50'+ fiberglass catamaran design plans

Discussion in 'Boat Design' started by mariobrothers88, Mar 6, 2022.

  1. fallguy
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    fallguy Senior Member

    This is not hard to understand.
     
  2. Rumars
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    Rumars Senior Member

    I am well aware that a resin rich laminate will be thicker but weaker. It also makes a difference if it's CSM/rowing or all stiched fabrics, or if it's hand layed or infused. The problem is that while strenght may be sufficient, stiffness may not, wich impacts the final panel weight, because one would have to add thickness or stringers.
    Just an example, Lagoon uses infused monolithic bottoms with cored topsides. While working on their saildrives some owners discovered that in that specific area the hull is only 3mm thick. That's because the engine and saildrive sit on a tall bracket, wich is a hollow fiberglass stringer that stiffens the area, so the actual skin can be much thinner then elsewhere (the bottom does not have the same thickness everywhere).
    With plywood it's the same game, most ply builds have stringers inside and use thinner skins. Spronk used 12mm ply on stringers for 60 and 70ft boats.

    The examples are to be found at Multihull Structure Thoughts https://www.boatdesign.net/threads/multihull-structure-thoughts.62361/ the first post contains an index to help navigate the thread.
     
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  3. sailhand
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    sailhand Senior Member

    Hi Im not an enthusiast when it comes to steel or aluminium for building and particularly not repairs let alone ongoing maintenance. My friend owned an aluminium cat for several years in the charter industry. It has had the hulls below the waterline completely replaced 3 times in 20 years and is currently being rebuilt again as I write this, all due to ongoing issues with electrolysis even after several rewires etc.
    When repairing these boats you usually have to remove the interior and also the insulation that is necessary to make the boat liveable. You then have to rebuild everything after youve finished welding. Construction is also much more tedious as you have to build a frame then skin the frame then insulate then build another boat inside a boat to complete the vessel. There is a place for metal boats no doubt, moreso in the commercial realm, tankers, trawlers, cargo ferries etc where the added robustness proves invaluable. When I walk into a boatyard or a marina however with mostly recreational vessels they are overwhelmingly fibreglass.
    I believe fibreglass is much easier to repair, and I have a welding ticket, than steel or aluminium. Also much easier to carry or source everything required for repairs in remote areas. I suppose with modern advances in electrical systems on boats and welding equipment and techniques this may have balanced up quite a bit. However I still believe its much easier to carry resin and glass on board than mig welders and gas and wire and spare aluminium sheets for example. Steel may be much easier to weld however you then have to deal with the corrosion issues. Also there is the weight issue and the fact that it is not buoyant, I think its foam core for me. Older foam had a few problems, however as with welding processes, newer materials and techniques are improving all the time. The floor of my monocoque travel trailer/caravan has no metal chassis under it at all. The entire floor panel is h110 pvc foam stitched through with carbon tow and then laminated with 800gm2 weft triax. It is then resin infused with the carbon tow forming a strong attachment between the two laminates helping to prevent any delamination. Testing has indicated a remarkeable improvement in strength using this process. The project is not complete at this stage however the early indications of the overall strength of the build are positive. Foam cored build techniques are typically much faster in construction lower in maintenance and simple to repair. Even given the added robustness of a metal build I dont think I would be willing to consider it.
     
  4. rxcomposite
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    rxcomposite Senior Member

    This is purely engineering exercise not bounded by Class or ISO rule. No skin thickness minimum, no standard thicknesses and weight of core and fiber.

    The goal was to optimize laminate by designing to the allowable limit of Tensile/Compressive stress of outermost fiber, Shear stress on core, and Deflection, then compare the weight.

    Let us take a bottom plate located below the bilge radius. This makes it a primary structure. A safety factor of 4 (stress ratio of 0.25) was used. A panel size of 1m x 0.5m was set with a bottom pressure of 40 kN/m2. See attached spreadsheet.

    1. 24 mm Okuome plywood was chosen. It passed the core shear test but failed on skin stress and deflection criteria. Panel weight is 6.1 Kg.

    2. A higher grade 24 mm Sappele plywood was chosen and it passed all criteria. It was further reduced to 21 mm thickness to optimize. Weight of the test panel is 6.7 Kg.

