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ChrisI View Drop Down
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    Posted: 07 Jun 22 at 3:17pm
Originally posted by The Q


Some of the punts have gone square top, not sure if they've found an advantage yet. We'll see if more go that way.


Square tops are in a sense just a modern interpretation of the traditional gaff rig.

But is it only carbon masts that they work with, that can flex and produce the desired 'gust-response'.... or can they work with aluminium/much stiffer spars?
Or could batten technology ensure that they do? (spring loaded battens!)......





Edited by ChrisI - 07 Jun 22 at 3:19pm
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Post Options Post Options   Quote Mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 3:43pm
I actually think square tops are easier to get to work with a solid mast, like those used by cats rather than a whippy carbon mast seen in some other classes (skiffs). So I think they can be made to work with a pretty solid mast which is designed with very little compliance. 

Cats go for rotating masts which are very stiff and need less in the way of stays to keep them upright. Then use sheet tension, Cunningham and battens to control the stability of sail (response to gusts). I think the use of carbon masts in cats is less about bend characteristics and more about weight. If you're using a stiff mast you don't have to rely on stays to keep it upright so much. And if you don't have to rely on stays you don't have bake in rig setting with stay tension and spreader length and deflection before going afloat. The penalty is having to rotate the tree trunk of a rig each tack. 

Trapeze skiffs with square tops don't have rotating rigs, so it pays to keep the mast diameter down. The downside of this is you need to rely on spreaders to keep the mast where you want it. It means the whole system is very complex and it's hard to get the control on that 4th corner so can mean more tuning on land.... but removes the faff of tacking the rotating the mast on the water. 

Personally, I don't want the tip of my mast for my square top bending dropping off to leeward in gusts at all. If the mast drops to leeward that is affectively sheeting you on. What you want is for the the fourth corner to open up or even the top batten to pop inverted. Getting that to happen is more a play with batten tension, batten stiffness, cunningham and sheet tension / kicker. 

Edited by Mozzy - 07 Jun 22 at 3:49pm
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Post Options Post Options   Quote eric_c Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 3:58pm
I think 'gust response' can happen without a square top sail.
Anything with a lot of roach will surely de-power as the wind loads it, bends the mast more and the leach falls away?
Doesn't tthst happen a little with a triangular sail, except it's the middle of the leach which falls away, not the top where it's wanted?

Cynically, I think the move to square top rigs is 90% about 'looking modern' by apeing the skiffs.
The skiffs I think partly use square tops for the same reasons Merlins have 4-sided sails, it's the most area within class rules and more of it is up where the wind is?

There's also an element that lots of sail development is done in the big money 18ft skiffs, a lot of knowledge gained by direct comparison in racing. So probably a class looking to tap into the latest thought on sail design will do well to tap into that and follow the skiffs?

Probably true that it's cheaper and easier to tune battens than masts?

Carbon masts can be made with all sorts of characteristics, which is less easy to achieve by tapering extruded ali tube. The carbon is also lighter so maybe springs back quicker for a given 'spring force' and the characteristics of the resin perhaps affect the damping?

Don't forget there is a lot of history with glass tipped masts in the B14 and AFAIK Finn and maybe OK? Likewise some catamarans have used carbon battens as well as glass.

For a cheapskate club sailor like me, I think some of these sails have a lot of stress in them, resulting in shorter usable life than you get from a more traditional roached sail?

And don't forget, the elliptical plaform is supposed to be most efficient?
(Blah Spitfire Wing blah......)

I'd be interested to know more about it, not sure what's been pubished since Bethwaite?
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Post Options Post Options   Quote eric_c Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 4:06pm
Mozzy makes some good points.


 
I'd  add that cats developed their rigs mostly without the masthead asy's that need a lot of stays to resist the forward halyard tension.
The upper shrouds on a skiff rig should be making the mast tip move back and not to leeward?
These issues make a lot of pre-bend in the upper mast a sound approach?
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Post Options Post Options   Quote ChrisI Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 4:15pm
Originally posted by Mozzy


I actually think square tops are easier to get to work with a solid mast, like those used by cats rather than a whippy carbon mast seen in some other classes (skiffs). So I think they can be made to work with a pretty solid mast which is designed with very little compliance.Cats go for rotating masts which are very stiff and need less in the way of stays to keep them upright. Then use sheet tension, Cunningham and battens to control the stability of sail (response to gusts).


But doesn't the 'gust response' you get in a cat with a stiff aluminium mast come from the fact there is no kicker tensioning the leach i.e. the gust hits and the mainsail clew rises slightly (and comes inboard slightly) opening the top of the sail...?

Edited by ChrisI - 07 Jun 22 at 4:16pm
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Post Options Post Options   Quote eric_c Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 4:15pm
Weight aloft is also worth thinking about, if you can put the same sail area mostly as high up with a fat head sail on a shorter mast, that is a big saving in pitching moment, which will help perforamce in choppy sea.
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Post Options Post Options   Quote eric_c Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 4:22pm
Originally posted by ChrisI

Originally posted by Mozzy


I actually think square tops are easier to get to work with a solid mast, like those used by cats rather than a whippy carbon mast seen in some other classes (skiffs). So I think they can be made to work with a pretty solid mast which is designed with very little compliance. Cats go for rotating masts which are very stiff and need less in the way of stays to keep them upright. Then use sheet tension, Cunningham and battens to control the stability of sail (response to gusts). 


But doesn't the 'gust response' you get in a cat with a stiff aluminium mast come from the fact there is no kicker tensioning the leach i.e. the gust hits and the mainsail clew rises slightly (and comes inboard slightly) opening the top of the sail...?


If you're thinking about e.g. Dart 18, there is no kicker, but high leach tension with a 6:1 main sheet on a traveller? Easing sheet is more like easing the vang on a dinghy?
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Post Options Post Options   Quote ChrisI Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 4:26pm
There's no question that for us a square top rather than a pin head is not fashion but a key part of the design - in light wind conditions and esp when tide and wind are from the same direction we go forwards when other boats are going backwards.
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Post Options Post Options   Quote eric_c Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 5:31pm
Originally posted by ChrisI

There's no question that for us a square top rather than a pin head is not fashion but a key part of the design - in light wind conditions and esp when tide and wind are from the same direction we go forwards when other boats are going backwards.


That's just area high up though?
You can think of a square top as either a triangular sail with roach added by a long top batten, or as a triangle with the top cut off.

You could get the same light air performance with a taller mast and more triangular sail?

It's all comparative and with a SMOD you don't really have a sensible framework to say square top is 'better'.  Better performace per sq ft of sail? Better performance within the limits of mast length? Better performance within a budget?                                                                         
                          

In a development class, we're talking about best performance within an arbitrary set of rules which 'tax' roach to arbitrary degrees.                                                    
                          

So what baseline do we use when evaluating a square top's all round performance?                                                                  

Presumably what we want is a rig which keeps the heeling moment constant as the wind increases above the design point of the crew hiking/trapezing to the max? Is that more about the squareness of the top or the characteristics of the mast etc? Can an elliptical sail ever be as good in this respect?                       
          
 How do we model this when designing a rig? How many rigs are actually rigorously designed and how many are just a case of sketching a sail and see how it goes?
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Post Options Post Options   Quote H2 Quote  Post ReplyReply Direct Link To This Post Posted: 07 Jun 22 at 7:28pm
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!

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