Scott,

I’m on thin ice when it comes to deciding if a cloth is air-tight enough or not for using as a sail, so take this with a large pinch of salt. I read somewhere, some time, that there was a sort of standardised way og measuring this sort of airtightness, but I have forgotten where I read it.

Just remember, the average differential pressure in a sail is mostly well less than a MilliBar (hekto Pascal, hPa, these days), which is only 1.0cm water column. Only on small spots of the sail will the pressure ever reach 2-3 hPa. In a sail, 1.0 hPa would mean about 10kp/sqm sail area so a 20sqm sail would produce 200kp force with that little pressure. It takes some wind to achieve that, somwhere around 30kts (again, double check it).

My point is that we can easily blow a pressure that is 50 times higher than that wind pressure .

Therefore, if you need to use some force to blow through the cloth, and you are unable to just breath through it, it may well be useful as a sail. If it is cheap and easy to work with, it may be worth a try. Make a couple of bags of it first to see if it sews well with your machine.

Good luck,
Arne

I appreciate the spirited discussion on materials and sail construction. There is much to think about.

I got my order of POLYESTER RIPSTOP (DWR) - 2.6 OZ. I like the way it feels (for what that is worth) and it definitely sheds water very well. It seems less stretchy and stronger than other materials. I would say it is much less stretch than any nylon I have looked at and slightly less stretch than Odyssey.

Unfortunately, if I give it a little effort, I can blow air though it. It seems that a coating of Polyurethane or Acrylic or something to make the fabric air tight is a necessary feature. This fabric does not have a coating.

I will probably use this to make some bags. I can't imagine it is good to have air blowing through a sail.

It seems the supplier used straight scissors or a razor to cut the fabric. A hot knife or pinking shears would have been considerate. When I unfolded the fabric large parts of the weave came apart in a long thread.

Anonymous wrote:

Anonymous wrote:

I wouldn't use odyssey for a cruising boat. The photos attached show a sail made of odyssey3 in 2011 and under new ownership a year later.

Sorry, can't flip the 2011 picture right way up.

Downloading and zooming the pics I don't see any obvious tears in the sail. An alternative explanation for the difference in the pictures is: with wind and without wind.

Another explamation that's just ocurred to me for the poor appearance of my sail after a year is that the new owner has omitted the Hong Kong parrels. These proved necessary with that sail. Branwen's sails don't need them - a firm tug on the throat hauling parrels gives her sails a nice shape. I fear additional deterioration may have resulted from the sail being left uncoated, exposed to sun and city air, in her Cardiff marina berth. 

Forget about western sailmaking in junkrigs

David,
Well, then I guess it is my turn to repeat that one should sort of un-learn western sailmaking when making a cambered panels junksail. The reason is that the load in the fabric is an order of magnitude lower than in western sails of the same sail area. This it had to be on the original Chinese sails, since the Chinese until recently did not have access to strong sail-material. To them the boltrope was probably not primarily to keep a shape in the sail, but to avoid ripping and destroying the whole thing.

But let’s say that I have misunderstood this. How come then that my sails don’t get distorted after several years of sailing? How can it be that Johanna was just as close-winded the day I sold her in 2014 as during the first sails in 2003? How can it be that her helm balance did not change over time as a result of the camber moving aft? And how can it be that my sails last quite well?
Could it be that my sails, with their stout webbing boltropes, and no fancy sailmaker craftsmanship in them, are good after all? I think so.
Practical sailing experience beats theories and words.

 

Arne

Johanna in 2014 with her new owner

Asmat,

Your photos clearly illustrate why I do not use, and strongly advocate against using  boltrope or webbing on the leech of a JR sail. When that sail was new, it is clear that the boltrope was just a little too tight, and the leech is hooking, just a little. When the sail has aged, the rope has stretched, and is no longer supporting the cloth, which has stretched and broken down. When two materials have different stretch characteristics, which then change as they age, it is impossible to get them to work together. Seatbelt webbing simply cannot be matched with common sailmaking cloths. This is not sailmaking; it is canvas work, where the finished item is rarely under the conditions of being tensioned and expected to take a pleasingly fair and well defined shape. I do not accept for one moment that a 3D JR sail panel needs this kind of canvasworker's approach, that Arne defends so eagerly.

I will say again, for what feels like the umpteenth time: the right way to construct a leech is the conventional sailmaker's way: firstly, to lay the cloths that make up the panel either parallel or perpendicular to the edge, within 5˚, and secondly, to finish the edge with a sailmaker's tabling about 50mm wide, either rolled (like a garment hem, turned in twice), or a separate piece of cloth, threadline parallel to that in the cloth of the panel, folded in half and sewn on over the edge). That is all that is needed, but a doubler, 200 - 300mm wide, added to the edge before the tabling, mitigates against breakdown of the cloth in the long term.

In the days of sailmaking in natural fibre, it was a very skilled job to rope a sail. The boltrope had to be just a little bit tight on the luff and foot (and the head of a gaff sail), but that was OK because the tension, and therefore the stretch, on those edges was adjusted with the halyard and outhauls. Boltrope was never applied to the leech, for the reasons given in my first paragraph (except for storm sails, when set was secondary to overall strength). In a JR sail, we do not have control over the tension in the luff and leech, so we cannot control stretch.

Just because traditional Chinese sailmakers used a boltrope does not make it right. A look at old photos of junks rarely impresses one with the set of the sails. Did they look at Western sailmaking practice and copy it without understanding? I don't know, but I suspect so. Their sails used to be made from rattan, but when they started using western canvas, their old methods would not apply. So, what to copy? The visiting tea clippers etc.