Polytarp lugsail 3D shaping

To continue PAR's advice, sailcut is a free program that you can get from here:
http://www.sailcut.com

The depth and location of max camber are nothing to do with darts or polytarp sails, but with any sail or wing. Books like "Aero-Hydrodynamics of Sailing" by Marchaj have extensive research on the best shape. 10% Camber is pretty standard, at around 40 to 50% of the chord length (for boats with a single sail). This is why the higher dart is smaller, to give the same 10% of the local chord at that location (which is a horizontal line, as the wind travels roughly horizontal across the sail).

I think the idea is that the sail will stretch around the dart, making it into a fairer curve. The one time I tried, it looked terrible!

I would try using sailcut, and keep in mind that polytarp is a woven material like dacron, so the orientation of the fibres is important.

 

There is a quote from somewhere that states "Golf is a game whose aim is to hit a very small ball into an even smaller hole, with weapons singularly ill-designed for the purpose."

There’s also a well known joke about a tourist in Ireland who asks one of the locals for directions to Dublin. The Irishman replies: "Well sir, if I were you, I wouldn't start from here".

A rag on a stick will get you sailing. However well the Polytarp is sliced & diced, its blue colour will still advertise "Polytarp".

In the end there can be only one. (Highlander)

http://www.polysail.com/article.htm

http://en.wikipedia.org/wiki/Polytarp_sails

http://forum.woodenboat.com/showthread.php?167770-Material-For-Making-White-Polytarp-Sail

http://www.pdracer.com/sail/make-polytarp-sail-1/

http://www.duckworksmagazine.com/04/s/articles/polysail/

http://www.thecheappages.com/oddsails.html

Just do it & have fun.

Perry

 

I see! Well, that explains why cotton sails went out of use.

Anyway, I've decided to use Tyvek instead of polytarp. I should rename the thread.

Well, that is one possible approach. Still, I've red that even properly made Dacron sails go out of shape in time. That implies that wind doesn't force the sail into correct shape; instead, it stretches it out in the wrong places. So I'm a bit afraid that with flat sail, I'll go straight from no shape to wrong shape.

Anyways, I could do this, but then again, my design calls for light wind performance, and several knots of wind won't be enough to force flat sail into any kind of shape, especially not the deep-drafted one, which works best in those conditions...

But thank you anyway for your input! The poetry stirs imagination

As I've mentioned several posts back, I have this program, but thanks anyway! It should be very hand if only I knew what shape I should aim for, except for what you already said. For example, this program asks to define bottom, middle and top profiles. There are a lot of options to tweak. Of course, I couldn't probably make a sail so precisely that any of such subtle things would matter, but I'd like not to add up errors in design AND actual sailmaking.

I'll look up the book. Any other material to read on shaping sail and transfering the shape to flattened panels without software (I want to know how it's done)?

Thanks for the links! I'll read through each.



An update. Since I still don't know how to properly shape a sail, neither how to transfer that shape into flat panels, I modeled my sail in Sailcut software, added draft of 9% at 35% chord length in the middle, 6% at 31% in the bottom, and 2% at .28 in the top (I have no idea if these numbers make any sense), and the software magically gave me flattened panels. I went for cross-cut layout, since it is the most popular nowadays, and I remember something in "Sailmaker's apprentice" that says this layout is one of the best (another alternative would be vertical cut, but I guess it is not a good thing having seams run perpendicular to wind flow).

Then I printed out these panels on A4, cut them out carefully, and joined edge by edge with sticky tape, because at this scale, it would be nearly impossible to join these edges any other way. It was a messy job, took me more than an hour, made some mistakes on the way, but the result is very promising, compared to my dart-shaping attempts.

Here is how it looks:





Although it ain't perfect, most of the irregularities in the shape are due to my clumsiness. It was a nightmare to cut those lines, as well as tape them.

Of course, it doesn't mean I would be able to do the same with Tyvek, sewn or taped.

So, I can go with fast approach, take some polytarp, stick up a flat sail with dart or two, and go sailing. Or, I can devote some time into learning, design a proper shape, and sew it with Tyvek like it's supposed to be.

The latter option is more attractive, but maybe it's not a good idea to over-complicate my first serious boat building project. I'm not sure yet.

 

One more question, and a plea, if I may. I am now heavily researching sail broadseaming. I know what shape I want, but I don't know how to translate that shape into broadseams. The only reference most people provide is "Sailmaker's Apprentice". It is a great book. And I have it. But there is a catch.
I got this book from my friend, in a PDF file, once copied from Google Books Preview (not available anymore). Sadly, there are a lot of pages missing, about 1/3 of the book. As fates would have it, the most important pages which deal with formulas of broadseaming are missing.

Being stationed in Europe, I have no chance to buy or find this book in libraries. The only option is to ship from US, which would cost trice as much as the book itself, even if from a second-hand source.

I am wondering if anybody here has this book in electronic format, and would be kind enough to share, or at least those missing pages (there are lots of them, but basically from page 250 to the end of the book). Or maybe a scan, if that wouldn't be too bold to ask..?

I am not entirely sure if what I'm asking is legal, but I can at least assure that my intentions are noble. I only wish to learn. And should my project be successful, I will make no attempts to make money from it. I shall release plans free of charge on the internet as a token of gratitude to people who helped me in the bumpy road of designing this little boat.

I know I'm asking a lot, but it is very, very important. PM me if needed. Thank you in advance

 

Its not legal, and writers of textbooks deserve to eat too.

