Using aluminiun masts sections as tranverse structure instead of bulkheads

Discussion in 'Multihulls' started by Roni, Jul 20, 2020.

  1. Roni
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    Roni Junior Member

    Hi there,

    I was talking to a boatbuilder and he mentioned that in a few projects he had used aluminium mast sections in place of composite bulkheads for the transverse strucuture for sailing and power cats around 35 - 40 ft. I tried to get him to teel me how he did de concection between hull and aluminium beam but that part he didn't tell me in details. Are there any examples out there I could check out?

    Thanks a lot
     
  2. gonzo
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    gonzo Senior Member

    If you mean a compression strut, that is fairly common.
     
  3. Roni
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    Roni Junior Member

    No I mean in place of the transverse bulkheads. To resist the transverse bending moment
     
  4. TANSL
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    TANSL Senior Member

    What are those bending moments ?: Could you show a sketch that allows us to locate those moments?
    Of the efforts that a bulkhead is supposed to undergo, which ones do you want to solve through pipes?
     
  5. Richard Woods
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    Richard Woods Woods Designs

    you mean like on a beach cat. Very common on thousands of bigger boats as well. Certainly up to 80ft or so. And lots of trimarans used mast sections before alloy got too expensive to extrude and carbon/epoxy got a lot cheaper

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  6. Roni
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    Roni Junior Member

    Hi TANSL,

    The transverse bending moment (TBM) is around 34 kNm, pitching connecting moment (OCMN around 77 kNm. I want the pipes to be the main transverse structure holding the hulls together and resist the TBM and PCM
     

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  7. Roni
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    Roni Junior Member

    I guess because of our currency exchange rate that maybe not the same here. Most quality fibers (glass and carbon) are imported. Our currency is not doing so well. Aluminium, on the other hand, is local, so less susceptible to the weak currency

    Do you know how the hull and beams were connected in these bigger boats?

    Thanks
     
  8. TANSL
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    TANSL Senior Member

    Forgive me Roni because I did not understand well what you wanted to do. I thought you were going to replace a transverse bulkhead with a piece of mast.
    For the transverse beams that join the two hulls you can use any kind of tube but the masts usually have various elements inside that do not contribute to reinforce the tube against the loads it is subjected to but increase its weight. So I would recommend completely hollow tubes, with a moments of inertia (or strength modules) with respect to the X-X and Y-Y axes that withstand the bending moments that you mention. Depending on the value of those moments, an elliptical section, similar to that of a mast, may be the most appropriate, although it would be more comfortable to put circular or square sections.
    The transverse beams between the two hulls must be embedded in a web frame of the hull which does not need to be a transverse bulkhead.
     
  9. fallguy
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    fallguy Senior Member

    I have done it using Wood's design. I can't go into great specifics because it is proprietary and would be wrong for me to share Richard's work.

    Basically, you build a socket of solid glass that fits the mast section.

    Rotational forces are a concern; so the mast must be held fast from rotation. This means a round mast is a no-no unless you can lock it in say via the hardware track, etc.

    The sockets are laid up with precise locating in the hulls.

    Then a mechanical fastening is used.

    We used 1/4" thick or so 316 stainless to create strapping. Those straps are then held fast with studs bonded into the hull.

    For larger vessels, it must be calculated well and a safety factor.

    Here is a picture of the setup I built. The thing could have been improved upon if we had rotated the mast upside down fir a better fitment and less chance of rotation.

    It should be noted; these need silencing or they will make noises.

    Again, consult with a NA for lamination schedule; this is a critical structure in the vessel.

    This picture is at a work in process point. The glass standoffs were bonded later to deal with strapping variations and to be able to torque down the strappings.

    A991FE75-25DE-4F26-8A5E-93941BA4716F.jpeg
     
    Last edited: Jul 21, 2020
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  10. fallguy
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    fallguy Senior Member

    I will add that laminating these sockets into the hull was quite a challenge. This work is not for the weak willed.

    We built a jig to hold prewetted mat backed glass; fyi.

    Richard, please let me know if I have gone too far sharing your expertise here and kind regards. Good things around the corner!
     
  11. Roni
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    Roni Junior Member

    Hey Fallguy,

    Nice job. These sockets are on top on a web frame? Are they above the main deck?

    Thanks
     
  12. fallguy
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    fallguy Senior Member

    59E14298-3305-4683-9DBD-579FF39C86D2.jpeg
    I don't know term web frame. They are above a bulkhead; perhaps same thing. Mounting the sockets above the bulkheads made installation easier since we only did one side each day. We did four total sockets, so 8 total jobs. We might have done a couple of them in one day, but the job was physically taxing. The sockets are mounted just under the decks. The sockets were built with flanges and a vacuum mould and one flange was always cut off to deal with deck height deltas. This boat actually has an intermediate socket midship for n aft that serves as a cabin seat as well. So we use four total beams. Three mains and a netting. The middle beam is laid sideways and the sockets are all around and there is a thru bolt. We actually slide the middle beam in from the port hull which is holed through. It requires duct taping silencing on and some electrical pull lube, but it goes together better than trying to install a middle beam with a crane. The middle beam sockets are all around. Those were entertaining to build. We used a method Richard Wood's advised.

    If you look close, you can see the forward socket and the middle socket which is tube-like. This is the star hull, so middle beam socket is a blind hole. The middle socket could not be mounted above so is alongside.

    Please be extra kind to Richard. This is all his engineering, small modifications may habe happened, but the core of the work is his. And if this helps you in some way; he is due at least your gratitude more than me.

    This is the Skoota 32 demountable design. Perhaps Richard's greatest pain in the rear ever, but is turning out to be a nice boat. My only worry now is how we will fair with the weights at the end. Our second hull is almost ready for paint, about a week away. The cabin exterior is done, so we are going to put the boat together August!
     
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  13. Roni
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    Roni Junior Member

    Thanks a lot Fallguy, and my regards to Richard too. I imagine all that knowledge didn't come easy.

    The bridgedeck in such a case would have to be supported by the beams, correct?
     
  14. fallguy
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    fallguy Senior Member

    yes

    in this case; the entire cabin is supported by the beams; one beam is on the edge; so the xx and the yy are opposite
     

  15. Roni
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    Roni Junior Member

    If not asking too much does the bridgedeck simply sit on the beams or are they bolted together?
     
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