stresses in hulls of catamaran

Discussion in 'Boat Design' started by kmeastman, Feb 28, 2008.

  1. kmeastman
    Joined: Dec 2007
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    Location: Omaha NE

    kmeastman Junior Member

    I have seen threads that discuss how to calculate stress in crossbracing of catamarans like this one: power and sail

    Those stresses must be transferred to the hull somehow. How do you calculate how this stress is transferred so that you know how big to make your brackets or how thick to make the hull. I know you would normally put a flate plate on either side of the hull to spread the load out when attaching things that will transmit a lot of force, but how do you know how big to make them?

    Any help is appreciated,
  2. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    you have to have a connection (or a group of connections) that combined will transfer the bending moment, and all other possible loads, from the beam into the hull. It is just a matter of following a load path through each component, and sizing each of the components to handle the design load. It helps if you draw a "free body diagram" of each component in the load path and try to determine the size and direction of every possible load in every direction (and in rotation) possible to size and design the component.

    Often a plate or flange must be sized by the number and size of fasteners used, if you only have a few (or one) fasteners, then you need a fairly thick and stout plate. If you have a lot of small fasteners, than you can use a much thinner (and lighter) plate since the total load is distributed over many fasteners.

    When you have a doubler or reinforcing thickening of a skin, the thickness has to be strong enough to take the point load of the fasteners, and the doubler size has to be large enough so the load is spread out over enough area that the adjoining thinner area can take the load. This is true when you use bonding with adhesives (as in a fiberglass hull) or if you use screws, or a combination of the two.

    Also make sure you accommodate all load directions. Where a beam meets a hull you not only have a pure bending moment but also rotation in two other axis, as well as tension/compression loads in three directions. You have to design each connection for possible loads in all directions, and moments in three axis of rotation. So you have to review the maximum possible loads in each direction, and the maximum possible moments in three axis of rotations. Often when designing for the primary axis, a simple review of the loads in the other directions will show there is more than adequate strength for the others. But not always, that is why you have to examine the connection in each direction, or it may fail in a way that did not occur to you. I have learned this the hard way, and many big manufacturers have as well. Do not assume the secondary loads in other directions are negligible.

    And then make sure where you attach these components to the hull it is strong enough to take the transfered loads. It would not be good to have your joint perfectly strong but then have the hull fail where it attaches to the hull. Again it is a matter of spreading the load out over a large enough area to accommodate the design loads.

    One last issue to consider is total defection. On short, shout components the defections are usually small enough to ignore, but on large/longer parts, like the beam or a mast, or thin components, like the hull skin, deflections could be so large they can fail by introducing loads and/or rotation angles to the connection that you did not accommodate. So either you have to add strength (and weight) to get additional stiffness to limit deflection, or you have to design the connection to accommodate the rotation angles without adding additional stress to the connection.

    As you can see detail design can be the most time consuming process part of the process. That is why most designers simply copy what other people are doing rather than developing new details.

    I hope this helps.
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