Cybernaught's Polytarp Sail Making Method (The quick, the dirty and the ugly)
By Steve Henshall #218 "Muthaducker" (4ar)
I take the same approach to boat building that I do to model airplanes, target shooting and most of my other hobbies. It's for fun and it should be enjoyable. If it isn't then I'm not interested. KISS and TLAR along with UWA are the only rules. I HATE WORK! Sorry, I didn't mean to shout.
KISS = Keep It Simple, Silly
TLAR = That Looks About Right
UWA = Use What's Available
I have made quite a few sails, more than 20 but not 100, and I started by reading all the online stuff and following instruction precisely until my head hurt. It wasn't fun so now I do it this way.
This is short and sweet and in pictorial form.
First, find some sticks. I used an old piece of bamboo that was a spar before for a boat that no longer exists. I got two 1" x 1" x 10' sticks from the local hardware and some Trapal (Polytarp) I got about 30 meters of skinny rope or thick chord. It's the stretchy nylon kind because that's what they had and made do with what I had on hand for the rest. I put the mast in the step on the PDR and laid one of the sticks next to it fore and aft to get an idea of the length I'd need for a sprit-boom. Allowing about 9" between the Deck and the Foot I marked the mast with a magic marker. This gave me about 6' for the Luff and bit over 7' for the Foot. Multiplying I get an area of around 42sq ft. Since this is a main-sail for my other boat and a second sail for high wind sailing on the PDR that area is good enough.
Now I took the rope and tied it to the top of the mast, used slip knots to tie the sprit and sprit-boom ends to the rope and tied the other end to the bottom of the mast at the point 9" above the deck. I arranged things using the TLAR method to get a shape that looked OK and gave me what seemed to be adequate head-room and tied off the other ends of the sprit and sprit-boom so it looked reasonable.
When I was satisfied that things were about right, (I got in the boat and moved the sail to make sure I had head room etc.) I lifted the whole assembly out of the mast step and laid it on what we laughingly call our lawn. (More rocks than grass.) I made sure the rope had tension and things looked OK then measured the sides and a diagonal to get the approximate dimensions.
In this picture you can see the carefully detailed sail-plan with accurate dimensions painstakingly drawn to scale.
At this point I went inside and logged onto the following site.
http://www.analyzemath.com/Geometry_calculators/herons_area_triangle.html
Now I didn't need to do this. I know the formulas and I taught math for 15 years before retiring. It would probably be a good exercise to do the calculations long hand just to give the old grey matter a bit of a work out but you know by now how I feel about WORK! Yea, let's just stick with Quick and Dirty.
Upper left on my detailed diagram shows the area given by the measured values as 43.3sq ft. That's fine.
I spread out the Trapal (Polytarp) on the garage/workshop floor and held the corners with empty epoxy cans full of sand in preparation for lofting the sail dimensions.
This is the KISS way to loft the sail onto the tarp. I marked the 4 corners with a whiteboard marker so I could wipe off the marks if needed and then took the spar-framework apart. I used a chalk line to do the four sides and then marked foot, luff and head at one third the dimension with a mark that was 3% of the dimension length outside the line and made the bolt-rope mark with a piece of ¾" plastic pipe. This curved the 3 sides that needed to be curved to put draft into the sail. I cut the tarp 2" outside that line for a glue-over flap to secure the bolt-rope and hem the sides of the sail.
Note: You can find all kinds of site's on the net that give directions for making proper cuts.
I cut an 18" dart at the bottom of the Luff and a 9" dart at the top then made a table with some left over 1/4" ply, saw horses and a couple of 8' boards. I made a loop at one end of the rope and hotglued it then whipped the rope with nylon kite string to make the loop strong. Then I used a 2" scraper and applied contact cement along the luff from the Tack up towards the head. I put an equal amount on both sides of the bolt rope line and then left it 20 minutes to dry. I cut the Luff at about 10" spaces from the outside to the bolt rope line and using cans of sand to keep everthing straight and taught I folded the hem over the bolt'rope. If you try to fold the whole length of the luff without the cuts it can winkle and spoil the looks of the job.
You can see in this picture the Luff is almost finished and you can see the lines of the cut where the 10" sections have been folded from the tack up. You cannot re-do the fold if you mess up. I know a lot of you can run to the hardware store and get double sided tape or you can sew the hems etc. I can't. I'm in the Philippines out in the provinces. I can get Rugby (Contact Cement) so that's what I use. Contact Cement or what we used to call "Rubber Cement" works good and when you get the system worked out for using it correctly it's good and it's cheap. Here one bottle will do at least 3 sails or more and it's about 35pesos which is about 75 cents US.
When I get round the sail I cut the rope about 10" past the Tack again and make a loop like the first one that will lie directly over the first loop. Then I finish off cementing and folding the hem.
You can see the loop and the nylon string whipping to secure it. You can also see the magic marker line for the bolt rope and the darker color to either side is the contact cement after it has dried and is ready for sticking.
When the bolt rope is in I put reinforcement patches of Trapal over the four corners and at 8" spacings along the luff for the holes that I will use to lash the sail. When that is done I use a soldering iron to melt holes in the corners and in the patches along the Luff for the securing the sail to the spars and mast. Be careful not to burn the bolt rope with the soldering iron.
Here you can see the new sail mounted on the PDR and when the wind blew it looked to have about 10% draft so the darts and curved sides must be about right. The proof of the pudding is in the eating though and I'll see how it works when I get to sail with it. Perhaps tomorrow.
That's the quick and dirty sail making method I use and even if you don't use it completely perhaps you will find a tip or two here that you can apply to your own sail making methods.
This may give you an idea of what you go through to pursue the sailing hobby in a third world country. It's all grins and giggles and I hope you enjoyed the read.
Remember, KISS, TLAR and UWA.
If you want to try this and have any questions you can contact me through the pdracer member list.
Clear skies and fair winds to all.
Steve. AKA: Cybernaught