Catamaran build with different core materials

Discussion in 'Multihulls' started by saltifinch, Mar 19, 2016.

  1. saltifinch
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    saltifinch Junior Member

    Hey there, just joined the forum today. Planning on building a 35+ cat.

    From all my research, as an amateur builder, plywood/glass seems to be the best material for construction. I am also impressed by its physical properties in use for hulls.

    I'm just curious where in construction I could use lighter core materials, such as polypropylene honeycomb, in order to save weight. From my research, the pros are

    Lighter than plywood
    High impact strength

    Whereas the cons are

    Low shear strength
    Poor qualities in high heat.
    Fiberglass can only bind to scrim.

    Any suggestions where it might be best utilized?
     
  2. Richard Woods
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    Richard Woods Woods Designs

    My customers who have used polypropylene honeycombs have all regretted it and said they should have used foam. Lighter, easier to work, better resale value

    But I agree it is hard to beat ply/epoxy/glass in that size range. The weight saving is never that much whatever hull material you use. Thats because the hulls are only a small part of the boat, rig, sails, deck gear, galley, tanks, engine, metalwork personal effects etc make up more than half the weight

    I have a number of ply/epoxy designs in that size range. Flica, Vardo, Mira, Mirage, Romany. And Skoota 36 powercat (One is building in Vancouver). Details on my website

    I will be back on Saturna in July

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  3. upchurchmr
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    upchurchmr Senior Member

    In a properly designed sandwich structure the core primarily requires shear strength.
    That is not to say that in specific areas you need other properties, like compression strength around fittings.

    But the majority of the area needs shear strength. I personally wouldn't use polypropylene core with a low shear strength, and you only get the shear load into the core by having good bonding, so if the glue only attaches to the scrim you have a second bad property.
     
  4. rob denney
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    rob denney Senior Member

    I used to use a fair bit of poly honeycomb and like it a lot. Lighter than ply, tough and rot proof. The trick is to use it properly. This involves a hot iron to bevel and seal the edges, make lap joins, and hard spots for fittings so the glass only ever has to bond to the scrim. It's fun stuff to work with and the bond between the glass, scrim and core is better than glass to ply or foam.

    Unfortunately, it is only good for infusing in less than sheet sized pieces and does not have the sheer properties of foam so we don't use it much. But in a ply boat you could use it for bulkheads (maybe not the mast one), furniture, bunks, ring frames, top sides (wet laminate), hatches, decks, etc. With a bit of reinforcing, and an awareness of it's shortcomings, you could build the whole boat from it. As far a I am aware, at least 2 40' cats have been. It is lighter than ply, not quite as light as foam.

    The temperature at which the core or scrim will be softened is considerably higher than that at which the epoxy will be.
    ................................
    Ply/glass is good if you enjoy hard physical labour, dust and sticky mess with toxic materials requiring you to wear gloves, overalls and mask for most of the build. And don't mind a finished result that is heavier, more time consuming, requires more maintenance and is possibly more expensive than it needs to be, with poor resale value.

    For those who don't think this is a good way to spend 2-3 years of their life with a high probability of giving up part way through, there is Intelligent Infusion.

    The majority of the work involves laying the foam and glass on a simple mdf table or mould (no sanding, waxing or polishing) and is done at waist height, with dry materials. Tools are a pair of scissors, utility knife, straight edge, tape measure, hot melt glue gun and an electric screwdriver.

    This is then covered with a plastic bag and the air sucked out with a $200 vacuum pump. Resin is mixed, and sucked into the bag. 40 minutes later, half the hull and deck is perfectly wet out, all the builder has to do is stand and watch. The process is repeated for the other half hulls/decks and all the flat or single curved surfaces, then everything is glued together into slots or matching joins. No sanding, filletting, measuring or aligning is required. Perfectly fitting doors, hatches, and their surrounds and rebates are in place, with all edges sealed.

    Surfaces are fair and ready for painting (no torture boarding), there is no hand laminating and no sticky, dusty or toxic mess. Boat building skill level is low, progress is rapid, waste is minimised.

    We have completed a video showing the process for a 7.5m/25' power cat, should be on www.harryproa.com in the next couple of days. Meanwhile, any questions, please ask.

    rob
    harryproa@gmail.com
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    What Rob Denney says about resale value of plywood boats certainly has currency in this part of the world, many people (maybe misguidedly in some instances) look askance at ply hulls.
     
