Building 17' wide 14' long deck

Discussion in 'Multihulls' started by DamianW86, Jan 7, 2022.

  1. DamianW86
    Joined: Nov 2021
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    DamianW86 Junior Member

    Hello!
    I am undertaking a project of making a deck for a project macgregor 36 cat.
    the boat's beam is 20' total. that's about 17' of deck space that will span between two hulls.
    The over length will span 14'.
    I plan on using epoxy glass laminate with divy core.

    Here are some questions I hope to get answered!
    - how many layers of glass on top/ bottom
    - should I run foam `studs` below deck for additional support?
    - how thick of a foam core do I need?
    - how dense of foam core should I use?


    deck will be reinforced with above deck full width span bench on front and back of the deck!
    I look forward to some help!!
    thanks!
     
  2. TANSL
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    TANSL Senior Member

    I don't know if you are talking about the normal deck, the wet deck, or both at the same time. In any case, that part of the structure of a catamaran is the most complicated to calculate and it would take a lot more data than you are providing us with. It would be necessary to know the main characteristics of the boat and have a cross section of it where the structure, the cross beams, their connection to the hulls, secondary longitudinal stiffeners, etc.... could be appreciated.
    Is there a special reason to use sandwich construction?
     
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  3. patzefran
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    patzefran patzefran

    Obviously, he want to replace a trampoline for a hard deck !
     
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  4. TANSL
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    TANSL Senior Member

    Yes, probably, but it all depends on how he wants to join the deck with the hulls, if he wants to join it. Maybe he also wants to put some longitudinal stiffener under the new deck. Anyway, the first thing to do is design the structure and then, only then, calculate its scantlings.
     
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  5. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    I think that a 'standard' Macgregor cat is only about 16' wide, so this sounds like a lot of customisation - have you fitted new aluminium crossbeams (in the same locations as the original beams) to make her wider?
    Can you post a photo or two showing the hulls as they are now?
    The scantlings of the laminate will depend on the load that it has to take - will you be carrying passengers on this new deck? If so, what would you anticipate to be the maximum number that might be on this deck at one time?
    How will the deck be attached to the aluminium cross beams and to the two hulls?

    Can you elaborate a bit more about this please Damian?
    Will these benches be located above (or attached to) the main cross beams, or will they be independent?
     
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  6. guzzis3
    Joined: Nov 2009
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    guzzis3 Senior Member

    If it were me I'd use 1/2" 80kg/m^3 foam with at least 600 gsm 45/45 either side then make L sections in solid glass, alternating layers of 45/45 and uniaxial running length wise and glue them to the underside of the foam deck longitudinally on about 12" centers.

    That will have some spring but it will be strong enough and fairly cheap. UTEK in china can supply stuff much cheaper and if you are in the USA apparently there is a retailer there with good prices.

    Any PVC foam to that spec will be fine. Don't use polyurethane foam for structural stuff like this. If foam is too dear you could make 2 layers of solid glass and glue them together with [ sections for stiffness.
     
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  7. fallguy
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    fallguy Senior Member

    You need to offer a drawing.

    A typical trampoline is supported from four sides.

    The idea to support the deck only on the hulls is less than ideal because deflection will be significant. So, building a couple of crossbeams means reducing the spans by 300%.
     
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  8. fallguy
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    fallguy Senior Member

    In general terms, I would probably build with 25mm core for spans of 3' or greater.

    The use of the vessel is important because a hard deck can slam/break in heavy stuff. I was advised against it.
     
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  9. DamianW86
    Joined: Nov 2021
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    DamianW86 Junior Member

    I apologize for the delay in response here and I appreciate your feedback!
    to clarify, here are more details.

    I bought new aluminum cross beams (to replace old ones)
    new boat width is 16' center to center hull, 19' edge to edge in widest spot.

    I am replacing original (no longer existent) trampoline into hard deck that will stretch from mast cross beam to back cross beam (traveller sits on it)
    new hard deck will be 14' length and 14' width.

    Cross beams provide structure but deck will not be attached to cross beams.
    New deck will add stiffness to hull flexing I presume.

    Deck will sit on hull's side flange and will be fiberglassed to both hulls.
    additionally I plan on gluing triangular reinforcing brackets from d80 divinycell to create rib like reinforcing structure supporting deck from below (against hulls)
    For longitudinal stiffness I plan making reinforcement underneath deck and similar rib structure then glass it all over to create more aerodynamic structure.


    On top of the deck I plan on building a hard deck cabin very similar in look (hopefully) to TS5 catamaran (inspiration)
    effectively the vertical walls will serve as lateral beams across the deck.
    Composite benches inside cabin will be hard mounted for added deck support (each bench will hose electric motor below deck)

    in areas where I plan on installing hardware (top of bimini as well as motor mounts) I would use plywood 5/8 as core.
    everywhere else (deck floor, vertical walls to cabin and hard top I would plan on using 5/8 divinycell foam (locally available)


    This is a huge project but I already started it so there is no going back :)
    already have 2x electric propulsion motors, rotating mast, new rigging (fractional) from colligo marine

    Also converting old rudders to kickup high aspect.
    If I do good job in timely matter, I would also convert bows to reverse bows as well as improve dagger board to high aspect, canted inward.

    Lots of work ahead of me :)

    but the pressing question for the time being is:
    -can d80 be used as sandwich core for flooring, walls and hard top?
    -what fiberglass to use for light/strong composite, 1708 biaxial 12/ or 17oz? (I plan on using epoxy)
    -how many layers top/bottom? 2/3?
    -With such a large flat area will vacuum bagging, or infusing make a huge difference? or can in be wet layup by hand?
    -should I use s2 fiberglass over e-glass biaxial?

    Thanks a lot for your responses!
     
