Lug Sail design for a beginner

Discussion in 'Boat Design' started by cpodest, Aug 1, 2014.

  1. cpodest
    Joined: Mar 2014
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    cpodest The Beginner

    Dear Guys,

    I'm working in the construction of a 3.4mts flattie. Solved the main part of the wood working, I put the focus in the sail design.

    As u can see in the attached pics, the LCR of the hull is located at x=1.93 mts aprox. I decided to put the CE of the sail 15% at bow from LCR ... so the CE of the sail should be located at x=2.16mts (all the mesurements from aft)

    Taking a look at similar boats, similars displaced vol and similar LWL... I decided the sail area will be between 7 and 8 sqrt meters.
    (my design: displ vol 0.250 mts3, lwl 3.150mts)

    I have two main problems ...
    - What kind of lug sail should I used? Balanced or Standing?

    My idea, is locate the mast in x=2,7mts... and design a lug sail in order to locate it's CE in the needed place (2,16 mts).... But, once located the mast, can i fit the position of the sail? I mean, move the sail aft or bow (some inches) in order to locate its CE in the right place?

    On the other hand
    Does anybody, a good reference for a balanced or standing lug design? There is a lot of information about how to calculate is center of effort, but i didn't find anything about proportions on the head/foot, or luff/leech???

    In order to have an idea, I would like to have a sail like the pic.

    I would like to have this old look with the Lug sail in my boat, so please, not recomend other kind of sail.

    Any question, recommendation or comment are welcomed¡

    Thanks a lot for your help
    Best Regards,
    Carlos
     

    Attached Files:

  2. gonzo
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    gonzo Senior Member

    The CE of lug sails change considerably from one tack to the other, unless you rig a tacking sail. That is, you pass the yard from one side to the other when tacking. On a small boat, that is not really practical. The more forward sail area forward of the mast, the more the CE will shift.
     
  3. Skyak
    Joined: Jul 2012
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    Skyak Senior Member


    That sounds like a big sail for a small (short) boat. What are your references and are you doing the righting calculations? You don't show the boat or beam so I can't say anything for sure.

    Here is a reference in the rough size.
    http://www.andrewlinn.com/plans/120301_michalaklug/index.htm
     
  4. cpodest
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    cpodest The Beginner

    Sorry Skyak,

    Boat's Beam is 1.3mts

    Could you please recomend me a method to calculated the sail area?


    Thanks
    Carlos
     
  5. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    The Lymington scow uses a standing lug. But I think you want a balanced lug. I doubt that you want a dipping lug

    Remember that, like a semi balanced rudder, you should not have more than about 20% of the total sailarea in front of the mast/pivot point. Otherwise the sail will be over balanced

    You probably also want to check the area and balance when reefed (if relevant)

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  6. Petros
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    Petros Senior Member

    you can fine tune the sail location by canting your mast forward or aft a bit. this can be done a number of ways, shims or spacers at the mast partner, or with an adjustable step or even a step with multiple sockets (forward and aft) for the base of the mast.

    if your mast is will have stays than you just need to have enough adjustment in the stay tension means to adjust the upper mast location forward or aft. you also can trim the mast forward or aft a bit by adjusting where the lower boom crosses the mast.

    Most small boat builders do exactly what you are doing: work it out on paper as accurately as you can, and than adjust it as needed after you get it in the water. this means you need to plan for a means to move the sail's CE forward or aft enough to trim it. If you end up redesigning the sail you will have to be prepared to relocate the mast as well.

    a swing keel also allows a small amount of adjustment to the balance (which is the only advance I think swinging centerboards have over a simple and light dagger board). And of course making a larger rudder, or cutting down the one you have, to get it to balance the way you want.
     
  7. Skyak
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    Skyak Senior Member

    I can tell you a rough starting point but I think you need to discuss your application with someone experienced with lug sails and hopefully familiar with local conditions.

    On dingys I start with the sum of the moments about a point on a freebody=0
    In english units (sorry) we estimate one pound of force/ft squared at 15 knot wind speed multiplied by the vertical distance between the centers of effort of the sail and the daggerboard should be equal to the weight of the crew times the horizontal distance between their center of mass and the center of flotation.
     
