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Thread: How to convert poly tarp mils to sail cloth oz. equivalents?

  1. #1
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    Default How to convert poly tarp mils to sail cloth oz. equivalents?

    I'd like to make a trial sail or two using polytarps but not sure how to convert mil thickness to an equivalent (or near) sail cloth oz weight.

    I'm looking at 4 or 5 mil for something thin/light for a junk sail (they use thinner/lighter sailcloth because each panel is a "single" sail and has small area/extra batten support. So thinking 4 or 5 mil poly tarp would be as close to 3.5-4 oz. sailcloth as I can get?
    I have an 8 and 10 mil tarp on hand too and thought maybe it could be used for a trial lug sail?

    Anyone know how to accurately compare tarp thickness to sail cloth (oz.) thickness?
    I'll be looking at nylon tent material and if Tyvek didn't have all that branding printing on it I'd consider that too. But...tarps are already on hand and cheap if I screw up while experimenting...
    Thanks

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    There used to be fellow who made these sails exclusively but I can't find his site anymore. There is however much information online. I think it has more to do with thread count/quality of the tarp material rather than sail cloth equivalents. There's some information here too
    https://www.duckworksmagazine.com/06...ails/index.htm

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    Don’t. Do. It.

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    I made the simplest tarp sail I could for a canoe. No stitching, just duct tape. It worked well up and down wind, but only lasted a season. I just used cheap hardware store tarp, but this was a small sail.

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    Probably the bias stability would be far more important in trying to get something similar in characteristics to real Dacron than the weight or thickness of the materials. Look, we know for a fact that the stability and durability or lifespan of a plastic tarp will never even be close to real sailcloth, but for its useful life (whatever that becomes) it needs to mimic the shape-holding ability of real sailcloth. If it can't do that, at least for a while, it is worthless (as is tent nylon for the same reason, so don't even bother with it). So testing for bias stability by grabbing a decent-sized hunk of the stuff and pulling on it diagonal to the weave may be the best way to compare potential suitability of different samples. The more stable, the better. Then again, you can't design and build decent sails if you don't know how - something extremely obvious in way too many polytarp sails, so that part can't be ignored.

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    Thanks as always, guys.

    I'm not expecting any real sail shaping,(the lst sail would be a traditional flat Junk sail anyway) mostly a mock up to see if the junk rig's physical reefing/placement of lines, length of boom, etc. would even be something I'd like, want to deal with, starting with a "driveway" sail session.

    I would never delude myself that these materials or my lack of skills in sailmaking would equal a quality sail built by a Journeyman sailmaker.

    (I've been a "big boat" sailer for 40 years so understand all of that) it's the dinghy experiment thang that's going a cheap and dirty route just to see if the concept is even worth pursuing for now.

    Toxophilite, thanks, I've seen a few pages of the old Polytarp guy's site but yes, I believe he's gone with no internet site existent now.

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    o5.jpg
    These prototype sails were made of 160g/m2 polytarp. I wouldn't use anything thinner. I sailed two seasons and they worked decently and are still usable They stretched a little in stronger winds (15 to 25kn) but didn't break and didn't lose their shape permanently which was surprising. Our summers are nothing compared to the amount of sun in e.g. Florida or so the longevity in there might be really different.

    As prototypes they worked very well: I could make a really well educated quess when discussing and ordering sails from professional sailmaker for season 2022. I am pleased I made these sails, also learned a lot about sails even if I didn't use broadseaming.

  8. #8
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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    Rather than mils, look at the weave. I just bought 3 tarps for shelters on my property, and searched-out 14x14 tpi weave.

    Making sails is a lot of work, even polytarp ones. Use the best material you can find/afford.

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    Timo and Dave, thank you. I hadn't previously been exposed to the weave factors and will check out the tarps I've got and/or look for something appropriate.
    I was basically feeling my existing (light-maybe 4 oz. or less) Dacron Sprit sail and just trying to get a similar feel by the very scientific method of pinching it between thumb and forefinger ;-).