  6. saltifinch
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    saltifinch Junior Member

    I feel a little starstruck, knowing that Rob Denney replied to my post! :)

    I actually found some link to your webpage on a different post today. Very interesting concepts, but I'm not entirely sure a proa is quite the style I want. Meditate on this, I shall.

    Another question about mixing different materials, if you please. Would it be advisable to make the hull bottom out of solid fiberglass? Seems like it would be pretty easy to do, as its flat, and therefore no real moulds required.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Solid glass is going to be heavier, but at least on the bottom you don't have the issue of crushing the core from hard groundings.
     
  8. saltifinch
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    saltifinch Junior Member

    Yeah that was my thought also. No worries about running up on sandbars or rocks. Well, obviously I'd be worried, but not the same amount ;)

    And as was mentioned earlier, the extra weight would be miniscule in comparison to the rest of the materials in the boat.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

  10. rob denney
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    rob denney Senior Member

    If you knew me, you would find this quite amusing.
    Unless you are being sarcastic, in which case, it is still amusing, but for other reasons. ;-)

    Meditate away. The harryproa makes sense from all viewpoints, except personal preference and resale value (which is almost as poor as ply boats, but will improve as more of them are built), so I won't be upset if you don't go for it. If it is just a styling thing, let me know, maybe we can change it to what you want.

    Not all the Intelligent Infusion concepts would carry over to a cat, but enough of them would to make it quicker and simpler than building in ply.

    Easy to do, but unless you intend sitting on a lot of rocks, not worth the considerable added weight. If you are sitting on rocks, a short, solidly built keel is a better bet, or a shallow enough draft that you could slip a couple of planks or tyres under the hulls. If you do want a solid bottom, infused ply wood is possibly the best non metal material for resisting impact and cutting. Heavy, but lighter than solid glass, rot proof and amazingly tough and resilient.
     
  11. gonzo
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    gonzo Senior Member

    Plywood/fiberglass is superior for puncture resistance compared to most foam cores. Some tests show that foam sandwich may remain watertight even after delamination from impact though. If you plan on sailing in remote areas, plywood can be easily patched with a piece of plywood and self-tapping screws. Sandwich construction is much more sophisticated and repairs are harder; particularly when conditions are not ideal.
     
  12. saltifinch
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    saltifinch Junior Member

    *I can't build a boat trying to envision repairing a major wreck. I gotta focus on fundamental properties of the core material.*

    After re-reviewing the debate on foam vs plywood, I am very much tempted to go with foam core insulation for the majority of my build.

    Pros

    Much easier to work with
    Better insulation ( I live in the Pacific Northwest)
    Lighter

    Cons
    Higher cost
    Lower shear strength
    Not as easy to repair
    Less ability to bolt/screw into

    I feel like the resale value might also be higher, just due to people's perceptions.

    I'm a welder, I'm very experienced in fabrication, so the concepts of boat building don't seem overwhelming to me.. But I have very little experience working with any of the materials. Foam seems like the easiest for an amateur builder. The only real concern to me is the overall strength compared to plywood, but it seems to be the consenus its a valid building material, assuming fiberglass is done properly. I'm pretty sure that motto goes the same for any core though.
     
  13. gonzo
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    gonzo Senior Member

    I can see the resale value being higher for foam sandwich. There is a general perception of plywood being a cheap material of lower quality. Catamarans have symmetry, so you only need to cut one set of molds. Are you going to hand lay the fiberglass?
     
  14. saltifinch
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    saltifinch Junior Member

    I suppose its how you define "quality" in people's minds. Plywood is by far the stronger material. But the concept of "rot" persists, even if it completely negated by proper fiberglassing and painting.

    I might choose foam simply due to the ease of cutting and shaping. Yes, I may spend a few thousand more, but I can make that money back in time saved I think. And all the tools I won't have to buy. And ease of moving/flipping.

    And a faster, lighter boat!

    Assuming foam has enough strength. I've read articles on delamination and S-curve issues with foam, but I wonder if this is just old technology issues....
     

  15. saltifinch
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    saltifinch Junior Member

    I'm still not certain about which technique I will use to fiberglass. I only have experience with hand lay-up, but the more videos I watch on vacuum bagged infusion, the more certain I feel its a viable option, and not necessarily that complicated, while producing the absolute best laminate if done correctly.

    It seems like foam is more suited to shape and build inside a female frame. At least that's how most of the Youtube videos seem to be done.

    I suppose the real question is: Which type of foam is best. I believe there are rigid and semi rigid brands, and different techniques used accordingly. Any thoughts?
     
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