  8. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Or you could consider a sliding centerboard, such as that used by Frederic Fenger on Yakaboo.
    ALONE IN THE CARIBBEAN by Frederic Fenger, CHAPTER I, THE "YAKABOO" IS BORN AND THE CRUISE BEGINS

    [​IMG]
     
  9. messabout
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    messabout Senior Member

    ^ The mizzen looks like a sprit sail and the main looks something like an odd combination of Gunter and sprit. Neither of them appear to be lugs.
     
  10. Milehog
    Joined: Aug 2006
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    Milehog Clever Quip

    I'm fond of my balanced lug rig. Absolutely simple and you can adjust the CE in minutes.
     
  11. cpodest
    Joined: Mar 2014
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    cpodest The Beginner

    Skyak,

    Thanks very much for your help. I'm taking in count your advices. Sadly, there is nobody with experiencie in Lug sails near to me, that's why i'm looking for help in the forum. I have never seen this kind of sail here, just in the internet and some forums like this.
    I'm reconsidering the sail's area considering the momentums as you said... and also i'm looking the information about average wind speed to do the calculation.
    I can do some sketch of my design... could you be so kind to take a look it and give your comments?

    Thanks a lot for your help
    Best regards,
    Carlos

    Do you have
    I really apreciatte your help¡
     
  12. lewisboats
    Joined: Oct 2002
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    lewisboats Obsessed Member

    I would aim for a sail area between 5.5 and 6 sq mt, with at least 2 reefs. If you design your mast partner to allow you to rake your mast you can build in sufficient "room" to adjust your CE to whatever you want. I would put the foot of your sail a bit higher for better visibility and room for the boom to be lower if you rake your mast aft. Also... if you plan on having a skeg along with whatever your fin or board will be, don't determing your starting CE before adding those to your design. Don't forget to add about 50% of your rudder area to the CLR mix too.
     
  13. cpodest
    Joined: Mar 2014
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    cpodest The Beginner

    Rake the mast

    Hi Lewis,

    Thanks very much, in fact, the boat will have a skeg, i will consider it in order to get the CLR , plus 50% of the rudder area... I was considering only the hull area.

    why rake the mast?... sorry for my ignorance, but why i can't only move the yard and the boom aft or bow? is there any others advantages raking the mast?

    On the other hand, I have seen a lot of Lug Sails... some looks very beautiful... and others not so much .... is there any formula, any kind of proportions Luff/leech, head/foot.. etc? Skyak gave a very useful link where a sail is made.... i would like to have more information about the design..... I have a lot of question and this kind of sail is not comun in Brazil ...

    Really appreciate your help
    Best Regards,
    Carlos
     
  14. lewisboats
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    lewisboats Obsessed Member

    There are a few benefits but the main one is adjustability. Having an adjustable mast rake can allow you to fine tune your balance rather than trying to hit it bang on with a fixed position. As your sail ages its performance will change also, so you can use rake to help keep it performing well for as long as possible, along with luff tension, out haul tension, and tension of the out haul on the spar. It also raises the clew end of the boom as the sail goes to leeward and the boat heels so there is less likelihood of it striking the water. As far as shape goes... there are some variables but mostly it determines the height of the CE of the sail... a lower CE provides less heeling moment but usually affects upwind performance. Peaking the yard higher will help with upwind performance but will also raise the CE and the CG and increase the heeling forces. Small boats are so limited by their waterline length... and in yours' case, the hull shape, that speed difference won't be much of a factor (you won't be getting up on plane). For your case, were it me, I would lower your peak somewhat and put a little roach with 2 or 3 short battens and run at about 6 mt sq for sail area. You'll have the power, you can move the mast a couple of inches forward due to the CE being further aft and you won't have the weight or CE as high as with the high aspect sail in the Photo. Yours isn't too bad, but I would still drop the peak down and aft by about 15-25 cm.
     

  15. Milehog
    Joined: Aug 2006
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    Milehog Clever Quip

    No need for adjusting the mast rake when the yard and boom (assuming a balanced lug) are able to be moved forward or aft by adjusting lashings.
    I'd never consider battens either on my balanced lug rig. It's genius is it's simplicity and surprising performance.
    KISS = Keep It Simple Stupid
     
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