Center of Effort Question

Discussion in 'Boat Design' started by John Larkin, Nov 7, 2013.

  1. John Larkin
    Joined: Jun 2006
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    John Larkin Junior Member

    Hi All,

    Bought plans for Joe Dobler's "Lissa", a 16 ft. oar and sail design. Am perplexed that the sail CE is aft of the daggerboard. Aren't most CE's usually placed forward of the daggerboard? I have attempted to contact someone who has sailed the design but have not gotten an answer.

    Any thoughts?

    John L
     

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  2. Petros
    Joined: Oct 2007
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    Petros Senior Member

    the center of lateral resistance includes the below water portions of the hull as well as the dagger board. Some designers also include the rudder area and any fin or skag as well. This can put force on the tiller to keep it in a straight line but will also act as a safety feature for beginner sailors, so the boat rounds up into the wind if the tiller is dropped.

    the forces over the sail and hull are more complex than just centroids of the areas. Using the centroids usually get you close enough, but usually some adjustments to mast step and rake have to be made, a good designer will adjust his plans based on actual sailing performance, and it may no longer line up with the calculated centroids of the areas.

    The best way to know is to find someone that has actually sailed one rather than just 'eyeballing' the plans. If built to those plans, it may or may not behave well on the water, if not there is always ways to adjust and tune the rig to make it perform better.
     
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  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The CE normally is located just forward of the CLP, which on many sail plans places the centerboard's lead edge just under of just aft of the CE, but with this boat, things are a little deceiving. Because the appendages are spread out more then usual and the CLP is farther forward then would be usual, she balances on her forward located sail plan. It's not the centerboard position, so much as the CLP. If you print out a copy of the hull, with the rudder and the centerboard all the way down, then cut out this profile, from the waterline down (it's important to cut the rudder in half vertically on this boat), you can very accurately estimate the CLP. Once the underwater profile is cutout (you're not making dolls, so the other half will just have to get over it), fold parallel along the LWL, which will provide enough stiffness to the paper to support itself. Next, balance the underwater profile on a pin, inside the fold. Move it back and forth until the profile is level. Poke the pin through the paper then open the paper up. A vertical line through this point will show where the CLP for this underwater profile is. Place this printed profile over another of the full sail plan and carry this line straight up, it'll probably fall a little aft of the CE. The distance away from the CE is the lead, which on this boat will likely be about 15% - 17%, as a percentage of the waterline length.

    There are a lot of folks that'll tell you to place the centerboard or dagger directly under the CE, but this isn't entirely correct. They tell you this, knowing the CLP will shift aft once you climb into the boat and she trims down by the stern. Each boat, depending on shapes used in the hull, appendage arrangements and sail plan will usually balance with some "positive lead" to the CE. Most boats need a positive lead (if they expect to get to windward), but some can work with negative (pretty rare).

    Petros is correct, this is an over simplification, but will get it close enough that tuning on the water will dial it in. With the little hull cutout trick, you will find she does balance out. Every single fin and rudder developed at NASA until after the Shuttle program received this type of physical test, so it's good enough for you.
     
  4. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    I can see two possible reasons why the centerboard is so far forward:

    1.) So it's out of the way of the rower, when under oar power, and
    2.) So the boat will balance well when reefed.

    With a Bermudan sail, the CA moves forward as the sail is reefed.

    If the boat had good balance under full sail, it could suffer from moderate to severe lee helm when fully reefed.

    So the designer draws it so it has the best balance when reefed, which lets the crew know it's time to reef when the bow starts pointing up wind uncontrollably. This way the boat ends up with the best balance when the conditions are most severe.

    This boat would make an excellent "raid" boat.
     
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  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The board on this boat (really a dagger, not a centerboard) can cant aft, which will accommodate weather helm and with that rig and mast rake, the CE will not move nearly as much as you'd think reefing. The rowing position is very likely another reason for this arrangement.
     
  6. boat fan
    Joined: Sep 2008
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    boat fan Senior Member

    Nice , brief discussion on balance guys , excellent !

    Quite a nice , simple little boat you are building too John.
     
  7. John Larkin
    Joined: Jun 2006
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    John Larkin Junior Member

    Thanks for the useful input. Another question:I want to swap the gunter rig the boat was designed for with a balanced lug sail, mostly because I think it looks shippier. I have figured out the CE for the lug (I think) and placed it right on the same CE location as the original design. i have also increased the size from 60 sq ft to 70 sq ft. Does this eyeball okay to you folks? Think I am over-canvassing this narrow (49") boat with the bigger sail?
     

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  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I think you can manage a 15% increase in area, though you'll be "sailing" her a lot more then the previous model. I do think it might be too much on this skinny gal, but mostly because the lug has more power up high, compared to the gunter. A skilled sailor can hold her down or dump as required in the larger sail plan. The CE location may require some on the water adjustment, so allow for an adjustable partner or mast step, so you can rake it around, until you find the sweet spot.
     
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