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Old 15-11-2012, 06:54   #16
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Re: Square Top Main vs Roachy Main

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Lord of the bleeding obvious. even fox news types can rehash formulae.

Technically (and i can tell you love being technical) the interpretation of a bruce number IS CONTINGENT ON A SAIL SHAPE ie the indicated performance from the bruce number is going to be based on some basic sail shape for the main.
I believe there is some miscommunication at work here.

Bash is referring to "Sail Shape" as the shape of the curved foil which is produced with the introduction of the wind. The shape of the sail when in use, viewed from above the mast.

Not the shape of the sail, when laying flat on the loft floor.

The effectiveness of a sail is based on how aerodynamic the sail is "shaped", not it's size. If you take two identical sails, and make one more effective, you get higher performance from it.

He is saying because of the square top, or roachy cut, the effectiveness of the sail has improved 10%. I can see that. It's general aerodynamics.

It's not always about 2 dimensional shape, the 3rd dimension is the secret.

James L
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Old 15-11-2012, 07:58   #17
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Re: Square Top Main vs Roachy Main

Throw in the "tip vortex" factor for the 2 shapes and see the comparison.
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Old 15-11-2012, 08:03   #18
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Re: Square Top Main vs Roachy Main

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Throw in the "tip vortex" factor for the 2 shapes and see the comparison.
Well, to be honest, you may not have much of a "tip vortex" for a standard main sail. You would probably get more mast turbulence than "tip vortex", at the top anyway.

With a roachy main or square top, the tuning of the sail is important, so knowing how to effectively tune the sail does play a part in the sails performance. Most who buy such a sail, do the research to know the different tuning to make the most of the sail.

James L
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Old 15-11-2012, 13:29   #19
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Re: Square Top Main vs Roachy Main

Seems to me that just a short time ago the "hot setup" for foils (above and below the water) was to get as close to an elliptical planform as possible. I believe that this had to do with vortex shedding and its effects on performance. A sail with a lot of roach can come closer to that shape than a square top, I think.

Balancing the added area against potential improvements in lift:drag ratios is well beyond my skill set (and I suspect most of the posters here), so it would be interesting to get some knowledgeable inputs. Any aerodynamicists present?

And of course the impracticality of squaretops for a cruising boat are (IMO) far more important than a minor performance improvement when reality is included in the equation!

Cheers,

Jim
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Old 15-11-2012, 13:53   #20
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Re: Square Top Main vs Roachy Main

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Seems to me that just a short time ago the "hot setup" for foils (above and below the water) was to get as close to an elliptical planform as possible. I believe that this had to do with vortex shedding and its effects on performance. A sail with a lot of roach can come closer to that shape than a square top, I think.

Balancing the added area against potential improvements in lift:drag ratios is well beyond my skill set (and I suspect most of the posters here), so it would be interesting to get some knowledgeable inputs. Any aerodynamicists present?

And of course the impracticality of squaretops for a cruising boat are (IMO) far more important than a minor performance improvement when reality is included in the equation!

Cheers,

Jim
While not an Aeronautical engineer. I do have a good understanding of aerodynamics (Military taught).

Elliptical tips are a reduction of drag with respect to speed, not just drag itself. The faster a wing travels through the relative wind, the higher the drag the tip votex creates.

In World War II this was helped by reducing the tip vortex from the end of the wing with an elliptical tip. The elliptical tip tends to break the wake (or vortex) effect into smaller pieces to reduce the drag.

This also happens in water, but at a much faster rate due to the density ratio of water to air (33 to 1).

Now, I'm not positive, since this is somewhat different. The engineering is sort of backwards. I do not think normal sailing vessels tip vortexes would create much drag due to their relative wind speed. I would think there would be some point where the top vortex would start creating a point of negative return. I just don't think anyone would be brave enough to find it (above 100 mph).

Now......if your talking about a sailing hydrofoils.....that is a little different. They can reach some tremendous speeds with relationship to the wind.

I am under the impression, if under 100 mph, the tip vortex is minor at most.

I will reiterate, I'm not an aeronautical engineer.

