Need fibreglass layup info

Discussion in 'Fiberglass and Composite Boat Building' started by Basjan, Apr 2, 2008.

  1. Basjan
    Joined: Sep 2007
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    Location: South Africa

    Basjan Basjan

    After much thought I have decided to get a boat, actually building my own.

    I can obtain a new mould for 5.5m (18ft) river boat , but I need some info on the layups..
    Beam is max 1.95m, freeboard fore 800mm and freeboard aft 550mm.

    I would like to know how many layers of CSM I should use for this size boat?
    what should the max hull thickness be?
    Do I start with a light CSM and work towards a heavier CSM?
    How many layers of what weight CSM should I use?

    :?: :?: :?:
  2. Basjan
    Joined: Sep 2007
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    Location: South Africa

    Basjan Basjan

    Well, it seems my post was not clear enough.

    I am trying to do some costing to check if it is worthwhile to buy the mould.
    I need info on the weight of the fibreglass chop strand mat to use and how many layers would be sufficient for a sturdy hull, eg. how thick should the hull be.
    Are 2 layers 450g csm enough or must additional layers be used?
    Will 3 layers of 300g csm be better than 2 x 450g layers?
    If gelcoat is applied by brush, is one layer enough or should a 2nd layer be applied?

    This will be a centre consol with max outboard of 50hp.

    :?: :confused: :?:
  3. KnottyBuoyz
    Joined: Jul 2006
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    Location: Iroquois, Ontario

    KnottyBuoyz Provocateur & Raconteur

    Ok, I'll bite and try to help you out although I'm no expert.

    Firstly it's my understanding that building a boat in a mold is normally reserved for larger production runs of more than 1 boat. The method to build a boat in a mold is radically different than a cold molded 1-off design. Normally the mold is waxed (or other release agent) then the outer coatings of gel-coat is sprayed into the mold. This outer layer forms the finish of the boat. Next a layer of chopped strand matt is added to prevent print through of the heaver layers of glass that must be used for structural integrity. Some builders will use woven rovings, knitted fabric or multi-axial fabrics. Exotic boats would use carbon fiber. I'm no engineer but there's plenty of discussions on here on engineering laminates but basically you have two skins, inner and outer, separated by a core.

    The core can be balsa, wood, plywood, foam, honeycomb (plastic or metal) and any other of a host of exotic materials. The idea is to separate the inner and outer core to achieve strength (and in some cases saving weight). Depending on the application, planing speed boat, performance sailboat, cruising sailboat, heavy displacement trawler, etc. the laminations required to achieve the desired strength characteristis will vary. Using only CSM in the hull will result in a very weak (and heavy) boat which will not likely perform as expected. The random orientation of the fibers provide little, if any, strength. The other glass fabrics, due to their weave and weight provide the most strength because the fibers are oriented in straight lines. The straighter the better. Layerd fabrics (multi-axial) with criss-crossing layers oriented at varying degrees to one another provide the best weight/strength ratios.
    A good basic primer on the use of various core materials and strengths is available in the boatdesign articles

    So basically the only use for the CSM will be to help prevent the weave of an underlying fabric from "printing through" to the surface of the gelcoat as the boat ages. Some boats were built (correct me if I'm wrong) with chopped disoriented fiberglass & resin shot from a gun type aparatus (chopper gun). This method, although cost effective, resulted in poor layups of boat hulls. I don't believe it is done this way anymore.

    For what it's worth, and remember I'm no expert, if I were you I'd look for a design (plans) that I could build using commonly available materials and tools. I'd choose marine plywood, epoxy resins and multiaxial fabrics to produce the (not the lightest) most economical boat possible. Something like this....


    Good luck with your project. Let us know how it goes.
  4. tinhorn
    Joined: Jan 2008
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    Location: Massachusetts South Shore.

    tinhorn Senior Member

    I pretty much agree with KnottyBouyz, except that when I cut up a jet ski hull, it was obviously all chopped strand, and very thick and heavy. (It's also the cheapest method for production boats.) If you've never run a chop gun, don't even think about it for a boat project - until you get the knack, your parts will be dangerously inconsistent in thickness.

    While the sandwich method KnottyBouyz described is the lightweight, hi-tech end of the spectrum, chopped strand mat is the low-end slug system. In between these extremes is the option of mat against the gelcoat, then your strength layer(s), with mat again on top of it all. I wish I was qualified to suggest a layup schedule but I'm not. Just let me observe that if your strength layer is resin-rich, you may very well see printhrough even though you used a layer of mat. Printhrough is resin shrink. Resin shrinks - glass doesn't.

    I AM qualified to make some observations on brushing gelcoat, having been too cheap for awhile to buy a spray gun. You can't allow the gelcoat to be too thick, nor do you want thin spots next to thick spots. (When I was making fiberglass parts, I strived for an 18-mil thickness built up in three spray-gun passes.) You will discover that a good gelcoat layer is impossible with a brush without leaving brushmarks, but attempting to apply a second coat is an invitation to "alligatoring" of the first coat - a very nasty condition. My solution to this dilemma was 1) brush on gelcoat and allow it to cure for a day. (If it takes longer than 30 minutes to "kick", allow it to cure for two or three days.) 2) Prepare the glass for the first layup. 3) Immediately before applying the first layer of glass, catalyze and brush in a second gelcoat layer. 4) Catalyze the resin for the glass slightly hot, and lay it into the wet gelcoat. Don't oversaturate the mat - it'll be absorbing the gelcoat. 5) If I was really concerned about brush marks I'd use some gelcoat to tint the resin in the first layer of glass. 6) Don't confess these methods to anyone because they'll consider you low-brow.

    Unless you really want to get all stinky and frustrated with chemicals and itchy glass, have you considered having a fiberglass shop make the parts you need from your mold?
  5. SamSam
    Joined: Feb 2005
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    Location: Coastal Georgia

    SamSam Senior Member

    Your post is still not clear enough. Layups aren't just a matter of slapping in x amount of x. It depends on the shape of the hull, curved areas are stonger than flat areas. It depends on the inner structure of stringers, bulkheads, decks, etc. that support the fiberglass hull. It depends on the use a boat will get, such as recreational or commercial, and where the use will be. It depends on the performance expected of the boat. Some parts of a boat get more wear and so get more laminations. Some parts of a boat get more stress and so get more laminations.

  6. Basjan
    Joined: Sep 2007
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    Location: South Africa

    Basjan Basjan

    Thanx for the reply's.

    I hope the pic of the mould are attached.
    It seems a bit dirty, but I will first check it out to see if it is usable.

    If I take on this project, it will be dirty hands all the way.
    I have acompressor so spraying gelcoat is not a problem, just need to get a gun, hopefully I wont need to "license it" again.(he he he)

    I still need to know how many layers of chop mat would give a sturdy hull.
    I know that at certain points (keel, transom, bow) additional layers must be added.
    Our local boats seems to be all chop mat.
    I'm not looking to build a speedboat or daring to go to sea, just something to go fishing with on the river.

    If all works out, I'l pull a few hulls and sell them for the DIY's guys who want to build there own boat.

    Attached Files:

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