building a polytarp gennaker

Discussion in 'Boat Design' started by peterchech, May 31, 2011.

  1. peterchech
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    peterchech Senior Member

    I have just finished a Dacron lugsail for my 24' outrigger canoe. I want a foresail, but I don't want to waste any more time sewing Dacron. I want to get back on the water quick!

    I experimented with a blue polytarp gennaker, just duct taped together, and it worked wonderfully. I am now going to build a somewhat more "refined" version of this, with about double the sail area. Aiming for about 90 sq feet here. Using the "better" polytarp material, the heavy duty UV resistant white colored stuff. I plan on just duct taping it together and punching in grommets, but may consider one of the two sided tapes available out there as well.

    My concern is limiting stretch here. The stay system will be somewhat flexible unfortunately, since the only place to run the stays to is the crossbeams and those are lashed on with polyester line. My questions are:

    How much round should I add to the luff, proportionately?


    If I use a bolt rope to limit stretch on the edges, which edges should get bolt rope? All three? or should the leech be left rope-less like the lugsail?


    I am on a budget here, so a roller furler isn't in the cards until I find a used one somewhere. I plan on running the genny up the forestay with jib hanks and a halyard, any recomendations on a simple reefing system for it? Is slab-type reefing possible (or desireable) here?
     
  2. Stumble
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    Stumble Senior Member

    Peter,

    I am not sure what your budget is, but call around to some of the yacht clubs and sail makers they likely have old sails just rotting they will give away. If that doesn't pan out I would go for as flat a sail as you can make, the material you are using will stretch, so you might as well at least start flat.

    I would only add tapes on the luff, and maybe the foot. But the leech should be left off. In commercial sails there is a leech line, but that is a pretty difficult thing to add to such a 'getting by' type of sail.
     
  3. Richard Woods
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    Richard Woods Woods Designs

    You don't want to add luff round, but subtract it. That's because the luff will sag more than the extra material needed to make a cambered sail. The amount to subtract will depend on your specific rig so I cannot really help there, not knowing the boat

    I'd run a line of duct tape down the leech and along the foot to help stop motor boating. And I'd use duct tape in long strips radiating from each corner to dissipate the loads.

    Have a look at photos of laminate sails and you'll see that is basically what they do

    You probably do need a 4-6mm luff rope

    If you hunt around online you'll find a couple of free sail cutting programs

    Having said that I'd go for the used sail approach (it's what I do)

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  4. TeddyDiver
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    TeddyDiver Gollywobbler

    Instead of polytarp check up Tyvek..
     
  5. peterchech
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    peterchech Senior Member

    It took alot to convince me to go to polytarp. Haha tyvek will just have to wait...
     
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    If you can find it, double-sided tape for outdoor use is great for tarp sail seams.
     
  7. peterchech
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    peterchech Senior Member

    Do u have a brand name for the tape so i can order it online? Ur not talking about outdoor mounting tape right?
     
  8. viking north
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    viking north VINLAND

    Listen to TeddyDiver on the tyveck (house wrap) it is indestructiable--very little stretch--totally UV resistant-- very light--Very similar to mylar. Many people use it for quick and dirty and emergency sails. Plus many building supply stores have their name and logo printed on it,you can sell this as an advertisement to buy a future proper sail, The Tarps will quickly break down from UV. --- Geo.
     
  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    I wish I did! I have been looking for a supplier in my area for a couple of years, since outdoor carpet went out of style it has been hard to find. But if you can get it, it's great stuff. The adhesive is very aggressive and sticks to scissors and knives so you have to keep cleaning it off with paint thinner or other solvent.
     
  10. viking north
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    viking north VINLAND

    Peterchech, before you go thru all the trouble of building this thing why not post the approx. acceptiable dimensions of the sail you need and see what pops up. The forum is not strickly for technical help, there have been many situations of free material help. I think i might still have an old but good sail kicking around here somewhere if i haven't already given it away---Geo.
     
  11. peterchech
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    peterchech Senior Member

    Good idea. Not positive yet on the dimensions. I calculated them, but to be honest most of my calculations often wind up being off when I actually go and measure ;)...

    I calculate the leech is about 15', foot is about 10' but could be more.
     
  12. viking north
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    viking north VINLAND

    Post the fore triangle dimensions it will give a better picture. The mast height, the distance from mast to bow attachment point at deck level and the fore stay length. Then give the foot length you want to achieve. Anything longer than past the mast at deck level is rated as a % of the bow to mast length.(foot) I.E. a sail's foot that reaches from bow attachment to mast is rated at 100% beyond the mast it is rated as a % of that length. examples--125%---150%---and so on. ---Geo.
     
  13. peterchech
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    peterchech Senior Member

    Does that formula hold up for a balanced lugsail? I mean, shouldn't the formula be altered to reflect % past the luff of the mainsail in this case?

    I am gonna have to take some more precise measurements with the boat rigged up I think...
     

  14. viking north
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    viking north VINLAND

    Good question--have not worked with a lug sail set up but the general rule is when calculating CE (center of effort) any overlap is disregarded. Maybe someone with more rig qualifications than I could fill you in there. Anyhow when you get more detail info on your requirements post it or e mail me privately and if I have a sail to suit, you are welcome to it -- Geo.
     
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