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eric_c View Drop Down
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Posts: 382 		Post Options Post Options   Quote eric_c Quote  Post ReplyReply Direct Link To This Post Topic: Square tops
    Posted: 07 Jun 22 at 7:29pm
See the old thread I bumped.
I also found a thread about the re-hash of the RS400 rig, when they tried a square top main and rowed back to just changing the cloth:    

 




https://www.yachtsandyachting.com/forum/forum_posts.asp?TID=7237&PN=4&title=rs400-proposed-changes



    
Divided opinion. some were very positive about the square top, but as I recall, it looked very nice but got spanked by Merlins around the cans?
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Posts: 382 		Post Options Post Options   Quote eric_c Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 7:33pm
Originally posted by H2

Never had a square top before my H2 which also has a bendy carbon mast, very easy to make work in medium and strong winds but has taken us time as a class to make the rig work really well in light winds if that helps!


What are the symptoms of it 'not working well' in light winds?
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Posts: 750 		Post Options Post Options   Quote H2 Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 7:42pm
Originally posted by eric_c

Originally posted by H2

Never had a square top before my H2 which also has a bendy carbon mast, very easy to make work in medium and strong winds but has taken us time as a class to make the rig work really well in light winds if that helps!


What are the symptoms of it 'not working well' in light winds?

The boat feels "sticky" in light winds, essentially its easy to stall the rig in light winds. We have had to learn how to flatten the fully battened sail so that the flow remains attached. This has been achieved by reducing rig tension to straighten the mast whilst also experimenting with kicker to control the roach twist. Put another way - when the H2 first came along it seemed like we could sail to our PY in medium or heavy winds but when it went light we got a total kicking in PY racing but after several years of trial and error we have found some new "gears" that keep the boat powered up as the breeze falls. Hope that helps?
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turnturtle View Drop Down
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Posts: 2538 		Post Options Post Options   Quote turnturtle Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 7:59pm
I always understood square tops were a development class thing to maximise sail area with a constraint on spar length in the class rules

If a new one design class has been developed with one, then it probably has more to do with aesthetics and commercialisation than good sail design

More than happy to be corrected if this isn’t true!!

Edited by turnturtle - 07 Jun 22 at 8:00pm
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eric_c View Drop Down
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Posts: 382 		Post Options Post Options   Quote eric_c Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 8:17pm
Re H2



Yes that's helpful.
Maybe that's general to sails with a lot of roach rather than being specific to flat-head, I've seen a few sails which are hard to make sense of when the wind gets too light.
Some boats you can leave the kicker off  in light air because the sheet gives the right blend of pulling the boom across and leach tension.
A lot of factors than can't be isolated from one another, too many variables.


Edited by eric_c - 07 Jun 22 at 8:22pm
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ohFFsake View Drop Down
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Posts: 219 		Post Options Post Options   Quote ohFFsake Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 9:33pm
If you watch the (truly excellent) 18 footers tv coverage on youtube, and pick a windy race you'll see plenty of examples of what they jokingly refer to as "flappage".

This is the top section of the fully battened mainsail completely inverting in the gusts, so the battens more or less pop the wrong way.

This is probably the clearest demonstration you will ever see of automatic gust response, and explains at a glance the whole reasoning behind square top sails.  In the gusts the bottom half of the sail is providing drive with leach tension giving good pointing, whilst the top of the sail is completely feathered for minimal drag and heeling moment.

Edited by ohFFsake - 07 Jun 22 at 9:36pm
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JimC View Drop Down
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Posts: 6661 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 9:47pm
Yes, one of the key things is the mid leech. If the mid leech doesn't stand up then the boat won't point. So in my dream world the top of the sail would flop off completely and the bottom 2/3 of the sail wouldn't move an inch. AFAIK that's pretty much unachievable, but with all full battens and stable cloth it goes a substantial way towards.
Overrotating rigs march to an utterly different drummer because the mast tip bends up to windward...
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Oatsandbeans View Drop Down
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Posts: 382 		Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jun 22 at 5:06am
I agree-the reason that the mid leach has such an effect on pointing is that it effects the upwash on the air hitting the jib luff. Having a mid leach close to the centre line makes the wind in front of the jib bend -in effect giving you a lift. If you ease the main and the mid leach moves away from the centre line you lose this.
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The Q View Drop Down
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Posts: 126 		Post Options Post Options   Quote The Q Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jun 22 at 6:28am
I think on the Norfolk Punt it's a case of getting sail area up high as in many areas of the broads we are badly affected by trees. 
Many of the Broads cruisers have jackyard tops'ls, some now have them permanently fitted, so their only reefing is at the bottom..

The Merlin has a Size limit (10sqm?)  so for them by moving as much area as they can up high, they should get stronger clearer winds.

Currently on my one off, I'm using an old Kestrel Sail (and mast), that has a long top batten, a sort of semi square top. Even with just that it sometimes decides to stay curved the wrong way, needing a sharp tug on the main to "pop" it. 

When  the new sail made I was thinking of going square top, but my one off is very round bilged with little form stability and also I need to move the sail area aft a bit. So now my current thinking is she needs a longer boom and just go for a conventional top.. But there's another couple months trials needed before a final decision needs to be made.
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H2 View Drop Down
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Posts: 750 		Post Options Post Options   Quote H2 Quote  Post ReplyReply Direct Link To This Post Posted: 08 Jun 22 at 8:31am
Originally posted by turnturtle

I always understood square tops were a development class thing to maximise sail area with a constraint on spar length in the class rules

If a new one design class has been developed with one, then it probably has more to do with aesthetics and commercialisation than good sail design

More than happy to be corrected if this isn’t true!!

The H2 was a new class designed with a square top from the outset. The rig was designed by Jim Hunt from HD sails and not a man I would use the words "aesthetics or commercialisation" in connection! Of course it is possible that you are a respected founder of a sail loft and multiple world champion TT - I know I am not - but I reckon Jim designed a rig that he felt would work well rather than look pretty.
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