    Timo's sails are the largest I've ever seen by far and wasn't aware such large projects had been done before!

    Dave, if I like where the experiment goes, I'd certainly move on to a Dacron sail designed and built by a sail loft. It's still likely a lug rig will be the final rig but can't stop scratching this particular itch (Junk Rig) until I play around with the concept.

    Thanks for the feedback, all.
    DP

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    Quote Originally Posted by Dinghy Pipedreams View Post
    Timo's sails are the largest I've ever seen by far and wasn't aware such large projects had been done before!
    DP
    The boat looks bigger than it is. The total sail area of the three lowers is 28m2. My main is 11m2 if memory serves well.

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    Quote Originally Posted by Timo_N62.9_E27.7 View Post
    The boat looks bigger than it is. The total sail area of the three lowers is 28m2. My main is 11m2 if memory serves well.
    Cool! Thanks Timo.
    I revisited ductwork's site thanks to Toxophilite's mention and found this if it helps anyone else...
    (It's written by the poly tarp sail biz guy):

    Polytarp is generally made from three to four layers of polyethylene. The inner layer is woven from strands of the same stuff, but the weave can be very loose or fairly tight. Only when the weave is fairly tight is the polytarp material much good for sailmaking. Much of the common blue polytarp that you buy from your friendly home improvement store has only about a 6 x 8 scrim, or weave, per square inch. My white and light gray tarps have at least a 12 x 12 scrim and a 14 x 14 scrim if I can get it. In terms of weight and thickness, the cheap polytarp might be only a 3 mil; 2.5-oz./sq. yd. material while the better grade is an 8 mil, 6.0-oz/sq. yd. material.
    Another differentiating factor is color. White and some other light colors of polytarp tend to be U-V (ultra violet ray) treated. Dark colors are not.


    I had a 12 mil white tarp o hand that I was thinking would be too heavy (especially for the Junk small panels) and from this it sounds like he used 8 mil as a maximum. Anyway thanks to you all I noticed his mention of weave/tightness/bias. Also, after looking at a kazillion amazon tarp listings I finally found this label that indicated oz. per sq. yard. Wondering if that literally compares with oz. per sq. yard sail cloth, it's a bit tricky comparing bias/weave with weights offered by different tarp manufacturers but.. .
    I've got enough info. to chose something appropriate now.

    https://www.amazon.com/Heavy-Duty-Wh...%2C178&sr=8-22

    Thanks,DP
    Last edited by Dinghy Pipedreams; 12-11-2022 at 05:56 PM.

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    Quote Originally Posted by Toxophilite View Post
    There used to be fellow who made these sails exclusively but I can't find his site anymore. There is however much information online. I think it has more to do with thread count/quality of the tarp material rather than sail cloth equivalents. There's some information here too
    https://www.duckworksmagazine.com/06...ails/index.htm
    Archive.org has some usable snapshots of that site from the early 2000's. Some of the pictures/buttons don't work but the info appears to be there.
    https://web.archive.org/web/20050130...ML/article.htm

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    Wondering if that literally compares with oz. per sq. yard sail cloth
    Nope. Sailmaker's yards are generally measured as 36" x 30". In any case, the two materials have such different physical characteristics that the weight comparison is meaningless. You are looking for something that performs similarly (as much as it can) not something which is just similar in gross weight. Bias stretch resistance is by far the biggest contributor in suitability, regardless of weight. Tear strength is mostly going to depend on the quality and frequency of the scrim yarns, not the weight of the plastic between them.

    If you plan on doing any sewing on the tarp you will want to do it the same way we do it on mylar/scrim sails, where any sewn seam is backed up with a version of seam tape that has a very light Dacron carrier, or with a piece of insignia Dacron stuck into the seam. otherwise, the stitch holes just elongate when only put through the plastic areas between the scrim yarns.

