Question about Catamaran rounding up

Discussion in 'Multihulls' started by getoutside, May 27, 2016.

  1. getoutside
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    getoutside New Member

    Question about sail area to boat size and minimizing a cats rounding up.
    For a school project, I am testing out some theories.
    First I am testing on a scale reproduction

    Specifically I want to find out if the boat will round up when the sail is out at 30 - 40 degrees off the wind based on the following information

    • The boat is a catamaran
    • The sail is mounted at the very front of a catamaran.
    • The sail area is 6.5 Sq. Ft.
    • The lwl is 7.6 feet
    • The beam is 2 feet
    • The boat is 110 lbs.
    • There is a keel dropping down from the center of the boat (stay with me here!) it drops 30"
    • The surface area of the keel is .79 Sq. Ft.
    • Keel weight at the bulb is 20 lbs
    • Rudder surface area is 1.2 Sq. Ft.
    I am trying to figure out if the boat will round up, or at what wind speed the boat may be likely to round up based on this information.


    I know this is just a model boat, but the same principles apply so I thought I would ask here first.

    Thanks and happy Memorial Day!
     
  2. Stumble
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    Stumble Senior Member

    Wait.... A centerline weighted keel on a catamaran? I question the sanity of whoever designed this question. Further there is no way to know with the information presented. You need to know the CE of the sail to even have a chance, and other than being told it is mounted at the very front no further info is given.

    My advice is to share the designer's drugs and it will all come to you, as if in a dream.



    Realistically if the sail is mounted at the very front of the boat it should never round up. It also won't sail upwind very well either unless there are other factors at play (like a kite sail).
     
  3. upchurchmr
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    upchurchmr Senior Member

    What is the sail type/ shape.
    Where is the "keel" located?
    What kind of hulls does this have - round or veed?
    Is the sail at 30 degrees to the Centerline of the boat? Or what relationship does it have to the boat and also the wind.
    Are you interested in whether the rudder can overcome any rounding up tendency?
    I'm assuming the rudder will be controlled as it always is, to counter any round up.

    Unless the CE and CLR are perfectly matched, the rudder will always be necessary.

    How did you ever make such a little boat that heavy?

    What are you actually doing? Is this some kind of autonomous sailboat design?
     
  4. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    Cat

    There have been many successful RC catamarans using a center-mounted daggerboard and/or ballasted keel. Will your boat be radio/satellite controlled or autonomous?
    But I know of none with that kind of weight on a 7.6' waterline. And I know of none with a rudder area larger than the keel/daggerboard area! The beam is extremely narrow for a sailing cat with that waterline length.
    This is so out of the "norm" you might want to post a sketch or two so people can get a realistic idea of what you are doing.
    Good Luck!

    For reference here is a bit on "Saildrone a 19' trimaran with a fixed ballasted keel: http://www.bloomberg.com/news/artic...ilboats-to-collect-research-data-aid-military

    http://saildrone.com/?p=

    [​IMG]

    1. The Wing As wind passes over it, the wing produces thrust. That force is concentrated on its axis of rotation, preventing the wing from spinning wildly.

    2. The Tail A *little tab at the back of the tail can be set to the left or right, causing the wing to rotate a few degrees and maintain an efficient angle of attack.

    3. The Counterweight Positioned at the end of a spar, it adjusts the wing’s equilibrium so its center of gravity is balanced, allowing it to rotate as needed.

    4. The Rudder While in theory it’s pos*sible to operate Saildrone by using only the sail, it’s more efficient to use a *rudder to point the boat where you want it to go.

    5. The Autopilot GPS provides speed data and location. That’s all Saildrone needs to know. Navigation instructions reach the autopilot via satellite.

    6. The Keel If Saildrone gets knocked over, it will right itself because of the keel’s weighting. Its steep angle sheds debris like kelp and lost fishing nets.
     
  5. getoutside
    Joined: May 2016
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    getoutside New Member

    The sail would be "A" sail for an RC laser
    https://www.sailrc.com/product/rig-a-rig/
    Since the hull is narrow the keel is weighted to re-right the boat
    The keel is mounted at the center on both axis of the boat.
    The rudder is from a kayak
    The sail will be restricted to 45 degrees to port or starboard.
    Radio Controlled at this time.
    I made a photo collage of what the basic components of the boat. They are not to scale.
     

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  6. Doug Lord
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    Doug Lord Flight Ready

    Your original question would be more likely to be accurately answered with scale drawings.......
     
  7. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    That position of the sail will not round up, it will round down. This will develop what is called Lee helm. This boat will always want to sail down wind.

    The center of effort of the sail and the center of effort of the Keel have to be almost matched in length wise position along the hull.
    Anything else is uncontrollable, except for sailing dead down wind.

    Get a basic book on sailing.
     
  8. redreuben
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    redreuben redreuben

    What's a foot ?
     

  9. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    Something with 5 toes - ??
     
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