Page 4 of 4 FirstFirst ... 34
Results 106 to 126 of 126

Thread: Polytarp Sailmaking

  1. #106
    Join Date
    Sep 2018
    Location
    Morris, IL, USA
    Posts
    13

    Default Re: Polytarp Sailmaking

    Quote Originally Posted by johnw View Post
    You can get a new jib for $170 from these folks: http://www.intensitysails.com/gesaforda.html
    Yes I know about the intensity sails and already stated that I could buy one. What fun is that though?

    I am prepared for those that will try to talk me out of it using arguments that polytarp sails are crap, or you might as well make one out of dacron because of the work involved, or "real" sails don't cost that much, or a dozen other arguments. However, I am not intending to make one just to get a sail. If that were my purpose, I would just buy one.

    For me it is the adventure and I want to make one out of polytarp simply because It intrigues me and I was able to buy the tarp and double sided carpet tape at a store I am frequently at for just under $20. I have all of the other materials needed from what was left over from adding reefing points (using dacron) to my main sail, so my total cost will be $20 and my time. I look at it as $20 spent for hours of enjoyment. Just ask my wife. She would tell you that I do plenty of stupid time wasting stuff. She would also tell you that I have a great time doing that stupid stuff too.

  2. #107
    Join Date
    Dec 2003
    Location
    Sound Beach, NY
    Posts
    3,565

    Default Re: Polytarp Sailmaking

    I made a tarp sail years ago for my canoe. It was a lug sail with darts cut in it and "stitched" with clear duct tape. It only lasted a season, but it was fun, cheap and effective. I was thrilled that I made a sail for $8. I don't have any experience making a headsail. You might try to reach the Polytarp guy, if he's still out there. Or check Duckworks, I think some guys over there have built polytarp sails. Good luck, I'm interested in your results.

  3. #108
    Join Date
    Jun 2000
    Location
    Madison Wisconsin
    Posts
    9,031

    Default Re: Polytarp Sailmaking

    Most of your plan sounds OK. Exceptions would be the idea of darts (real sails don't have darts) and 3" of leech hollow is probably more than needed (1" of hollow per 10' of leech length will usually do it). It's hard to say how long a polytarp leech is likely to last before stretching out of shape, but time will tell. Don't rope the leech or foot. The shaping of small jibs like that is done with broadseaming and can often be mostly accomplished with a combination of the luff curve, and broadseaming the tack seam and a vertical seam or two on the bottom panel. These verticals give the bottom a bit of cup shape and keep the foot roach from flapping.

    I'm not sure the computer program is going to make the process any easier or better for a small simple jibsail. I'd rather see you order a copy of Jim Grants "Jibsail Manual" from Sairite (not very expensive) follow the steps and learn to do it properly - and most of it is (or should be) the same basic process with either Dacron or tarp plastic.

    Simple small jib with tack and foot broadseams.

    jib2a.jpg

  4. #109
    Join Date
    Sep 2018
    Location
    Morris, IL, USA
    Posts
    13

    Default Re: Polytarp Sailmaking

    Thank you. That is helpful information. If I understand you right, what you are saying is that a reasonable attempt would be to use one broadseam from the tack and one that intersects the middle of that one going to the foot. That yields a problem on my particular sail, but I'll get into that once I see if I understand the broadseaming first.

    Doing some research it seems the max camber depth for a jib should be around 30%, so I would mark the tack broadseam at the 30% point and this is where the seam overlap is the minimum. Is this correct?

    From there, how is it determined how much to increase the seam overlap. I think I read somewhere about a 1:60 ratio?

    Using an imaginary 100" long tack broadseam, It seems I would mark about 30" back. Lets say I use a 1" seam lap there, then at the luff the seam overlap would be 1 + 30/60 = 1.5" and at the leech end the overlap would be 1 + 70/60 = 2.16" (about 1-1/8" more). Of course the edge of one panel would be cut on a curve to make the final seam a uniform 1" width. Does this sound right.

