage of the head of the sail operates in the disturbed air off coming of the mast. Another of the shortfalls of Bermuda mainsails (vs gaff mainsails) is that the gaff sail has more sail area up higher so it can work a little better when reaching in terms of greater drive for a given heeling moment. 

Roach fairs out the air flow at the leach near the head of the sail, allows a greater sail area higher on the sail and which is further away from the mast, which then addresses both of these issues. 

I would suggest that where practical, maximizing roach may actually make more sense on most cruising boats than race boats.

Jeff


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## JonEisberg (Dec 3, 2010)

Jeff_H said:


> I would suggest that where practical, maximizing roach may actually make more sense on most cruising boats than race boats.
> 
> Jeff


I agree wholeheartedly, but I must confess - I'm a bit surprised to hear a guy as 'racy' as you are, admit to that... (grin)

The resultant loss of roach area is one of the biggest downsides of in-mast furling on a cruising boat, for me...


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## knuterikt (Aug 7, 2006)

JonEisberg said:


> I agree wholeheartedly, but I must confess - I'm a bit surprised to hear a guy as 'racy' as you are, admit to that... (grin)
> 
> The resultant loss of roach area is one of the biggest downsides of in-mast furling on a cruising boat, for me...


What do you call this? Furling main with roach
I'm no big fan of furling mains but that problem has a solution.


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## SVTatia (May 7, 2007)

IMHO the purpose of roach is to make you spend lots of money on battens, cars, chafe protection and sailmaker repairs.

My mainsail was straight cut, no roach no battens no problems, ever - after 50K sea miles. The only drawback - I may have arrived perhaps a few hours later?

Another plus - a big one - you can reef on any point of sail.


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## JonEisberg (Dec 3, 2010)

knuterikt said:


> What do you call this? Furling main with roach
> I'm no big fan of furling mains but that problem has a solution.


Yes, I should have noted that, and it's probably a decent solution, given that H-R is now using it... One major advantage I can envision, seems it would allow a main of even greater roach to clear a backstay when tacking, than a sail with horizontal battens...










Still an extremely rare setup on this side of the Atlantic, however - I don't think I've ever actually seen vertical battens on a cruising boat yet over here... Do you know whether such battened mains can be retro-fitted to most existing rigs, or do they require a purpose-built spar with a wider slot? Some of the Hood Stowaway masts I've used, I can't imagine the slot being able to accommodate the additional bulk of battens, without a major modification...


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## Sabreman (Sep 23, 2006)

> I don't think I've ever actually seen vertical battens on a cruising boat yet over here


I saw them last year on a sail at the Easton, MD Evolution loft. It's not a great solution because it's very difficult to control the shape of the sail with vertical battens. As with anything, it's a compromise. Proper shape vs. the ease of in mast furling. IMO, it's a _sail_boat, so I opt for shape.


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## knuterikt (Aug 7, 2006)

JonEisberg said:


> Yes, I should have noted that, and it's probably a decent solution, given that H-R is now using it... One major advantage I can envision, seems it would allow a main of even greater roach to clear a backstay when tacking, than a sail with horizontal battens...
> 
> 
> 
> ...


The boat in the picture is a standard Bavaria 32 with furling mast.
I'm going to my sail maker soon - will try to remember asking him what the limitation's are 
Here is a translation of what one owner wrote 


> We were class winner in Færdern (One of the largest over night races in the world) and No. 2 over-all of all boats rounding at Mefjord!
> So it seem's like Bavaria + Gran furling main is not so bad!
> We received many comments during the ceremony, how can that be possible? What have you done with the boat? It is not possible to win with Bavaria, short keel and furling main when 90% of the race is tacking!
> My only comment was: Talk with Henning at Gran if you are wondering what is possible.
> Bavaria 32 Owner


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## MedSailor (Mar 30, 2008)

The other solution to battens and mainsail furling is in-boom furling and horizontal battens. 

I appreciate all the explanations so far about roachy heads of mainsails. Now what about further down? Seems like if we wanted more sail area down low, we'd all have longer booms right? 

I'm thinking that there may be wisdom to the "first 2 battens full" and the rest partial, for a cruising main. 

MedSailor


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## Stumble (Feb 2, 2012)

Med,

A longer boom does two things. One helpful, one harmful. 

First it increases sail area, this is good.

Second it reduces aspect ratio, this is bad.

A good designer must balance these things out when designing the rig. As well as a lot of other stuff like: running backs instead of a backstay, mast weight, mast taper, expected loads, boom length and traveler placement, ect...


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## Brent Swain (Jan 16, 2012)

John Lecher , author of the self steering book and an aircraft engineer, said without full length battens a roach increases the performance of a mainsail from zero to three percent, but is responsible for 80% of sail repairs.


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## SchockT (May 21, 2012)

knuterikt said:


> What do you call this? Furling main with roach
> I'm no big fan of furling mains but that problem has a solution.


Do some of those battens go right from the boom to the leech? It looks like the ones in the HR pic does! That must be $1000 worth of battens! And what happens if your furling fails and you have to take the main down the old fashioned way? Battens that long would be pretty awkward. It seems like with the increased risk of the battens fouling inside the mast, and the disturbance the batten pockets cause to flow across the sail the benefit of the little bit of roach it gives you is hardly worth it.


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