 

Tyvek works alright, I've made several sails out of it and had decent results. The trick is you have to actually shape the sail or it's junk, as are most sails that are completely flat. Be sure to add reinforcement patches or grommets can pull out. The sail can't take too much strain, but it works well for lower-stress stuff like a lugsail. This is my boat:

 

Well, I figured since this book was freely available on Google Books (larger part of it), it has no copyright anymore...

Anyway, there is a practice of lending books to friends, and so on. If it were available in my country, I'd be the first to buy it, but now it's very inconvenient. I hope you understand the situation.

 

Found these three videos about using Trimaco Eliminator drop cloth for making sails. It's a tough butyl rubber backed, cross-woven canvas dust sheet, 9 feet x 12 feet.

http://www.keepturningleft.co.uk/blo...-plans-arrive/

I'd be interested in comments.

H/T Dylan Winter.

 

Your link doesn't open, could you please re-post it?

 

Sorry about that. Here is the Youtube link.

https://www.youtube.com/watch?v=NHdlrT3VPp4

Link to supplier in UK.

http://www.decoratingdirect.co.uk/viewprod/t/TRIMACO_ELIMINATOR_DROP_CLOTH_DUST_SHEET/

Even heavier.

http://www.decoratingdirect.co.uk/viewprod/t/TRIMACO_HEAVYWEIGHT_BUTYL_DROP_CLOTH/

 

Having 'played' around with some 3D modelled sails of real size for a fully battened una rig of around 8.4 Sq meters, the ammount of broadseam you need is pretty small, the luff curve is the main control. However the broadseam is very important in keeping the shape where you want it in the sail as the mast bends and you start to use the various controls too. Broadseam on this small sail is likely to be in the 1 to 4mm range, but the shape of that curve will matter ie wher max chord is etc.

Why not look at some of the Hitech building tarps with reinforcement laminated in? Some of these are strong enough for a decent sail and a step up from Tyvek and certainly just a polythene tarp. Yes, a little more expensive but much more stable. Cotton is OK, but rots too easily when wet, so you have to dry it almost every time. Dacron is OK especially properly heat rollered and set stuff.

With reference to PAE's comment about the noise of sails, my new seamless custom woven main is like rubbing a large crisp packet every time you tack... problem is everyone else can hear you - wish tere was a silent mode!

 

That seems like a very strong stuff. But also very expensive, compared to Tyvek. I definitely wouldn't be able to afford it...

I'm surprised people still say that broadseaming only helps locating the draft, while the draft itself is provided by luff curve after I specifically emphasize that I'm talking about lugsail. Every kind of lug sail must have it's luff straight or hollow. Foot is most often loose. Therefore, only head can provide any kind of curve, and it is not located in suitable position.

That is why almost all of of the draft must be provided by broadseaming in this particular case. This is why I'm dedicating so much time in researching it.

 

As I understand it, the rig requires luff curve in the top - you call it the head and broadseam in panels which will be dependent on panel orientation. Effectively you have a bermudan type at the head running into a jib luff - the leading edge of the sail which needs to be strong enough to take significant tension. It also gains draft from use of the outhaul to give it depth.

I'd be inclined to just make a couple of prototypes from thrown away packaging and tape just to see how the shapes work in reality. You'll learn more in two hours with some scissors and a knife and tape than a couple of days in front of the computer.

 

The Trimaco Eliminator is £29-42 for a sheet measuring 2.74 x 3.65 metres, a fraction over 10 square metres. In the UK, Tyvek is available only in rolls of 100 metres length, either 1.4 or 2.8 metres in width, for £119-20 or £226-00 respectively. I would expect the 2.8 metre width to be more useful & if you could buy 4 linear metres that works out to be £9-04 for 11.20 square metres, a saving of £20-38 or €28.11. How big is your sail?

For me personally, I prefer the Eliminator cloth as it would be much quieter than the Tyvek. Also, Du Pont would not be getting free advertising each time I set sail. OTOH, if you can blag some Tyvek from the manager of a building site. :?: :?: :?:

Regards,

Perry

 

since you plan to use this sail in light weather it will not matter much between tyvek and polytarp. Use which ever you can get cheaper, your first sail will not be your last one, so go cheap and just plan on learning on it. Take what you learn to the next one you make. I have used both, both work just fine for low cost sail. in a heavy blow you will stretch and damage either one, so just go with what ever you can get cheap.

I have used cheap duct tape on tyvek, one sided. you do not need two sided for a test sail, it just will show on the outside. you can put another strip on the back side if you want more strength. I have used cheap clear packing tape on polytarp, but the adhesive does not last very long in sun light, the duct tape is much more durable, and stronger. You can make it really strong by using duck tape reinforcement at the corners and along the seams, not pretty nor light, but it will make a strong and durable sail.

I think you can find images of the different types of seams on the internet, I have seen them. You do not need the book really (I have read it), but it mostly talks about making durable permanant sails, not quick polytarp sails. there is also not a lot about design in it, but how to detail the seams, attachments, and other important details. I have not used any of the methods he outlines becasue they are costly and very time consuming. I will stick with duck tape and tyvek for now, some day I may actually buy some real sail cloth and actually sew a permannat sail. but really, putting together a sail with duck tape is very fast.

I would keep the seams to a minimum, just put the amount in you need to get the shape. The darts will work fine.

good luck.

attached is a gray poly tarp sail with gray duct tape, you can not see it but it is there, and a green poly tarp sail that worked well, it uses yellow duct tape for the color, but cheap gray would work too. the other is a Tyvek sail using white duck tape, the shape came from a curved luff, no seams. It too worked well.