Controlling draft on flat cut sail?

Discussion in 'Boat Design' started by dustman, Aug 27, 2020.

  1. dustman
    Joined: Jun 2019
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    dustman Junior Member

    Can the draft of a flat cut sail be controlled effectively with vertical tension on the luff and leech?
     
  2. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    Sure. The less stretch in the sail cloth, the less effect it will have, but there will always be some stretch and shape to play with.

    Can you offer some specifics? What is this flat cut sail being flown on? How big is it, why do you have your sail cut flat, then want to flatten even more? Is this for racing or cruising?

    -Will (Dragonfly)
     
  3. dustman
    Joined: Jun 2019
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    dustman Junior Member

    I'm trying to build a decently performing small catamaran as simply and economically as possible. I'd be using it to go from Corpus
    Christi, TX along the coast to Florida then on to island hopping in the Bahamas. It would be a gaff rigged cat ketch, sails: 18' luff, 60" foot, 30" gaff(approx 70ft2). Free standing masts. I figure a flat cut sail would be much easier to implement, I could use one piece of material for the entire sail, with reinforcements sown on for reefing and other attachments. I was thinking of 4oz dacron. Not sure if that will have a low enough modulus to get decent shape in lighter winds though, but too much stretch might cause poor shape in higher winds. My thinking is that tensioning the entire sail vertically may decrease the draft when needed and vice versa. Looking to have as few lines as possible. Here is a simple sketch of my thought on how to rig the sails:
    IMG_20200827_175139571.jpg

    Thoughts?
     
  4. Will Gilmore
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    Will Gilmore Senior Member

    It looks good to me. Make it loose footed so it is free to belly out when you loosen the outhaul. I suggest that at the gaff, as well.
    I see little in that design that would benefit from any shaping. It's a pretty straight forward 2D foil. Maybe think about half-battens to move the camber forward, or allow for loading of a vang to tension the roach in stead of the luff.

    -Will (Dragonfly)
     
  5. Zilver
    Joined: Nov 2007
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    Zilver Junior Member

    Your drawing is very similar to a Wharram wing sail. They are cut flat, but the wide luff pocket around the mast provides the camber (and quasi wing shape). These sails are very practical and easy to make.
    Hans

    PS this would work better with a stayed mast, as the bend in the unstayed mast will alter the sail profile. Also a stayed mast is easier and cheaper on a catamaran.
     
    Last edited: Aug 29, 2020
  6. messabout
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    messabout Senior Member

    You had better have some more sail area if you plan to "island hop" in the Bahamas. If you have only the main that is pictured, the boat had better be small and very light. Going across the Gulf stream in such a boat is an adventure that needs careful planning and proven equipment. At times the stream is as calm as a pond and often it is a vicious expanse of water to challenge the best of small boat sailors. Why not use a proven sail plan for your boat? What size cat are you thinking of?
     
  7. dustman
    Joined: Jun 2019
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    dustman Junior Member

    There will be 140ft2 of sail total, approx 1200# displacement, making for 19.8 SA/disp ratio. 32' LWL x 16' beam. I am actually thinking of bumping that up to 160ft2 for better light air performance without the need to carry extra sails. Before you smash me on that small a displacement for that size cat, I have run the numbers on all the materials. I'll figuratively be tent camping on a giant hobie 16.

    As far as safety, I have read up thoroughly on that passage and understand the threats. I won't be crossing with wind out of the north, that's for sure. Also, I've been looking on Windy.com almost every day for the last year or so, studying weather patterns around the world, but especially around the Bahamas. I'm risk averse, so won't be taking any unnecessary chances.

    With the sail plan, I intend to make it as simple and as bulletproof as possible, so I can pull it down, put it up, and reef easily and quickly in any condition I might find myself. I don't want any extra doodads to complicate things or break, and I don't want to constantly pour money into the boat like a lot of sailors seem to do.
     
  8. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Buy a complete second hand rig from a Hobie 18 or Tornado and modify it to enable standard slab reefing. It's the cheapest and best way forward, you don't have to mess with making your own rig and the sails are guaranteed to work. If you go for a Hobie rig search for an early all Al one, not the comptip version.
     