    3. Single skin poly laminate was configured to pass the criteria. At 8.45 mm thick, it passed the criteria. (It fails in 3 decimals accuracy). Note that I used 3000 gr/m2 of CSM as a core to build up bulk. Even with calculated core shear stress of 3.2 N/mm2 it passes the shear test with flying colors. Panel weight is 6.3 Kg.

    4. A PVC cored laminated was investigated. 20 mm 130 kg core thickness was found to be inadequate as it fails the deflection criteria. It was raised to 22 mm thickness and passed. Slight penalty on compressive side skin. Panel weight is 2.2 Kg.

    The procedure involved analyzing the pressure at midpoint of laminate across the short side.

    To analyze the greater stress under the stiffener base would require additional local reinforcement as the region of tensile/compressive stress reverses. This is all about local stiffening at fixity. No attempt was made as it is beyond the scope of the goal.

    Hope this settles issues.
     

    Attached Files:

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  5. mariobrothers88
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    mariobrothers88 Senior Member

    Wow rxcomposite that was very helpful thank you for that!!
     
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  6. rxcomposite
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    rxcomposite Senior Member

    Thanks
     
  7. mariobrothers88
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    mariobrothers88 Senior Member

    Was the plywood reinforced with fiberglass for the testing or was it just simply the plywood by itself?

     
  8. rxcomposite
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    rxcomposite Senior Member

    No. It is not encapsulated. That is a different dog and hard to calculate as it is deceiving. While fiberglass is strong, it stretches 2.5 to 4%. Wood stretches 0.4 to 0.6% only. It is like combining carbon fiber and Kevlar laminate.
     
  9. mariobrothers88
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    mariobrothers88 Senior Member

    According to your results, fiberglass alone seems to have a similar or perhaps slightly better strength to weight ratio than plywood alone. Am I interpreting the results correctly?
     
  10. fallguy
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    fallguy Senior Member

    No. Because the reinforcements are not included.

    Or, yes, but. The amount of time and cost to build a flat fiberglass panel must not be overlooked.

    In the amount of time it would take to layup and cure and release one or two of the glass panels; you would be able to have an entire hull cut from plywood. The work to build solid frp panels is more than building foam core panels even. A perfectly flat mould surface is needed with constant prep for release for each panel.

    other considerations:

    fiberglass sinks
    foam and ply do not

    reinforcement methods as we mentioned; reinforcing solid frp would also add to the work/time


    I would say 3 months for large cat hulls in ply.

    2-3 years for solid frp
     
  11. garydierking
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    garydierking Senior Member

    I would suggest that you investigate C-Flex fiberglass. It's a roll of stiff fiberglass rods woven together with lighter fibers that can be laid over a simple plywood station and stringer mold. Additional layers of fiberglass are added to get the required thickness. I build three boats with this material. Below is a Rudy Choy 44' catamaran.

    [​IMG]

    [​IMG]

    [​IMG]
     
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  12. rxcomposite
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    rxcomposite Senior Member

    Yes you are correct. It boils down to if fiberglass or marine ply is available in the area of build. The final choice is how long do you want your boat to last? According to the book Marine Composites, 25 years and still reliable for fiberglass.

    The corresponding reinforcement are proportional in weight to wood or fiberglass and both are 40% of the weight of the hull and deck. The structure dimension is almost proportional.

    Wood is just faster material to build as it is a semi prepared material while fiberglass is all goo and thin and flimsy fabric.
     
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  13. fallguy
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    fallguy Senior Member

    I'm more concerned with the work and materials associated after the reinforcements are applied. Had always assumed ply reinforcements to be less. A ply hull say 12mm versus 8.5mm solid frp same tabbing?

    Also, care to expound on building solid frp panels? And time and effort required, etc?
     
  14. rxcomposite
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    rxcomposite Senior Member

    That has been an issue here in the Philippines in the early stage of the construction of wooden bancas as traditional build. It has been exploited to the fullest by filling fiberglass boat with water. But that is for open canoe type vessels.

    Now the Authority have changed their position. They outlawed the traditional woodenboat construction for commercial use. Wood is allowed for private or near shore use and constructor must be inspected by the Authorities. The statutory rules that is has been adopted is ABS and complies with Solas standards. Small boats, I think it is patterned after NK standard.
     

  15. rxcomposite
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    rxcomposite Senior Member

    Sorry FG do not have data for comparison. My data or rather my standards are for production use and covers construction standards for FRP.
     
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