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Thread: Polytarp Sailmaking

  1. #106
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    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

  2. #107
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    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.

  3. #108
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    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%

  4. #109
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    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.

  5. #110
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    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

  6. #111
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    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.

  7. #112
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    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

  8. #113
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    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.

  9. #114
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    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.

  10. #115
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    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.

  11. #116
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    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.

  12. #117
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    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.

  13. #118
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    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

  14. #119
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    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

  15. #120
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    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

  16. #121
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    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.

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