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Square Top Rigs on 18's |
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Granite 
Far too distracted from work 
Joined: 12 May 04 Location: Scotland Online Status: Offline Posts: 476 |
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 Topic: Square Top Rigs on 18'sPosted: 25 Feb 05 at 1:54pm |
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I was looking through the pictures of the 18's at the JJ Giltinan (on Y&Y site) and I noticed that some of the boats had Square Top mainsails. I thought that the eliptical shape was best for efficiency and that the reason for the square tops in other classes was to squeze in area where it is not measured. As the 18's have unlimited sail area this would not be the case. So why do the 18's have them? |
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If it doesn't break it's too heavy; if it does it wasn't built right
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redback
Really should get out more 
Joined: 16 Mar 04 Location: Tunbridge Wells Online Status: Offline Posts: 1502 |
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Posted: 25 Feb 05 at 1:57pm |
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More sail area for same length of mast?
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JimC
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Joined: 17 May 04 Location: United Kingdom Online Status: Offline Posts: 6661 |
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Posted: 25 Feb 05 at 2:06pm |
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Its much much more complicated than that! An elliptical load distribution is considered to give the lowest induced drag. However that isn't necessarilly the same as an elliptical profile sail, especially as the wind force isn't constant from bottom to top of the rig and also because the sail will normally be twisted.
In practice a rather fatter than elliptical head on the sail will more nearly approach an elliptical load distribution more of the time. So on most development boats where the profile isn't restricted you'll see over elliptiocal sails - 12foot skiffs and UK Cherubs are obvious examples. But apart from that you also have to consider gust reponse, and we're learning all the time that gust response is mega critical. On a fully battened sail (but not nearly so much if at all on a soft or semi soft sail) its possible to have loads of leech tension in the middle of the sail so the mid sail stands up firmly, but to have the upper leech really loose so it will twist on and off in the slightest wind change. Modern board sails are the easiest examples to look at. And this means that you can have plenty of area up top to respond to gusts but still have the mid leech standing up firm to the wind and not flooping off so you can still point. So all these are reasons why modern skiff rigs are tending to have a lot of area high up. |
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Granite
Far too distracted from work
Joined: 12 May 04 Location: Scotland Online Status: Offline Posts: 476 |
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Posted: 25 Feb 05 at 2:12pm |
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Have a look at this picture (Picture 16 on Y&Y slide show incase it does not show up) redback you could be right the square top looks slightly larger than the RMW one but the ammount of area you get with the square top could eisily be added spread over the full length of the leach without requiring a taller mast and still giving an eliptical shape. besides they could just have a taller mast if they want |
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If it doesn't break it's too heavy; if it does it wasn't built right
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redback
Really should get out more
Joined: 16 Mar 04 Location: Tunbridge Wells Online Status: Offline Posts: 1502 |
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Posted: 25 Feb 05 at 4:18pm |
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Yes I looked at 16. JimC is right I'm sure. Have a look at 16 again and you'll see that tha line drawn from the end of the boom to the top of the mast is where all the tension is. Quite a bit of the battens in the square top are aft of that line which means any wind pressure there has more leaverage to open the leech. So in the lighter breezes this contributes to drive but as the breeze increases the top part of the leech opens and provides less. The trick is to get the leech tension and batten stiffnes just right to match mast stiffness and crew weight. If you watch the modern Formula boards you'll see they have a similar rig, the top part of which is just limp in any sort of breeze, but as the breeze gets less you can see it make more contribution to drive. I guess that's how they manage to carry about 11sqm even in a 5.
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hurricane
Really should get out more
Joined: 15 Mar 04 Location: United Kingdom Online Status: Offline Posts: 1047 |
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Posted: 25 Feb 05 at 6:37pm |
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tornado's and f18's have them!!!
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Twin Poles
Newbie 
Joined: 06 Nov 04 Location: United Kingdom Online Status: Offline Posts: 33 |
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Posted: 25 Feb 05 at 7:04pm |
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Having a large roach that depowers itself in the gusts does make alot of sense, as the 18s are now limited to just 2 rigs (huge and enormous), so each rig has to cover a larger wind range than previously. |
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Chris 249
Really should get out more
Joined: 10 May 04 Online Status: Offline Posts: 2041 |
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Posted: 25 Feb 05 at 9:18pm |
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18 rigs are no longer unrestricted; maximum height is a wimpy 33'
7". Until '93/'94 the big rig was restricted to a poncey 37'.5' and the
#2 rig was 34.4'.
Of course, back in the 1980s when men were men, the 18's #1 rigs reached 13.2m (44') and wings spanned up to 31'. An interesting comment re twist and square tops comes from Steve Clark (Cogito C Class, A class, Int Canoe) who reckons that despite all the shouting about how effective their rigs are, what the skiffs prove is that flat and twisted is fast FOR UNLIMITED AREA. Once you limit the area (class rules, production limitations etc) deeper shapes are faster. Seems right to me, those super-floppy leach windsurfer sails are often slow FOR THEIR AREA. All the quizzing I've done to skiffies about their sails confirms what JimC says, BTW. The big head in board sails also, some say, reduces tip vortexes by sort of providing an area that lies along the apparent without "bending" it. Therefore the airflow over the windward and leeward sides of the upper leach get to re-align with each other and with the airflow above the masthead before they spill off the leach/head; it's the differences between the pressure of these areas that causes tip vortex and apparently the twist cures it. In a board, you can feel the effects quite dramatically but you lose a lot in low-down power in exchange for the efficiency at higher speeds. |
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JimC
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Joined: 17 May 04 Location: United Kingdom Online Status: Offline Posts: 6661 |
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Posted: 25 Feb 05 at 10:16pm |
Note that I'm not a sailmaker and I don't pretend to fully understand how this all works, but AIUI its more subtle even that that - if it were just the string to top mast load then you would be losing mid leech tension as soon as the top mast bent back. Because you have the load in the sail coming through the built in panel curvature and seam shaping the battens in the middle of the sail hold the leech up at mid height even with the top mast bending back. That's why again AIUI you won't get this effect from a part short batten sail, its the battens in the middle and bottom of the sail that make the mid leech stand up, the top ones are largely irrelevant to this. Don't forget the opening leech/mid leech bit happens pretty well on a 49er with a much more moderate roach than the modern development rigs. And again AIUI it its the same effect that's being sought for with the Byte CII rig. You want the top mast to be able to twist off and blade while the leech up as close to hounds height as possible stands firm so you can still point. |
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Chris 249
Really should get out more
Joined: 10 May 04 Online Status: Offline Posts: 2041 |
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Posted: 26 Feb 05 at 10:49am |
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Of interest (perhaps) AFAIK there's a swing back away from the bend
concentrated in the topmast in skiffs; 14s, 16s and 12s have
moved back towards more even bend I'm told. Dunno about 18s.
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