  10. bajansailor
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    bajansailor Marine Surveyor

    I will just add a comment re this - fallguy, guzzi and others are more qualified and experienced when it comes to your proposed lay-ups etc.
    By not attaching the deck to the cross beams, then you have to make the deck stronger and hence heavier - the deck structure would be more efficient really if it was tied into the hulls and cross beams on all four sides.
     
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  11. fallguy
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    fallguy Senior Member

    There are innumerable problems with the entire concept.

    Design loads for transverse beams need defining. Current, future, etc,

    Hull design for tophat loads not likely.

    Extra transverse beam likely needed.

    Significant additional weigh to vessel.

    No connection to transverse beams could allow loads to be more dynamic.

    Vertical walls will deflect and twist witbout proper support; they are not beams here.

    etc, etc, etc

    Sorry, but you need to pay someone to help and to avoid potential loss of life.
     
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  12. DamianW86
    Joined: Nov 2021
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    DamianW86 Junior Member

    perhaps I have not expressed myself fully. my apologize for the miscommunication.

    The transverse beams that I got now are 6" marine grade aluminum with 1/4" wall. (so very slightly heavier but significantly stronger)
    Original design macgregor was 18' beam. previous owner of this boat shortened it to 16'. Many previous owners increased the beam of the boat to 20'.
    my boat is effectively 19'.

    As to connecting the deck to transverse beams I always excluded that idea but for no good reason. this can easily be accomplished and the deck can indeed be attached to (mast) center and aft beams.

    As to vertical walls for cockpit I mean a full cockpit structure with 4 walls. wouldn't such design serve as bulkhead perhaps further stabilizing the entire deck space?
    regards
     
  13. rob denney
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    rob denney Senior Member

    A drawing of the floor with the frames, supports and stiffening drawn on it will help you see where there is not enough. With 5/8 H80 foam, you may get away with 900mm x 900mm/36"x 36" unsupported panels with 1 layer of 17 ounce db each side.
    It would be nice to attach it to the alloy beams, even if it was only sitting on them. Or the glass on each side was laid up to go over them.
    You will probably get away with thinner foam for the walls and maybe the top, depending on the unsupported areas and load cases.

    I'd infuse it, for the weight saving and ability to include cutouts, rebates, extra laminate, top hats, plywood (drill it the same as the foam if infusing) etc in the lay up. Depends on what you have in the way of table and vac pump.

    There would be some added stiffness with the S2. May or may not be worth the effort and cost.

    As with everything you do that is untried, the best way to proceed is to make some samples to test technique and properties.
    The huge advantage of composites is how easy it is to add more material where required if the original is not stuff enough.
    I would also jump up and down on the finished job to test the attachments. Overall, it will be plenty strong enough, locally, it will depend on load spreading.
     
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  14. fallguy
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    fallguy Senior Member

    You are assuming too many things. It is not a problem of communication, but assumptions.

    The first step in the process is not to ask the core and layup, but to determine the allowable loading of the beams.. It is not arbitrary Damien. In addition; you must provide a reasonable safety margin because at sea the loads are dynamic. That margin is significant and reduces the ability of the beams greatly. It may be the subject of great arguments here, but probably a factor of 3 and not less than 2.

    You can install a beautiful 50mm core and epoxy deck and a cabin over and get out to sea; hit a wake and the entire thing accordions on you. Or, you can have 10 friends joins you on the deck at dock and it does the same because now, not only is is supporting them, but also the deck and cabin loads.

    Additionally, at 14' span; you need to consider a wave pushing up on the deck or slamming loads.

    My gut hunch is you are asking too much in the y plane from the beams and assuming the load will move laterally toward the hulls from the triangles.

    The first step is to understand the limits of the beams.

    I have the same exact beam for my netting beam. It is probably too heavy for my use and 1/8" would have been fine. But I can tell you that I do not feel confident the beam is sufficient for your plans.

    I am using a much stronger beam for a much shorter span for the cabin here.

    I don't have time to do the calc now, but it is uniform loading calc. But even that is problematic here. If the loads are not attached to the beams; then that probably drives the safety margin up to 3 again because of concerns about the load being uniform.. Also, if the cabin load is only 8' wide, now the loading is no longer uniform..

    It is easy to attach the beams to the load with u bolts, btw.

    But you need drawings and calcs done. For anyone to advise you otherwise or to give you a deck laminate would be wrong.

    ps-I may be wrong about treating the beam case with a safety factor; it may be worse than that and I am not a naval architect; just a guy who likes boats; draw it first

    Also, find out the ppi of the boat or immersion data if you are adding weight.. if you are at 600#, and you add 1200#; you drop 2", can you get that back elsewhere, etc. this adds to the slamming potential, etc as well.

    In Honolulu I see, so you cannot overlook rough seas.
     
    Last edited: Feb 2, 2022
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  15. guzzis3
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    guzzis3 Senior Member

    Before I replied this was a simple flat deck with a couple of benches. Now it's a full width bridgedeck cabin cruiser.

    As I recall those macs are not load carriers, they are basically giant beach cats.

    The hulls and beam structure should be ok as long as you don't add much weight anywhere. To achieve this and keep costs down you could build 2 decks 12mm foam with say 600gsm either side and join them one above the other with [ section solid glass with lots of uni in the top and bottom sections and 45/45 right round the [.

    You could sit the bridgedeck on the beams and glass it to the hulls to stiffen things up but as said above that structure is getting heavy and that's before you fill it up with "stuff".

    So with such elaborate modifications I wonder why you aren't just building new hulls ? It sounds like you want something like Richard Woods Tamar and trying to turn a mac into one to somehow save money or time. I can tell you for nothing you won't save either and it's likely to end up a dogs breakfast.

    Sorry to be discouraging but the mac is just the wrong starting point.
     
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