    Then, I'm not sure about the how to figure the foot broadseam. Would it be the same 1:60, so say the length of the seam was 15", the overlap at the foot would be 1+15/60 = 1.25", and 1" where it intersects the tack broadseam.

    As far as my sail goes and the broadseaming issue, if I draw a line from the tack going perpendicular to the leech, I would basically be cutting off a small sliver from the foot of the sail. My basic sail for this boat is basically a right triangle as shown in the picture below. The red line is from the tack corner perpendicular to the leech. What would be done for a case like this? Would you just make the seam a little higher than the tack, like the blue line?

    Thanks again.
    Attached Images Attached Images

  5. #110
    Join Date
    Sep 2018
    Location
    Morris, IL, USA
    Posts
    13

    Default Re: Polytarp Sailmaking

    I just found a picture of a stock 100% jib and it looks like the tack broadseam is a few inches up from the tack corner, and the round looks like it is about 6" rather than the 3" I have drawn.

    I also want to confirm that the fabric grain should run perpendicular to the leech. Correct?
    Last edited by 109jb; 10-02-2018 at 02:36 AM.

  6. #111
    Join Date
    Jun 2000
    Location
    Madison Wisconsin
    Posts
    9,031

    Default Re: Polytarp Sailmaking

    Foot round amount is seldom critical and usually done to please the eye, but six inches is way more than needed there and the more you add, the more you have to broadseam for in order to keep it from flapping. I don't know where the 1:60 ratio for seam overlap came from. It's gobbledygook and not how real sailmaking is done, or ever has been. We don't use such ratios. Broadseaming begins by determining a broadseam curve from the mid-ish foot, up to around 2/3 of the way up the luff. The broadening of the seam would begin where it crosses that curve and proceed toward the luff, widening at a rate based on the dimensional stability of the cloth used. Typical small boat, medium firm Dacron would be an increase in seam width of around one half inch for every 30" of the broadseam's length. Polytarp (at least before it eventually stretches out) should probably be broadseamed at a similar rate.

    Yes, you can move the tack seam up a little bit on a foot which has that sort of angle in relation to the cloth weave and leech. Foot broadseams are pretty small amounts, as they are also overlap increases determined by the length of the broadseamed area, which won't be much here. In traditional sailmaking there is also a different and interesting way to handle that sort of foot/leech/panel seam angle We work over a full-sized lofting and first we would panel the sail all the way down from the top to that tack seam, basting the panels together. Then we would lay the foot panel over the lofting and cut the foot curve, but leave excess length at the luff and leech ends of the panel. Next, we flip it over and basically baste the foot curve we just cut to the straight bottom edge of the panel above it. Once it is attached, we cut the luff and leech to match our lofting. It creates a shelf foot - straight on the bottom, but maintaining some good lower sail draft. I remember being really befuddled when I started learning sailmaking by this. Why would I want to cut out a piece to fit my lofting and then baste it on upside down? After trying it and seeing how it worked, it got a lot more clear.

    I really have a problem with a lot of these internet and computer-based home sailmaking instructions. In most cases, they are far more confusing and more difficult than the real, traditional methods, and in the process, the students don't learn much at all.

    This is the sort of thing you can come up with by simply getting a decent sailmaking book and just following the instructions. I looked in the "big box O rigs" in my old Macintosh and found a typical plan for a similar jib with typical draft of about 2.5% (about 12" draft in every 10' of chord width). This is the way jibs have been designed for many decades and it still works just fine. The vertical curve is the broadseam curve, done by eye, but there really isn't anything very complex involved in the design of such a sail.

    Jib-plan.jpg

  7. #112
    Join Date
    Jun 2013
    Posts
    2,373

    Default Re: Polytarp Sailmaking

    Quote Originally Posted by 109jb View Post

    Doing some research it seems the max camber depth for a jib should be around 30%, .
    Jeezers, that's some depth! North Sails used to use around 18% maximum for jibs. These days, 30% draft is quite deep even for a spinnaker in some classes.