  9. Will Gilmore
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    Will Gilmore Senior Member

    I use to trailer a Hobie 18 all up and down the East Coast of the US. Sailed off Key West in 25 knot winds. I had to pull my mask out of my dive bag because we were going through the waves, not over them. It was a blast, but having to put up with that all the way to Bimini from Miami would be really tough.

    In 30 knot winds, the day before, I couldn't even get the peak ring to catch on the mast head hook. The wind held it off while I tried to set the main. Thanks for roller furling headsails and an anchor or I wouldn't have been able to stay off the bridge at Bahia Honda Key, while I tried to set the main over and over. I suppose it was for the better that I couldn't get the sail up. The winds were incredible.

    -Will (Dragonfly)
     
  10. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    What part of an "un-stayed gaff rigged ketch" is either bullet proof or simple?

    As Rumers said

    GET A USED HOBIE RIG.
    It will be bullet proof simple and easily reefed.
     
  11. dustman
    Joined: Jun 2019
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    dustman Junior Member

    NO
     
  12. dustman
    Joined: Jun 2019
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    dustman Junior Member

    Part of the whole point of this endeavor is to build everything from scratch and to design something uniquely my own. I'm not ignoring all the input, in fact my design has changed a lot since I first decided to do this, based on all the input, reading, study, calculation and thought. When I hear "get a used hobie rig", I hear "give up on yourself". Not happening...

    An unstayed mast for such a small sail seems much simpler than lines going every which way and having to build all those attachments into my design, not to mention the maintenance and potential failure points of such a system. What's complicated about a gaff rig? What's complicated about a cat-ketch? I'd rather not deal with a jib, poles, 2 sheets, having to adjust it just right to make it work with the main, etc. With a ketch I can balance the boat better, have a lower CE, a mast that isn't 30 feet tall, and I can spread the mast loads between 2 masts, simplifying the design. I can mount the masts on already existing load bearing structures.

    If there is a serious flaw in my thinking, let me know, otherwise I'm moving forward with it. What I do want is to make sure I design a relatively efficient aerofoil, but am willing to compromise a bit on efficiency for simplicity of construction, operation and maintenance. This is what I'm asking this community to help with. I don't mean to sound ungrateful for all the input, because I value all the input I have received on these forums immensely and have learned a lot.
     
  13. gonzo
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    gonzo Senior Member

    I don't understand why you would make a flat cut sail.
     
  14. Rumars
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    Rumars Senior Member

    Ok, let's focus on the sails. You have two of them so 2 sheets, and you have to adjust them just right to make them work togheter, there is no way around it, it's how sailing works. They don't have chamber so your only way of inducing it tensioning the downhaul, sheet, and peak hailyard. Your proposed vang will only bunch up the foot of the sail. The whole concept is unworkable unless you use some form of sail shaping. It's either an airfoil or a flat sheet, you can not have both at the same time.

    The only way to put unstayed masts on catamarans in a simple and easy way is to go biplane. Ketch requires a support structure for the mast partners to take all the loads. Does you cat have a cabin or are you planing a pyramidal strut structure?
    A gaff rig requires a stiff spar, and a cat has massive amounts of rightning moment, the resulting masts will be heavy (unless you go carbon).
    You want 18 feet of luff, you need to add at least 2 feet on top so the hailyards can work, plus at least another 2 feet for the bury. That's 24 feet of mast from the start, and probably more in practice, all for 70-80sqft of sail. And you need two of this masts.

    It's a noble cause you pursue, but it has nothing to do with the words simple, cheap or efficient.
     

  15. gonzo
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    gonzo Senior Member

    I don't think you are hearing right. It means that you are reinventing the wheel and claiming square is better than round. Your design is negating millennia of research and development of sail design. If you want a flat cut, which really makes no sense in this century, the maximum ratio is about 1.5:1. Instead, you are drawing a modern extreme high aspect sail, which also makes no sense.
     
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