Square Top Rigs on 18's
Printed From: Yachts and Yachting Online
Category: Dinghy classes
Forum Name: Dinghy development
Forum Discription: The latest moves in the dinghy market
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=493
Printed Date: 27 Jun 25 at 12:13am
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com
Topic: Square Top Rigs on 18's
Posted By: Granite
Subject: Square Top Rigs on 18's
Date Posted: 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? ------------- If it doesn't break it's too heavy; if it does it wasn't built right |
Replies:
Posted By: redback
Date Posted: 25 Feb 05 at 1:57pm
| More sail area for same length of mast? |
Posted By: JimC
Date 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. |
Posted By: Granite
Date Posted: 25 Feb 05 at 2:12pm
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Have a look at this picture http://www.yachtsandyachting.com/default2.asp?section=64&PID=5719"> (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 ------------- If it doesn't break it's too heavy; if it does it wasn't built right |
Posted By: redback
Date Posted: 25 Feb 05 at 4:18pm
| 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. |
Posted By: hurricane
Date Posted: 25 Feb 05 at 6:37pm
| tornado's and f18's have them!!! |
Posted By: Twin Poles
Date 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. ------------- |
Posted By: Chris 249
Date 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. |
Posted By: JimC
Date 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. |
Posted By: Chris 249
Date Posted: 26 Feb 05 at 10:49am
| 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. |
Posted By: Granite
Date Posted: 01 Mar 05 at 1:27pm
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That would seem to be the case in the 12' inspired C-Tech/Fife rigs that have been appearing on some cherubs in the UK, Jim C can correct me but some of them seem to be set up with the main shrouds not connected to the lower spreaders and with the caps being static bend controll on the masts. With the Kicker and Cuningham bending the mast while sailing. ------------- If it doesn't break it's too heavy; if it does it wasn't built right |
Posted By: JimC
Date Posted: 01 Mar 05 at 5:08pm
| Mine is set up with caps and main shrouds both through lower spreaders and locked in place by the checks to the spreader roots. Both caps and main shrouds are bending the lower mast forward, only the checks bring it back straight. I suppose you could adjust the checks to control prebend statically but I've never felt the need. Because I've got a mast stump the kicker doesn't try and put bend into the lower part of the mast. |
Posted By: Blobby
Date Posted: 02 Mar 05 at 12:32am
Is this not just a reaction to the increase in roach and the really fat-headed sails that are being cut now? These must put more load on the tip of the mast meaning a relatively stiffer section will still provide the same gust response as the more flexible tips do when coupled with more pointy headed sails? |
Posted By: eric_c
Date Posted: 07 Jun 22 at 5:39pm
| Bump! |
Posted By: 423zero
Date Posted: 07 Jun 22 at 6:54pm
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Bump? ------------- Robert |
Posted By: eric_c
Date Posted: 07 Jun 22 at 7:04pm
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I thought it was useful info (and opinion) in the context of today's other thread. |
Posted By: 423zero
Date Posted: 07 Jun 22 at 7:10pm
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Got it, read both pages before I noticed date. ------------- Robert |
Posted By: ChrisI
Date Posted: 08 Jun 22 at 10:53am
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Interesting thanks.
JimCs comment suggest that if we used fully battened that some 'gust response' could be designed into the top part of the sail more easily. Unfortunately fully battened might be unmanageable for us for rigging/launching/large wind changes. |
Posted By: eric_c
Date Posted: 08 Jun 22 at 11:29am
I've never noticed fully battened being much worse than anything else for rigging and launching.
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