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    Thanks Todd. Appreciate your response.
    DP

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    The difference between tarp and real sail cloth can be seen in my picture: no real sails would show such wrinkles around tack and clew. Even if the strength is enough the tarp flexes when loaded and bounces back once the stress is relieved.

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    One thing worth noting is that a polytarp sail is what it is, with some of its own fairly unique characteristics, due strictly to the material itself. It does not, however, usually make a very good model for later building a real Dacron sail while using the polytarp one as a template. You may find out, for example, that a 150 sq. ft. sail seems to work pretty well on your boat, but when you try to duplicate that tarp sail in real Dacron, things connecting it to the spars may need to be in very different places, angles, etc.

    I had this sort of thing happen to me on our trimaran, when I used the old Dacron Genoa, which had worked fine, as a template for a new X-ply mylar Genoa. It was a gorgeous sail with Dacron scrim on clear Mylar with a black Technora X-ply added for even more stability. The radial panel seams were all 3-M Super seam taped with no sewing needed so it was incredibly smooth. The only problem was that the new fabric was so much more stable than the old Dacron that I couldn't get the sheeting angles right and eventually had to move the Genoa tracks about two feet in order to get it to sheet properly. Bummer.

    So, you can build to gross dimensions of a tarp sail, but take the time to do the math and calculate the details for a Dacron "copy" using real sailmaking techniques, because what seems to work OK in one material may not work well at all in another. The time spent doing it right will be worth it.

  17. #17
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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    The main thing I wanted to test with the tarp sails was the sail area. The designer (Paul Riccelli) didn't like my idea of needed area (signicantly more) so I wanted to try it out. Turned out I was right: the winds on our lake area are so much lighter than the ones he had in mind (Florida coast?) that I wouldn't have got anywhere with the sails as planned. Doing that kind of change in plans without a cheap test would have been foolish.

    I also learned that I wanted to have the jib club footed and not overlapping as I dreamed. Tacking two sails (jib and fore) was too much when single handing. This kind of things can be tested with tarp and all I submitted to the sail maker was the size of the sails. He designed what draft was needed and how to produce it.

    My topsail is still tarp and I plan making a fisherman out of tarp, too. It is going to be more of a toy than real sail anyway.

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    So, you can build to gross dimensions of a tarp sail, but take the time to do the math and calculate the details for a Dacron "copy" using real sailmaking techniques, because what seems to work OK in one material may not work well at all in another. The time spent doing it right will be worth it.[/QUOTE]

    Things like this, keep me in the "I'd much rather have an experienced Junk sailmaker design and build a Dacron sail & rig for me" zone.
    I just can't find anyone in the PNW region who builds this sort of thing. Same goes for a balanced lug rig & sail.

    So... I'll likely go only as far as a mock up mast/yard rig and sheet leads in the driveway and keep looking for a sailmaker in the region to view what I've come up with and if they think it'd be workable. Haven't been able to find Roger and Jo's contact info. online yet but may try calling the Woodenboat Center in P.T. and see if they have that contact info. and to recommend a rig builder/sailmaker to me.

    Man, learned a lot in this thread, thank you.
    It's enough to reconsider picking up a used Minto rig and sail and building a new step for it. I owned and sailed one for years and it could still be carried in the (present 10.5 after) if needed.... it would be much easier to find and put into service. Plus I could borrow one to see if it would work-in place. Sigh...
    DP
    Last edited by Dinghy Pipedreams; 12-12-2022 at 04:16 PM.

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    The flat junk sail is fairly easy to design in either tarp or Dacron, as the only shaping is going to be the 2-D perimeter. It is well worth the extra work though to orient your fabric weave properly, in order to aim the yarns involved in either material properly. Some of the old junk sails were built with the cloth weave simply straight up and down, and horizontal, side to side. This is an awful thing to do to any fabric as it is placing the maximum stress diagonally on some areas and just asking for them to stretch out. You are much better off paneling your sail in sections which keep the weave at 90 degrees / 0 degrees to the leech edge. This sail could be made using only the five wide horizontal panels, or each of those sections can be divided up as shown here, but the weave is square to the local leech edge for each individual main section. That way, there is no area where leech tension is pulling against a diagonally cut piece of cloth. We just used regular sheet systems on these, rather than the individual sheetlets off the batten tips, mostly because it was in a confined space with enough going on already. Though I believe they did eventually add a rudder, the original test runs were steered by just adjusting the sails' relationships.

