Composite Beam Calculations

Discussion in 'Boat Design' started by LarryMcI, Apr 19, 2010.

  1. LarryMcI
    Joined: Mar 2010
    Posts: 25
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Florida

    LarryMcI Junior Member


    I'm designing a new cross-member which will span the Bows on a catamaran. Off-the-shelf rectangular pultrusions (6.7" x 10" x 0.3-0.5" wall) would do the job, but are are ridiculously heavy. Round pultrusions need even thicker walls to accomplish the same goals. I modeled this triangular cross-section, tapered beam which, intuitively, should be more suitable. Nothing in my engineering library hints at how to calculate beam-deflection for such a beast.

    Construction details: total length = 184.5" Nida-Core H8PP 1/2" honeycomb core, 12 ounce carbon fiber bi-axial cloth and 12 ounce (cf) uni-axial cloth are available for use with vinylester resin (or epoxy if necessary) using vacuum-assisted resin-infusion. An autoclave is not available, so pre-preg is not an option. Both halves are molded from the same CNC-machined mold. An 1/8" joggle is molded-in on top (6" wide) and bottom (full-radius) so the joining laminates will lay flush with the outer surfaces. This also doubles wall-thickness, at these high-stress areas, to 1/4".
    Triangular ribs would have 1/2" Nida-Core with bi-axial carbon fiber laminated to both sides.

    The dilemma is how to calculate beam-stress loads when the vessel is at anchor and pitching violently (hurricane conditions) or retrieving a stuck anchor (vertical pull) using a 10,000 lb rated winch. I want to minimize weight without sacrificing functionality and durability.

    The attached JPEG and IGES file represent the assembly without the joining-laminates applied.

    Any suggestions would be appreciated.

    Larry McInerney

    Attached Files:

  2. sandingRon
    Joined: Apr 2010
    Posts: 1
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Canada

    sandingRon New Member

    I just don't understand why the triangular member is more suited than a round one. Retrieving your anchor will put a downward force, forces at anchorage in rough conditions will be forward or at low angle down the horizontal, other force may be from the forestay.

    So you will have forces is much directions. Hence, a round profile, same capacity is all direction, is more suited I think.

    If you have a small profile that have to be strong, no need to have a sandwich construction since rigidity will be given by the profile itself. Unless you have a very large flat face that you want to be stiff. Your member won't be stronger using a sandwich construction on a small section. Then it much simpler to calculate. Just find the I from the section area.
  3. mcollins07
    Joined: Jan 2006
    Posts: 220
    Likes: 11, Points: 0, Legacy Rep: 166
    Location: Texas

    mcollins07 Senior Member

    Do you have text book examples for sandwich construction in your library and sperate examples for a "T" crossection beam of deflection. Sandwich construction requires considering compression and tensil strengths in different places. Your V can probably be approximated as a T for the purpose of considering those strengths and forces. So, you would need to combine the two methods from the simpler problems to model your beam.

    I would work towards a requirement from the 30 foot drop test. The beam should support the weight of the boat accelerated 30 feet by gravity, as a deflecting force.

    This is just some ideas. I have no certifications in civil engineering or NA.

    ~ Michael
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.