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Old 29-05-2009, 09:04   #1
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Vertical-Cut Panels in Sails

Hi all. Just wondering if any knowledgable person could give me the last word on panel cuts for sails. The last 20 words actually! We are about to order a set of sails for steel gaff rigged pinky schooner. The sail plan shows all sails cut with vertical panels. This is a low aspect rig on a heavy displacement hull. Would this make a difference to the cut of the sails? What are the advantages/ disadvantages over other cuts? Does modern dacron change the way a designer would approach his sail plan? I've noticed vertical cut on very old rigs and photos from the time of canvas. Any help would be appreciated. Thanks.
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Old 29-05-2009, 12:27   #2
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In the old times, sails had vertical seams for safety reasons: if a seam failed, it was still possible to keep control over the yard at the head of the sail. I've read that Nathanael Herreshoff switched to horizontal seams (with stronger thread) when he realised that vertical seams disturbed the airflow over the sails.

From your post, I don't understand if the gaff is made of steel, like on early 20th-century windjammers. In this case, it might be worthwhile to retain vertical seams, to resist the shock loads when gybing.

Alain
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Old 29-05-2009, 13:10   #3
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Thanks Hydra,
The gaffs themselves are alloy spars, the masts and booms are steel. Interesting about airflow over the seams, I guess it would make a bit of difference. So if one wasn't concerned about interrupted airflow, would vertical panels give one the strongest gaff sail?.. So, what about the jib?, what is the strongest panel configuration of a headsail? On the pinky the jib is not huge and has 2 reef points and a club boom.thanks again
Charlie
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Old 29-05-2009, 13:19   #4
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"Regular" Dacron stretches less across it's width than it does lengthwise. This is called the "Fill" direction. Most most Dacron sails are made with a cross cut panel layout (seams perpendicular to the leech) because it orients the cloth in the direction it will stretch least. If you order the sail "leech cut" like you are talking about, the sailmaker should use a "warp oriented" cloth (one that stretched less lengthwise). So it really comes down to making sure the sailmaker uses the best cloth cloth the layout.
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Old 29-05-2009, 22:49   #5
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Thanks Sail Warehouse for that info. But, what about the question. 'which panel configuration will give you the strongest sail possible'? Still a little confused. Thanks.
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Old 30-05-2009, 00:43   #6
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Strength isn't the only question. Stretch is the biggest question, because it determines the shape of the sail which, in turn, affects how much of the wind's force is caught, how much heel, and how much acceleration.

The sail's fibres need to be oriented to get the greatest resistance to stretch, which is usually roughly parallel to the leech of the sail. For most dacron fabric sails, the 'woof' or 'fill' direction is better able to resist stretch, so seams are laid out to be as close to perpendicular to the leech as possible.

Gaff sails, though, will have the stress aligned in the sail slightly differently. The greatest stretch will usually be paralleling a line from the throat to the clew - the same as if the sail were triangular - but that won't necessarily mean the seams will be striking the leech perpendicularly, especially if your peak is pretty low. (This is why it's best to have a gaff sail designed by a loft that has a reputation for their gaff sails, because the panel layout has some different design requirements.)

However, keep in mind that a gaff main's shape can be modified a lot more than a bermudan, with head, luff, and foot tensions and the ability to peak up. Each of these can change the direction of stress in the sail as well, altering the shape of the sail.

The reality is that modern high-tech fabrics are lower stretch and stronger than a traditional gaff rig will ever need plus gaff mainsails tend to built more heavily than equivalent thimble-headed mains.
Some traditional vertical cut sails are made after splitting panels to get twice as many vertical seams, but there's very little direct benefit beyond the aesthetics. Be wary of a heavy metal gaff crashing about aloft - the lightest gaff you can afford (hollow wood, carbon fiber) will really pay huge dividends in mainsail life if you sail in light airs; but you probably already know that if you're gaffer builder!

Head sails on a modern racer and on a pinky are the same: panels cut so seams strike the leech nearly perpendicularly, unless the sail is cut radially or star. There are many other variations - dutch cut, mitre cut, etc. - but they're becoming more rare as the benefit seldom is as great as the expense in calculating all the small panels, cutting them, and sewing them. (I don't think most of the unusual cuts, which are about controlling the shape of the foot of the headsail, reef as well as the plain cross-cut.) Most of the unusual cuts were designed for comparatively very flexible/unstable fabrics, and most have been superseded by computer developed panel layouts which align the panels and seams for minimal stretch and maximum strength and shape stability.
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Old 30-05-2009, 09:31   #7
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I agree with Amgine's synopsis. The question of what is the "strongest" really needs to be clarified. Stretch is probably more of the real issue. When you speak of strength it could mean a lot of things; flog resistance? tear resistance? chafe etc. Sail designers should match the panel layout to the Dacron type and the load map of the sail. Then use the appropriate corner reinforcing, chafe protection etc.

Many sail makers have multiple product tiers. If you want the "strongest" sail I recommend not over thinking the panel layout so much as going with the best "Offshore" product tier because it will have the heaviest corner reinforcing, chafe protection multiple rows of stitching etc.

If the esthetics are a big concern you might want to consider using either Tanbark or Creme colored Dacron for a classic look.

Cheers,
Jim
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Old 30-05-2009, 10:36   #8
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Thanks Amgine and Jim, very helpful. I realise now that many factors come into play when analysing a sail. Cheers.
Charlie
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