    Chinese lug building copy.jpg

    Though similar, the basic balanced lugsail pretty much requires getting into real sail broadseaming, edge rounds and other sailmaking skills that there really are no shortcuts for other than buying a pre-cut sail kit where somebody else and their computer already did the work. So that one may not be worth messing with at this point.

    The cambered junk sail is pretty complex, but you might enjoy playing with the basic design principles which can produce such a panel. Essentially, you are building a rather aerodynamic-looking "hump" in the spaces between the battens. This is basically the same way we build puffy vertical gores on balloons, and it is a cool concept. This balloon is lying on its side and a fan in the mouth is blowing it up. It was built from leftover cloth from a couple of real balloons that we repaired. The balloon's top with the blue deflation panel is on the right. As you can see, each gore is made from a stack of smaller sub-panels. What gives the gores their puffy shape is that the upper and lower edges of the sub-panels are convex curves. The deeper the curves, the more puffy that area becomes, and by adjusting the depths of the curves we can transition from no puffiness at the bottom, to maximum puffiness upper middle and gradually back to none at the very top by the deflation port.

    balloon 001.jpg

    The "trick" as it were, that makes this work and will generate a smooth shape is that each sub-panel seam to the sub-panel seam above or below it is the same curve. The maximum puffiness is around the bottom of the red panel - which has the same curvature as the top of the black sub-panel below it. The top of the red panel is also curved, but slightly less, and the same as the lower edge of the black sub-panel above it. As we move upward and downward from this area, the curves gradually diminish, but the edges they join are always the same, with each sub-panel having one curve on its top side and a slightly different one on its bottom side.

    Confused yet? Hope not, but this is a concept that can be played with using cut paper, polytarp or fabric to generate some really interesting panel shapes. You would essentially lay out the plan on the floor as shown above, but the small vertical lines dividing up the horizontal sections would be slightly convex curves. How much? That would need to be a product of some experimentation.
    Last edited by Todd Bradshaw; 12-13-2022 at 03:59 AM.

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    Wow, thanks, Todd-so much for taking the time and effort to present all of this to us. I want you to know that even if my personal project doesn't prove to be something I go ahead with "all the way", I want you to know how appreciated it is and that you aren't wasting your time and breath presenting your experience and teaching to/for our benefit(s).

    As a music educator, I sometimes have had students who never "followed through" by becoming good/great players, but I could tell that they were still enriched and inspired by
    the instruction/ intellectual theory in of itself and carried it over to other aspects of their lives and their other practical and creative efforts.

    I assure you that even if the Junk thang doesn't go "all the way", that your excellent teaching does this for a ton of people here and elsewhere so if you wonder if it's worth the hassle, sometimes, well...yes, it is.

    Speaking of creativity, wondered if the shelf behind the balloon was a model RR switching layout? The rocks and trees suggested that...

    Cheers, DP

  21. #21
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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    It's more like a running layout in N Scale along the tops of the shelf units in my office. In most parts it's only 12" wide, so there isn't much room for switching, but it has two track loops, each around 50' long, so the trains can actually go away and come back, rather than just chasing their tails around in circles. It detours briefly through the living room wall and there is another in the kitchen at the other end in order to get enough radius for turning the trains around and sending them back.

    Along the living room wall, 6" wide.

    gn2 005.jpg

    Back in my office on top of the bookshelves, 12" wide.

    3985-train.jpg

    jkhyg 009.jpg

    69a.jpg

    Rock faces and the base are all hot-wire-cut and wash-painted pink styrofoam. I'm not really a hard-core train geek, but I enjoy having them run around the shelves when I'm supposed to be working. I used to do custom weathering and paint jobs on locomotives during the winter when the sail business was dead, but haven't done any for a while.