    When you say "max depth" is that when you allow for forestay sag, which will increase depth? Excess depth is IMHO far far worse than lack of depth.
    Last edited by Chris249; 10-02-2018 at 05:46 AM.
    Has BigFella and SkyBlue on ignore.

  8. #113
    Join Date
    Sep 2018
    Location
    Morris, IL, USA
    Posts
    13

    Default Re: Polytarp Sailmaking

    Quote Originally Posted by Chris249 View Post
    Jeezers, that's some depth! North Sails used to use around 18% maximum for jibs. These days, 30% draft is quite deep even for a spinnaker in some classes.

    When you say "max depth" is that when you allow for forestay sag, which will increase depth? Excess depth is IMHO far far worse than lack of depth.
    I think you misunderstood. The 32% figure is where the maximum depth lies along the chord line, not the depth itself. The actual depth itself I'm thinking should be more along the lines of about 12%

  9. #114
    Join Date
    Oct 2009
    Location
    South Puget Sound/summer Eastern carib./winter
    Posts
    15,143

    Default Re: Polytarp Sailmaking

    Poly tarp schmolly tarp... why not just use an ACTUAL bed sheet?
    I called this one "Serta Perfect Sleeper".
    Sailed her from Smith Sound to St Georges (Bermuda) daily for months. A reach both ways...obviously.

  10. #115
    Join Date
    Jun 2000
    Location
    Madison Wisconsin
    Posts
    9,031

    Default Re: Polytarp Sailmaking

    Bedsheets can work, though allowing for their bias stretch is a pretty different ball game and nearly all edges need to be bound or have rolled hems and lap feld seams. This one is made from genuine Martha Stewart, high thread count, plain weave Egyptian cotton sheets. If you really want good cotton though, it will cost more than Dacron would. I paid something like $90 for the sheet set that that sail was built from, but it was made for an old museum quality wooden canoe and had to replicate the original sail.

    03a2.jpg

    From a practical and cosmetic perspective though, I kind of lean toward the Dacron version I made later.

    003a.jpg

  11. #116
    Join Date
    Jun 2013
    Posts
    2,373

    Default Re: Polytarp Sailmaking

    Quote Originally Posted by 109jb View Post
    I think you misunderstood. The 32% figure is where the maximum depth lies along the chord line, not the depth itself. The actual depth itself I'm thinking should be more along the lines of about 12%
    Ah, OK. In my experience that's referred to with different terms.
    Has BigFella and SkyBlue on ignore.

  12. #117
    Join Date
    Sep 2018
    Location
    Morris, IL, USA
    Posts
    13

    Default Re: Polytarp Sailmaking

    Thanks for the help Todd. I have been busy with work and just now had time to digest your last posting.

    Quote Originally Posted by Todd Bradshaw View Post
    I don't know where the 1:60 ratio for seam overlap came from.
    Quote Originally Posted by Todd Bradshaw View Post
    Typical small boat, medium firm Dacron would be an increase in seam width of around one half inch for every 30" of the broadseam's length. Polytarp (at least before it eventually stretches out) should probably be broadseamed at a similar rate.
    The way I presented it may have been deceiving, but I took the 1:60 that I found to mean the same as what you posted above. I think this was just my mistake in terminology.

    I believe with your last post I have enough to make the sail shape work out.

    I did have a question regarding broadseams and darts. First let me say I have no problem with making a broadseam or two in the sail I'm making if that is what it needs. However, I wonder about seam strength, particularly in polytarp. In dacron it is tried and true to seam and the stitches have proven to provide sufficient strength. In polytarp, I think this is more of an unknown. It is probably fine strength wise, but I also wonder about edge fraying, so am thinking it may be best to only cut the material if absolutely necessary.

    I have read that you don't like darts on sails and have been thinking about the pros and cons of them. In terms of darting in general, I don't like the idea of simple straight V shape darts, which would result in a peaked shape to the sail rather than a smooth shape. I also don't think cutting to shape a sail in the case of a tarp sail is entirely necessary. How I would do it would be to just gather the excess material in a curved V and stitch it down. I'll still refer to this as a dart, but that probaly isn't the correct term. In terms of shape, if the "dart" is done right, I don't see how the shape would be different. In the picture below I have crudely sketched what a broadseam might look like and what a dart like I am proposing would look like. If the curved black lower line is just brought up to the straight black upper line then the shape resulting from this dart seems like it would come out the same as the broadseam, and if done by the gathering method results in no cutting needed there. The green line would be a fold in the material that would be stitched down flat to the sail.

