sailing tri-hull structure stresses

Discussion in 'Boat Design' started by allenk, Aug 4, 2009.

  1. allenk
    Joined: Aug 2009
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: tampa

    allenk New Member

    to whom it may concern,

    I am interested in sailing catamarans and trimarans.

    However, I cannot afford the cost of a Ferrier/Corsair or a rebuilt Stiletto.

    There are very close designs available Scarab trimarans and Wharram catamarans, but there also is a large time commitment:

    Then I saw this design of adding hobie hulls to a center hull to make a trimaran:

    This brings up a question for me -
    If you cut the hobie-cat in half and attach the frame halves with the hulls to a conventional hull sailboat for a non folding solid trimaran;

    Will this work? what stresses will you have added to the frame?

    My assumption the upward force that would be applied to a port and starboard stay would be applied and intensified to the location on the hull where the side support is located, correct?

    Therefore needing very strong reinforcment there, perhaps large metal plates to reinforce and spread out the force where the hobie frames to?

    Any other concerns to watch out for?

  2. Petros
    Joined: Oct 2007
    Posts: 2,936
    Likes: 148, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    this is a complicated question because it all depends: the size and weight of the center hull, the size and height of the sail, the overall width of the finished craft. But it can be done, the loading on a Hobie beams is very different than that on a tri-hull.

    It does not mean the Hobie beams are not good enough, it just means you have to check what the loads are on your new configuration will be, as well as the loads in the center hull.

    It would be a way of building a low cost tri, sounds like fun: building a tri from salvaged old boat parts.

    Good luck with it, let us know what you find out. ONce you have specifics of what you are building someone on this list can likely help you out with the loads on your proposed configuration.
  3. allenk
    Joined: Aug 2009
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: tampa

    allenk New Member

    thanks much
    I could see a huge concern is changing the load on a port/starboard stay compared to the same stay moved out from the center over 5 ft, then that upward load with the beam as a lever putting an upward force on the side of the hull - I am guessing large plates need to be added to compensate and spread out that force.
    ..just a laymans guess at it :)

    thanks again
  4. allenk
    Joined: Aug 2009
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: tampa

    allenk New Member

    Petros thanks again -

    I was trying to draw it without a good cad package, and realized an easier way :)

    I am trying to build something similar to this:

    it does not have to fold
    concerned about how to attach the beams/tubing to the hull of an existing boat
    assuming need lots of reinforcement but how much
    current fore/aft stay and sails could be used?

    also realizing most tri-hulls have a very skinny hull at the waterline and wider above. However a standard day sailor is very wide at the waterline - will this slow it down considerably?

    Any thoughts - thanks
    Joined: Jul 2008
    Posts: 63
    Likes: 3, Points: 0, Legacy Rep: 40
    Location: ST LOUIS MO


    Its Been Done

    Go to 'STABILITY" section and see photos from GOTTABSOMEWHERE
    Check back with me for detaials

  6. peterAustralia
    Joined: Mar 2006
    Posts: 430
    Likes: 52, Points: 28, Legacy Rep: 233
    Location: Melbourne Australia

    peterAustralia Senior Member


    I think I follow. You are trying to build a low cost trimaran using recycled hobie floats, rudder and rig. You are concerned about the loads on the crossbeams. You are looking at a non folding design.

    What you plan do to seems pretty sound. In terms of the crossbeams, I think the issue here is sufficient bury. The maximum load that can be delivered by a float is its volume (in theory), so a 400L float can only exert 400kg of force. In practice peak loads are higher from things like wave impact, pounding and collisions, so a good factor of safety needs to be built in.

    I would be looking at finding aluminium tube which has an interior diameter the same size as the exterior diameter of the hobie beams. Then slide them in and secure them. How to secure them? Lots of bolts and epoxy, maybe welding? This bigger aluminium tube you want to be strong. So thicker wall thickness than the hobie aluminium beams would be prudent. I think hobie beams are curved, so how far you can thread the 'sleeve' on is hard to say.

    Tremolino by Dick Newick did exactly as you describe, but he added water stays in addition. So maybe some sort of strut or stay would not hurt. I like the strut idea, but that is just me.

    For a simple center hull maybe a dory shape, easy to make.
    Another idea is to use an old daysailer and make that the center hull. Please find a link with the conversion done with a Soling and Tornado hulls

    Examples of both these ideas are shown here
    what you plan to do sounds feasible and not too over the top.

    a tremolino site
    be quick, all the geocities free websites are closing soon!

    I guess you can look at the Soling tri conversion and ask the builder what strength crossbeams be used, maybe also ask him for advice. what would be the appropriate center hull is hobies where the outriggers, and not Tornados.... maybe a dragon hull?

    A simple dory is easy enough to make,, even I managed. If you need help with working out an appropriate hull shape for your dory, just ask...someone, somewhere will help you out.

    n peter evans
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.