    Kitchen, next to the refrigerator.

    khd 003.jpg

  22. #22
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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    I quess you couldn't help but add a wooden boat there as well

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    Very nice work, Todd. I don't have a 3rd party photo bucket/etc. page but if you'd be interested in seeing my modeling/scenicking work, I'd love to share some with you.
    Is your track plan prototypical (sounds like it isn't to allow the return loops, etc.).
    My big renewed interest in the hobby is derived from OPs groups doing prototypical job practices and switching lists, waybills, the works in my region. (Vancouver B.C. down to Seattle). Tons of fun, learned history and camaraderie prior to the pandemic.

    I model mid '40's on a track plan close to "Black River Junction" between Tacoma, WA and Seattle, Wa where most of the N.W. "Roads" met up. The majority of my rolling stock is Milwaukee Rd. I cheated by bringing UP and SP a bit further N. though. It's a switching layout with an upper and lower yard/city surrounded by N. Cascades Mt. Range scenery.

    The rock faces/mt.s are a resin based product two part non toxic product put out by Joel Bragdon. http://www.bragdonent.com/Amazing system.

    The finished product backed by a low expanding foam are very light (almost weightless) and one is able to cast l'X2' or larger cliffs with them all alone climbing atop the benchwork. Entering the train room is like walking through a mountain canyon.

    I don't know if you'd be comfortable doing this, but:
    If you'd like some pics, send me an email addy via a PM. I can send a couple of the short scale six string bass I designed and had built for me by Carey Nordstrand (Norstrand Pickups) with the lovely Macassar Ebony face & Eucalyptus back. We managed to get a good low B on a shorty. I wanted to do string through the body to bring out the low B more but Carey felt it was enough with just he brass nut/brass bridge. It constantly amazes people in the studios when I record with them.

    Ok, back to poly tarps instead of polyrythyms ;-)

  24. #24
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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    Train geeks…



    This layout never went anywhere, as I decided to move to Michigan.


    Jeff C
    Last edited by leikec; 12-15-2022 at 08:12 PM.
    Don’t expect much, and you won’t be disappointed…

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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    [QUOTE=leikec;6771287]"Train geeks…"
    We have a lot in common with boat & ship geeks.
    A moving vehicle/vessel that evokes a love of history, design, job practices and a rich vocabulary that rivals nautical lingo! (diamond pusher=fireman (coal shoveler), goes in the hole= (yard switching), etc.

    Heck, even those round tailed passenger end cars were known as "Boat Tail cars". ;-) Prototypical operations is fun with the guys n' gals, beer or coffees in hand, with a group of old or new friends.

    Check out OPSIG:https://www.opsig.org/About/Index

    Being a boat guy, I have a river based/bay port layout to encourage the building of vessels too. (Tugs, barges, the occasional schooner...)

    Any more of "us" M.R.R.s here? Especially port oriented layouts? One could squeeze one in alongside the building cradle and workbench ya know... ;-)

  26. #26
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    Default Re: How to convert poly tarp mils to sail cloth oz. equivalents?

    Nothing wrong with experimenting.

    I made a successful squaresail for Drake III as a test, which was the right decision. It was cheap, worked fine, and allowed me to explore the concept before spending the money on a professionally-made one. Which I never did -- a squaresail takes an additional crewmember when travelling in the Great Lakes, and I never seem to have one (all those people on board except one grew up and moved away).

    drake under squaresail.jpg

    I also made a mainbackstayspritsail, which again worked surprisingly well, but was a bit of a set-up requiring labour I don't have. So I didn't get a pro one made.

    I made made a polytarp leg-o-mutton spritsail for the 10ft dinghy I designed (the one in the picture) which is great.

    Anyway, these sails don't last too long, but they can be great to aid a learning process.

    Dave

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