Lightning bottom shape

Discussion in 'Boat Design' started by mikes49, Apr 4, 2013.

  1. mikes49
    Joined: Apr 2013
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    Location: Vermont

    mikes49 Junior Member

    Hello -

    I have been working up to building a wooden Lightning Class sailboat for a couple of Vermont Winters. It will be built in accord with the class specs and with traditional materials such as WR cedar planking over mahogany and cedar frames, but with a modern aluminum rig. The edges of the planks will be hard glued together and sealed on the outside with epoxy like Nickels and Holman did in the 1960s.

    The class has established offsets for each Station but with tolerances that allow between 3/8 and 1/2" of variation at each station. The Lightning is a hard chined boat with a rounded bottom arcs between the keel and the chines that are between 8 and 15 ft. Building a fast Lightning involves finding best bottom shape using the variations allowed in the offsets and the bottom arcs, and building it to the 700 lb class minimum.

    While exact specs of factory Lightnings are a bit of a trade secret, I have measured some boats and have a good idea how they are shaped. For example, the centerline height is generally as low as is allowed in the ends, low at the mast step, and high between the mast step and the stern. The chine height is generally as low as allowed in the ends and high elsewhere. The chine half-breadths are generally narrow except for at the ends.

    The bottom arcs on factory boats are generally 8 ft from the bow to midships. In between the midships and the stern, they transition to either 12 ft or 15 ft in the aft sections and then back to 8 ft at the transom. Most of the fast racers use hulls that go to the 15 ft arc. On the factory boats, they take the 8 ft arcs back to Station 5 or 5 1/2, and gradually flatten them to 15 ft at Station 7 1/2, which is about the mid point between Station 5 and the transom.

    I have been lofting my boat manually on my 20 ft basement table and also using the Rhino computer program. At present I have lofted my Lightning to as much as possible duplicate the factory boats with the 15 ft arcs at about Station 7 1/2.

    However, I am seriously considering moving the 15 ft arcs further forward in my boat to Station 5, so that it would be flattest in the middle 1/3 of the boat, and then letting Rhino create a surface that would transition to 8 ft arcs at the ends. This would make my boat somewhat flatter in forward sections and rounder in the aft sections. A very smart friend tells me this would reduce both the rocker and resistance and change the volume so that it would be faster. The same friend also recommended making the chines as low as possible from midships to the stern, and also making the chine H/B as wide as possible along the same run, which would make them lower and wider than factory boats.

    Anyway, as you can probably tell, I am not a sailboat designer and just have the most basic grasp of the things I have outlined above. I am also fully aware that I am fighting an uphill battle to make my wooden Lightning competitive, but the work on this project has helped the Vermont Winters fly by since I retired, and I won't be crushed if my Lightning doesn't turn out to be a rocket ship.

    I would appreciate any comments on my plans, and especially advice on whether moving the flatter arcs to the middle of the hull and also changing the chines would be advisable.

    Thanks very much.

    Mike
     
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  2. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Curious...do they measure a lightning when its rig is set up or without a mast

    Can you bend the boat into hull measurement condition ?

    This was a typical trick for IOR race boats
     
  3. mikes49
    Joined: Apr 2013
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    mikes49 Junior Member

    As I understand it, they measure the hull without the rig set up so I don't think you would be able to bend it to the measurements.

    Mike
     
  4. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Makes sense.

    With race boats we would haul the boat...rest on keel, block up.. put screw jacks under the transom edge and bow knuckle then crank the keel off the ground with the screw jacks...bend the boat to make it look short and heavy


    You should talk to Lightning class aficionados with wood boats for advice.

    Measureng a plastic boat may give insights. A wooden boat may change shape differently with age. In the perfect world it would change into a fast shape.

    Only a Lightning guy would know these secrets
     
  5. michael pierzga
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    michael pierzga Senior Member

    Perhaps you could write Sparkman and Stephens and ask ?

    Designers take pride in their work. The Lightning is a classic boat.

    Perhaps they will pass on tips ?
     
  6. Petros
    Joined: Oct 2007
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    Petros Senior Member

    it seems to me making the bottom more flat would allow it come up on plane sooner, and that is the direction your proposed changes will take it. So you appear to be doing the right thing in terms of making it a faster boat.
     
  7. mikes49
    Joined: Apr 2013
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    mikes49 Junior Member

    I wonder why the factory Lightnings haven't flattened the middle of the bottom. Would there be tradeoffs in performance other than when planing? Would the changes cause performance in other areas to suffer?

    By the way, I did write an email to Sparkman and Stephens. I will be interested in the reply.
     
  8. Petros
    Joined: Oct 2007
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    Petros Senior Member

    Yes, the way I understand it, when not in planing mode a flat bottom has more drag and is less manuverable. More rocker for example will make it turn faster.

    Typically there is a trade off between displacement performance and planing performance, a pure planing hull is draggy in displacement mode, and a hull for minimum drag in displacement mode will be difficult to get up on plan, so will be slower until there is enough wind to get it on plane. The more the boat can plane the faster it will be, but if the wind is real light, it will stay in displacement mode.

    It appears the trend in dingys is to make them come up on plan as early as possible, and hopefully keep it there. So a wide flat bottom with sharp edges and transom will allow it to plan sooner, at the expense of displacement speed performance.
     
  9. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Do Lightings typically plane?
     

  10. mikes49
    Joined: Apr 2013
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    mikes49 Junior Member

    Yes, Lightnings will plane in a good wind. Not sure exactly what wind velocity is needed. Also not sure if that would be just downwind with the spinnaker flying or not.

    I wonder if the factory Lightnings have kept the arcs rounded in the front half of the boat and near the transom in order to strike a balance between heavy and light air, so they can plane in a good wind but don't lose as much light wind performance as they would have if the arcs had been flattened further?
     
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