    I'm interested in what you think of this. I'm not trying to be argumentative, but rather trying to understand your point of view on darts. It is entirely possible I am missing something detrimental in terms of darts and am just wanting to learn.

    Thanks again for all of your help.
    Attached Images Attached Images

  13. #118
    Join Date
    Jun 2000
    Location
    Madison Wisconsin
    Posts
    9,031

    Default Re: Polytarp Sailmaking

    Your broadseam drawing is not the way a typical panel seam, luff or foot broadseam would be done. Rather than having one straight side and one tapered side, they would both taper, the tapers would be much more gradual and straighter and finally as you approach the sail's edge, there would be a slight flare, increasing the overlap rate a bit. A leech broadseam would be tiny and pretty much straight sided with no additional flare. Darts are not used because (1) they tend to make "hard spots" (spots at their beginnings, which don't smoothly fit into the shape around them) and (2) it is usually not possible to baste or sew the beginning of a dart properly.

    If it matters... (3) They look like crap and clearly indicate that the person who made that sail doesn't know what they are doing.

    In dacron it is tried and true to seam and the stitches have proven to provide sufficient strength.
    Actually, these days the basting tape is doing most of the work, with the stitching acting as backup. After sufficient dwell time for the tape's adhesive to set, it is very difficult to get seams apart without damaging the cloth. With decent basting tape, it is likely quite possible to exceed the tear or distortion strength of polytarp. Stitching on that stuff is pretty much a waste of time and may be more likely to reduce tear strength than increase it. The closest scrim fabrics we have in real sailmaking are the Mylar/scrim laminates. Proper seams in these fabrics are basted using Dacron seam tape. It has a light woven Dacron carrier, which keeps the stitch holes from tearing and distorting. If you plan to sew anything on polytarp it would be smart to incorporate something similar into the seams.

  14. #119
    Join Date
    Sep 2018
    Location
    Morris, IL, USA
    Posts
    13

    Default Re: Polytarp Sailmaking

    I understand your points. I have a roll of basting tape from adding reef points to my main. I'll have to do a little experiment to see how well it sticks to the tarp material. TheTbasting tape sure is some sticky stuff. From what I understand the tarp material can have some oils and other tape doesn't stick well to it. I'll take a couple pieces and clean and stick and sew them together and see how it looks and how hard it is to pull apart. If it works ok I'll likely do a broadseam in it.

  15. #120
    Join Date
    Jun 2000
    Location
    Madison Wisconsin
    Posts
    9,031

    Default Re: Polytarp Sailmaking

    I think the sewing will be the weak point - basically a couple of perforated lines which essentially say "tear here", or stitches which will tend to stretch the stitch holes in the plastic under a load until you get to the next scrim line. That's the same problem we find on Mylar sailcloth - and it's a lot stronger than polytarp plastic. Look for something woven like the Dac-basting-tape or strips cut from insignia Dacron to stick into the seams if they will be sewn. Some people use double-stick carpet tape on polytarp sails, but I have no experience with it.

  16. #121
    Join Date
    Sep 2018
    Location
    Morris, IL, USA
    Posts
    13

    Default Re: Polytarp Sailmaking

    I did a few tests with some cutoffs from the unused part of my tarp. I cut strips about 2" wide and did various seams strategies with the qualitative results below. The various seams were all connected together in one long strip and pulled. None of them failed and I was pulling with about 50-75 pounds of force.

    1. Folded fell seam with one zig-zag stitch down the middle - Not good. when pulled the fell seam distorted and tightened up the outside layers had obvious distortion of the cross fibers. Unusable.

