Lightweight GRP construction techniques

Discussion in 'Fiberglass and Composite Boat Building' started by johnnythefish, May 17, 2022.

  1. johnnythefish
    Joined: May 2016
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    johnnythefish Junior Member

    So I hear this a lot - almost always in relation to a “stitch and glue” type hull build - ie. Tolman skiff etc …. That using wood and epoxy results in a much lighter hull (half the weight is often cited)…

    Now I completely understand the reasons behind epoxy wood composites as a great way to build a one off boat …

    BUT …. are there really not techniques/ ways to maximise the efficiency/ cheapness of grp/ ease of finishing from a mold with a way to keep weights down.

    I am talking specifically building a boat from a mold in grp - not a one off foam core build - or using expensive composites ….
  2. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Of course, a GRP hull can be optimized to achieve the lowest possible weight, but it requires a series of calculations that are not easy. The analysis of the distribution of the layers through the thickness of the laminate and the study of the stresses on each one of them allow us to greatly improve the results and arrive at compounds with very tight weights. The use of sandwich construction can also reduce weight and lamination techniques, to achieve the lowest possible contribution of resin, also greatly improve the results.
    DogCavalry likes this.
  3. baeckmo
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    baeckmo Hydrodynamics

    Achieving a structure where all three parameters reach their respective maximum simultaneously is seldom possible.
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  4. rxcomposite
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    rxcomposite Senior Member

    To build from a hull mold, you will spend more than twice the cost of building your hull. Plus you need to build a plug, which would cost you more that if you build a wooden boat and spend more hours fairing and polishing the male plug.
  5. fallguy
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    fallguy Senior Member

    The conversation is an open door for polemics.

    BW, then Ranger boats found a way to inject foam between two relatively thin grp shells or hull and liner. But the capital cost and time to develop these methods are for production boats. Two moulds or more!

    There is no easier one off method than plywood s&g. Perhaps if you get small enough for skin on frame..

    For round chines, the next easiest is probably strip planking and epoxy glass skins.

    After that would be foam single side skins laid up like s&g and then laminate the inside or out in a single go. This method I plan to use for a dinghy.

    Or factory finished cores, etc. But solid glass has no build it easily at home innovation. It is a lot of resin and fabric to get to reasonable thicknesses. And spraying chopper is not really diy or light.

    Fellow wants an alternative to plywood because ply is so expensive. But unless you strip plank with cheap material; nothing comes close.

    But it rots..frp won't. Light frp is too flimsy...The tradeoffs won't leave us alone..
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  6. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    If you have a mold, and have successfully produced hulls out of it, you have a starting point from which you can begin subtracting structure until it fails.
    Or you can get proficient with the some of the software so passionately discussed on these pages, and scientifically determine the lightest possible layup schedule, and compare materials.
    So many factors in determining strength needed, the subject cannot be discussed in general terms, you must produce plans and specifications in order to have a meaningful discussion.
    bajansailor, TANSL and DogCavalry like this.
  7. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    Good fast cheap, as applied to GRP. I always laughed when someone said "pick two". You can rarely get one these days.
    redreuben and bajansailor like this.

  8. redreuben
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    redreuben redreuben

    Couple of things, design wise stress analysis can maximise fibre orientation and so eliminate a lot dead weight that’s there with the one size fits all laminate.
    The process can vary from hand laid to infusion, with any process measuring, weighing and recording is where you find the savings. For example spraying gelcoat, very few operators actually use a gauge to see how much is on the mould most just blaze away till it looks right. When using minimal thicknesses optimal catalyst must be used or it will tripe.
    esearch the material going on, calculate, measure and keep measuring as you spray so your application is optimised.
    And so on, each application method has its tricks.
    You can make very good moulds for flat surfaces with glossy melamine finished mdf or chipboard. Spend the time to install strips on the edges to house the joining tapes and you save a heap of fairing weight. etc etc etc.
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