Plug construction choice

Discussion in 'Fiberglass and Composite Boat Building' started by keith66, Feb 21, 2012.

  1. keith66
    Joined: Sep 2007
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    Location: Essex UK

    keith66 Senior Member

    I have been reading through this forum for quite a while & truly learn something new every day.
    Im just about to embark on building a new project, 26ft rowing Gig, design is done & the numbers look good, plus it looks smart. Now as its intended to be a production boat albeit low volume at first so will be GRP.
    Initially i thought about building a Cedar strip boat and taking a mould from her but the initial cost is too high,
    A cheaper option is to build a foam sandwhich boat and pull a mould from that
    I have also been offered a pile of Polyurethane foam blocks at a very good price & its a fairly good density so should serve as a plug base material. That option is a throwaway plug. Im pretty set on using tooling resin or equivalent to avoid shrinkage
    My main question would be, Is the exotherm of the curing mould likely to affect or damage a foam sandwich hull? I am thinking that being able to use the plug as prototype saves waste!
     
  2. michael pierzga
    Joined: Dec 2008
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    Location: spain

    michael pierzga Senior Member

    Cool project ! Many Gig clubs seem to be appearing these days. Good market for a few boats.

    I don't know much about mold making but if you stick around a while there are a few crafty composite builders on BDN
     
  3. iceboater
    Joined: Jan 2011
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    Location: Iceland

    iceboater Junior Member

    You can control the exotherm by adding fewer layers each round and you can also play a little with the amount of hardener. Depending a little bit on what resin you are using it might be better for shrinkage to build up the thickness in smaller steps. If you do this right,there won't be any problem with temp.
    But be aware of the maximum recommended time between layers for good mechanical bonding. Your resin supplier can advice you on that.

    Axel
     
  4. groper
    Joined: Jun 2011
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    Location: australia

    groper Senior Member

    I dont understand how "the initial cost is too high"? Because you have your first boat already as opposed to a throw away plug where you have nothing yet...

    Anyway, im making a mold at the moment for a 35ft catamaran hull shoe - which is very similar to a long outrigger canoe... i got the transverse frames CNC cut from MDF board which were lofted by the CAD program i used... I stripped over these with thin pine battens... You can then bend PVC foam over this to glass your hull, or go one step further and strip plank the frames completely, and sand and fair all the battens to form your plug.

    Im looking at doing somthing slightly different again, covering the battens with a special PVC foam thats used in sign writing and has a glossy surface for printing on. Im going to cover the battens with this material which seems to fair naturally quite well and form a rough mold from it. Ill let you know how it goes... I initially experimented with formica laminate, but its too thin and doesnt sit fair on the battens.

    If the PVC foam works, the entire mold would only take around 2 hours to build. However, its only a temporary mold and i would take a proper mold from the first boat if i wanted to reproduce this hull many times...
     
  5. keith66
    Joined: Sep 2007
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    Location: Essex UK

    keith66 Senior Member

    To clarify, If building a plug that will be used as prototype boat, costs for a cedar strip hull alone plus epoxy will be in the region of £2000, Thats high compared to foam sandwhich construction which i can source for considerably less.
    For the mould itself im looking towards a Tooling gel, Vinylester skincoat, then DCPD resin filled with ATH to avoid shrinkage & distortion. My local supplier plus everything i have read up on regarding using tooling resins reckons that if using them the layup should be done in one hit as they dont bond too well betweeen layers once cured. im mostly curious what the high exotherm would do to a plug / hull built in foam sandwhich.
     
  6. groper
    Joined: Jun 2011
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    Location: australia

    groper Senior Member

    i have no experience with DCPD resin... i would be surprised that it cant be done "wet on wet" each layer added just after teh resin tacks off, to avoid excessive exotherm?

    Over here, strip plank ceder is considerably cheaper than foam sandwich... go figure...?
     

  7. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    I have not done it but I have talked with several production boat manufacturers how they build their plugs and I have seen it done. The plug must be sturdy and durable because the molds you pull from them will wear out and get distorted, if your production run is long lived you will make several production molds off the same plug. The ones I have seen are heavy plywood profiles at about 16 or 18 inch spacing in a heavy wood frame, and than solid foam blocks are glued between the profiles and rough carved down to contour, than a layer of reinforced filler putty (like auto body filler) is skim coated over it and it is sanded to final shape. They use a giant milling machine to bring the hull to exact contour, than final finish is hand sanded. For a small low production hull this carving to final contour can be done by hand.

    If you want to test sail your design I would just use the simplest moldless construction method of plywood and glass, or foam and glass, to get it on the water and test it out. A boat hull can be used as a plug but it will not hold up to making a number of molds off of it.
     
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