    2. 1" overlap with 2 rows of zig-zag stitching - Very little distortion and the stitch holes elongate on a teeny tiny bit with all that force. Definitely usable.

    3. 0.5" overlap with 1 row of zig-zag stitching - Lots of needle hole distortion and the 2 pieces of fabric slid a little bit. Unusable.

    I also did not use any kind of basting tape on the above seams. I did a simple test with the basting tape I got from Sailrite, and it sticks ok but I don't think near as well as it does to dacron.

    As for the cut edges. They don't appear to want to fray at all because of the coating. I will have to keep and eye on them and will tape them if they begin to fray.

    I decided to go ahead with doing a broadseam from just above the tack to just above the clew (perpendicular to leech), and a single foot broadseam. I got the basic sail layout done, cut the sail oversize, and got the foot and tack-clew broadseams done. I attached the head to a string and made my wife pull the clew while I pulled the tack corner and lo and behold the sail has a bit of shape to it. I then was able to verify the layout after doing the broadseams and drew the luff, foot and leech curves. I'll hopefully finish the sail tomorrow night.

  17. #122
    Join Date
    Sep 2018
    Location
    Morris, IL, USA
    Posts
    13

    Default Re: Polytarp Sailmaking

    I got my jib sail done last night and it went pretty well. An indoor horizontal hang test looks good, and when I have time I'll hoist it on the boat and see how it looks. I'll post some pictures when I do.

    i used carpet tape as the basting tape for the perimeter and then sewed with 2 rows of stitching. The material sews well except that it is so slippery that my home sewing machine would slip when there was any feed resistance. So, the stitches didn't come out real uniform but they will do.

    BTW, much thanks to Todd Bradshaw for the design help.

  18. #123
    Join Date
    Sep 2018
    Location
    Morris, IL, USA
    Posts
    13

    Default Re: Polytarp Sailmaking

    As promised, here are some pics of the jib I just made in a hoisted position. Unfortunately there was virtually no wind today to see what kind of shape it might take. Just enough wind to kind of push it to the starboard side but that was about it.

    Jib 1.jpg
    jib 2.jpg

  19. #124
    Join Date
    Oct 2005
    Location
    The Netherlands
    Posts
    572

    Default Re: Polytarp Sailmaking

    Not bad, I would like to see that jib with more wind.
    Another way to get cheap sailcloth is to buy 'seconds' from Contender. It may have stains, coating problems, creases but most of it is perfect for your purpose and the price is about 40% of the normal price. Contender only sells to pro's so you will have to ask your friendly local sailmaker and he might have it for repair, or could order it for you.
    And have you tried Sailcut 4 for designing sails? I used it years ago and liked the results and now they have more possibilities in their software. Robert Lainee, who started it, wrote about a 'default' sail which would be fine for most of us.
    Www.oarandsail.nl

  20. #125
    Join Date
    Jun 2000
    Location
    Madison Wisconsin
    Posts
    9,031

    Default Re: Polytarp Sailmaking

    Looks good. For the next one, you might go a little shorter on the luff. Usually about 90% on the hoist is good and it leaves you some room to mess with luff tension as a draft adjustment tool, as well as some insurance in case the luff stretches over time.

    FF, Contender cloth is pretty expensive over here because it's imported. It's good stuff and I love that dark, almost burgundy tanbark shade they have, but even at 40% off you can still probably do better with something domestic from Challenge.


    Contender tanbark Dacron:

    contender-bark.jpg

  21. #126
    Join Date
    Sep 2018
    Location
    Morris, IL, USA
    Posts
    13

    Default Re: Polytarp Sailmaking

    Yeah. I've wondered about the luff length. I made the luff on this one the same length as the specs shown on various places including the day sailer assn., and the sail rite specs. It also matches my Genoa, but I've always wondered if it should be a little shorter. In the picture I just used a couple ss carabiners at the head and tack, so there is a wasted couple inches compared to my normal halyard knot at top and short shackle at bottom. Even so, I still had a few inches from the block at the head with the sail hoisted pretty tight. I definitely would shorten it a few more inches if I did it again. Thanks again for all you help.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •