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Old 04-12-2012, 13:33   #1
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Mainsail Shape for Bluewater Cruising

We are about to order a new main sail for our cat. It should be made of Hydranet and last for a circumnavigation.
The sailmaker now offers two options for the shape, both have the same sail area. One more modern, wider in the top and one more conventional. We have been told that both would be quite easy to reef.
I am not sure about the advantages and disadvantages- what do you think?
The boat will be sailed by the two of us with two kids on board. So easy handling is important. Please look at the pics.
Any comments?
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Old 04-12-2012, 13:49   #2
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Re: Mainsail Shape for Bluewater Cruising

I would keep it simple. There really is no benefit to radial design in this case and I would not like to continually worry about supporting a fat head, especially in a breeze. I would choose the elliptical.
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Old 04-12-2012, 14:29   #3
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Re: Mainsail Shape for Bluewater Cruising

A fat-head sail is supposed to depower by itself in strong gusts, reducing the need for manual adjustments, provided that the uppermost batten has the right stiffness (sometimes, the sailmaker makes an error here).

Of course, it isn't compatible with a permanently tensioned backstay...

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Old 04-12-2012, 14:38   #4
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Re: Mainsail Shape for Bluewater Cruising

neither of the two as you will be predominately down wind on a circum nav,with the main against the shrouds,
either of those sails the tendancy is for the battens to bend against the rigging,a better sail would be one with no roach,and cut as flat as possable.

spend your money on two good head sails .
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Old 04-12-2012, 14:56   #5
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Re: Mainsail Shape for Bluewater Cruising

Did I say keep it simple? It's a cat lot's of reaching downwind. It's all about reefing and full battens are heavy. Maybe two full upper battens a two half battens lower. Lot's of roach up top to support. Maybe less roach too if one concerned about batten length. Who is your sailmaker? How many crew?
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Old 04-12-2012, 16:01   #6
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Re: Mainsail Shape for Bluewater Cruising

Each shape is OK. The square one will have an extra load at the top batten that is supporting most of loading from the leech. The rounded one will be more evenly loaded but it will be more prone to leech deformation over time.

Personally, I would go with a cross-cut, moderate roach sail (less roach than in any of the images). I would make up for the 'lost' SA with a rollable kite.

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Old 04-12-2012, 23:10   #7
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Re: Mainsail Shape for Bluewater Cruising

I actually would go with the crosscut square top. I like the extra power it develops, and when reefing a square top generates less helm, since the CE stays closer to the designed CE compared to a more traditional shape.

I also like the full batons, particularly on cats. The extra power while reaching is really nice, and the self depose ring in gusts is great on multihulls where the boat can't heel over to depower.

Finally, because the loads are higher on a multihull I would absolutely go with a tri-radial. The better load bearing characteristics of the tri-radial will really help in long term durability. This is much more of a performance option, but if you are the type that cares about decent sail shape it will really make a difference.
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Old 05-12-2012, 06:14   #8
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Re: Mainsail Shape for Bluewater Cruising

Thank you for your inputs! They are asking 14900 USD for a triradial version with 82 m2 (882 ft2).
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Old 05-12-2012, 06:51   #9
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Re: Mainsail Shape for Bluewater Cruising

You contradict yourself. First you choose a cross cut, then absolutely go with a radial. You choose a square top for power then explain away it's limitation, in this application, it's questionable de-powering aspect.

Lastly, I see no evidence of a square top keeping the CE properly positioned over a conventional elliptical on the same mast. Perhaps in a more highly engineered low stretch fabric to prevent stretch and CE creeping aft. Would not one prefer less sail aloft when deeply reefed off one of the capes? I know I would. The incremental benefit of square top "in this application" would not be worth the worry or the expense.
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Old 05-12-2012, 07:32   #10
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Re: Mainsail Shape for Bluewater Cruising

Quote:
Originally Posted by Tar34 View Post
Would not one prefer less sail aloft when deeply reefed off one of the capes? I know I would.
This sounds quite convincing!
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Old 05-12-2012, 08:57   #11
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Re: Mainsail Shape for Bluewater Cruising

I would go with the tri-radial design over panels.
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Old 05-12-2012, 14:21   #12
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Re: Mainsail Shape for Bluewater Cruising

Tar,

You are right, I misspoke (mis-typed?) my fingers clearly out typed my brain.

To be clear. I would go with a tri-radial square top.

On great sailing weather a square top generates more power than a pin head main, for a number of reasons, but mostly because it has less drag, and a better air foil shape.

As for reefing. When a pin head is reefed, not only do you decrease the sail area, you also move the CE of the sail forward. This occurs because the foot length decreases relatively fast. This can cause a boat to develop significant lee helm, particularly if the CE of the normal sail plan is relatively far back.

On the other hand when you reef a square top main the CE stays mostly the same, because the foot stays almost the same, you just reduce the hoist height. This keeps the CE in roughly the same place on the boat, regardless of the amount of sail that is projected.

A good way to see this visually is to take a piece of paper and fold it in half so you have a piece that is 4"x11" and fold another so you have a triangle with one side 11". Place them on top of each other. Now cover up the same amount of area from the bottom of them. What you will see is that the triangular shape looses a lot of area from the back first.

So in my estimation, a square top is a better performer in all situations. The only place where it is a problem is dealing with a backstay (which may be the deciding factor), since a square top main cant be tacked easily thru a normal backstay.

This is exacally the reason why all offshore racing classes use square top sails. It isn't really to maximize sail area (they are pretty much all limited in sail area anyway) but to back the boats perform better, and more controllably with what they have.
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Old 05-12-2012, 18:56   #13
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Re: Mainsail Shape for Bluewater Cruising

Stumble

What you describe is correct. For the most part. What we have been presented with is a static rig. There is no mention of rig changes. A flat top main will increase sail area and setting it on the existing spar will actually cause the CE to be higher on the sail plan. How this affects stability is something for the designers. Shortening the boom might be the easiest fix. That being said, aesthetically, I prefer the radial. If going radial, I assume stepped construction and a benefit of lighter low stretch fabrics. I like the rugged taffetas. The cross cut was the option shown with the elliptical. I'm always on a budget and the thought of a well constructed cross cut adequate for my purposes. I am not convinced full battens might not be a liability, but would opt for uppers to support the roach. And half battens lower to decrease weight and ease of handling. But I'm not the one circumnavigating. We are, for the most part arguing nuances. I'm thinking simplicity and easy repairs. A busted full batten could compromise the sail over a period of time, especially off the wind when the roach or flat top is loaded. I do respect your opinion though.
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Old 05-12-2012, 20:19   #14
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Re: Mainsail Shape for Bluewater Cruising

I'd go for whichever one is least likely to break. Lets face it, you aren't (I assume) trying to break any records. Losing 1/4 of a knot of speed is less important than breaking the main... So longevity is more important that squeezing the last iota of power out of your sail.
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Old 05-12-2012, 20:47   #15
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Re: Mainsail Shape for Bluewater Cruising

That's a good point. If something goes wrong with that fancy (highly loaded) square top during the circumnavigation the mainsail will become useless. Away from a the few big sailing centers sail maintenance reverts to 1950's technology. Do catamaran circumnavigators carry spare mainsails?
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