James L
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Old 15-11-2012, 14:11   #21
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Re: Square Top Main vs Roachy Main

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That shows it nicely markpierce on the seawind 1160 you pictured - not a lot of area gained from this roachy main to a squaretop.

Good point Bash on the efficiency gained but just look at the seawind main and tell me it can really be a 10% gain in performance is ive heard claimed. I mean in light winds thats gonna take me from 6 knots to over 6.5 and thats a pretty big gain from what looks like just a few square feet of canvas....
If you draw s traight line from the head to the clew it's a huge gain...? am I missing something?
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Old 15-11-2012, 14:31   #22
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Re: Square Top Main vs Roachy Main

From the accumulated wisdom archives of Micah's Plagiarisation Project:

WB-Sails Ltd
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Old 15-11-2012, 14:34   #23
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Re: Square Top Main vs Roachy Main

I race on a very high performance 46 footer. All carbon hull, mast, rigging, ect. The difference between the pin-top beer can main and the square top racing main is between 1/2 and a full knot depending on the wind conditions and point of sail. This is not your average cruising mono though but we have real world data to show difference between to the 2 sails on the same boat.
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Old 15-11-2012, 15:08   #24
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Re: Square Top Main vs Roachy Main

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Originally Posted by Cheechako View Post
If you draw s traight line from the head to the clew it's a huge gain...? am I missing something?
I think it was supposed to be compared no a "normal" roachy catamaran sail. Like the title says, I guess.
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Old 15-11-2012, 15:31   #25
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Re: Square Top Main vs Roachy Main

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I think it was supposed to be compared no a "normal" roachy catamaran sail. Like the title says, I guess.

Spot on. ie I am trying to better understand if there is any significant benefit between an already high roached main as found on most cats already due to no backstay(seawind pic case in point) vs going a bit further to square top.

I was told by one sailmaker they can make make up to 10% difference to performance but that seems a bit of a stretch. ie per Thank you dads post moving up from a pin top on a mono does make a big difference to a performance racer (maybe even 10%) but how about a cruising cat thats already got a heap of roach?

Good point on the potential stacking issues too but I presume thats been solved as like i said a heap of the new production cats not bound for charter are coming standard with square tops now. Is it mainly just for show as suggested by an earlier poster?

thanks
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Old 15-11-2012, 16:09   #26
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Re: Square Top Main vs Roachy Main

it seems with the sq top main you could get rid of much of the roach and related battens and issues on a cruising boat with about the same sail area.... I've always wondered what possible good that top 3 ft of sail with the poor shape hidden behind the mast could possibly be good for.....
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Old 15-11-2012, 16:22   #27
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Re: Square Top Main vs Roachy Main

I would think the 'square-top' (as many ugly nautical things ... IOR) is the result of some rule. Maybe a racing rule punishing mast height more than sail area. Or on a cat some capsize safety rule. Or perhaps on productions cats just maximizing sail area on what tend to be undersized sail plans ... for improved light air performance without extending the mast.
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Old 15-11-2012, 16:25   #28
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Re: Square Top Main vs Roachy Main

The bahamas boats competing in Raceweek sure fly with them!
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Old 15-11-2012, 16:34   #29
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Re: Square Top Main vs Roachy Main

The REAL Bermuda rig, not the Marconi, was referred to as the "leg o' mutton" because of its large headboard, which looked very much like the "new"shape.
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Old 15-11-2012, 16:43   #30
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Re: Square Top Main vs Roachy Main

I look to the newest technology...Open 60's, Americas Cup boats that all have the square head sails cause they are not cruising. Most racing boats have loose footed sails now too that is like sailing a jib...not a main.
Racing is a gazillion dollar engineering thing that require lots of research that makes them fast. I don't think cruising and racing have much in common.
The analogy of a vortex in airplanes is for much greater speeds. We don't get that cause we have to reef way before getting there.
Having sail up top makes great sense as that is where the best wind is at and probably makes the difference for the racers...Not to mention, most jets have square wingtips these days.
I would go for a square shaped top of the sail on my boat but would get another reef point put in to get the head of the sail down where it would work. Maybe 4 reefs...but that's another thread I guess.
I gotta love the people who design the fast racing machines who let us see what makes a boat really fast. Mine is a cruiser.
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