How many layers of which type of fiberglass

Discussion in 'Fiberglass and Composite Boat Building' started by Tom Peach, Feb 13, 2024.

  1. Tom Peach
    Joined: Oct 2020
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    Tom Peach Junior Member

    So here is my question, I am building a houseboat, it's not really going to go anywhere, it a timber frame, ie osb3 and a timber frame.

    And then I will coat it in fibreglass. So the first question is shall I use chopped strand mat in sheets to coat it or twill?

    Next question is if you suggested CSM then should I go with 300, 450, or 600 grammes per square metre? And how many layers do you think it might take to get a decent hull?

    I think that covers it, Ciao!
     
  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    I had to google 'OSB3' to see what type of wood it is -
    OSB/3 | Oriented Strand Board | Wood Panels | Hanson Plywood https://hanson-plywood.co.uk/products/osb3/
    Please do not use this for the hull of your houseboat.

    In the link above they mention "The trade term OSB/3 comes from the technical class denoted in BS EN 300 for ‘load-bearing boards for use in humid conditions’. Humid in this instance meaning environments where boards are protected from external conditions but moisture content can increase because of humidity."

    External conditions here would be water (fresh or salt) - and sheathing this stuff in fibreglass is no guarantee that you will keep the water out.
    And you would have to then sheath both sides of the panel, as well as seal the edges - and you do not want to use CSM with polyester resin.

    It would be much better to use a proper waterproof plywood, and sheath it with cloth using epoxy resin rather than polyester.
    OK, this will be more expensive, but it would probably be cheaper in the long run than sheathing OSB3 with epoxy and cloth on both sides.
     
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  3. wet feet
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    wet feet Senior Member

    The advice above might be unwelcome if you have just finished cladding the frames with OSB,but it is good advice.I know of several houseboat hulls locally that were built from 18mm WBP ply and covered with 3 plies of 600 gsm mat and they are now over 30 years old.As for weight of glass,it is quite normal to apply the heaviest material that will take the shape as this will save the time needed to apply a greater number of plies of lighter material.The limiting factor is usually the tightest curve on the entire surface,but it is quite possible to use light plies on the extreme curve only and heavier material generally if the total laminate remains uniform.

    Twill would give a stronger hull,but that should not really be a consideration for a houseboat and csm tends to give a thicker laminate for the same weight of reinforcement.In your situation,the thicker laminate would probably be a good thing as it puts the water further from the wood.Epoxy can't be used with csm as it won't dissolve the binder you may get a nasty shock if you price epoxy versus polyester.....
     
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  4. rwatson
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    rwatson Senior Member

    First thought, if you are using a poor substrate, make sure you use Epoxy, not Polyester Resin.
    Polyester is not waterproof.
     
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  5. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    If the structures strength is provided by the OSB and wood all you need is waterproofing, in wich case lots of epoxy inside and outside. Glass is optional inside, just a light layer of plain or twill outside, mainly for abrasion. Longevity depends on workmanship and care, let water into the OSB and it's over fast even with OSB4.

    If fiberglass is to provide the structural strength then you know the needed thickness and alternate CSM and woven rowings until you reach it. To plan you can use the old rule of thumb, 1mm thickness needs 1kg glass. In this case OSB is a waste of money, there are cheaper mold materials like for example cardboard.
     
  6. fallguy
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    fallguy Senior Member

    x4 on not using osb; especially if not using epoxy; too easy for a pinhole and early failure of the hull

    if you want to gamble or already applied osb to frame; then I'd sheath it with 1708 mat in...but gonna need to do a filler putty coat to avoid air in the laminate...the fill coat might be something like epoxy and fumed silica with maybe some balloons so you can sand it flat and epoxy for the outside...inside you'll want to do neat coats of epoxy and lots of them and even perhaps pouring it at the frame/osb interfaces

    If you combine osb with too little laminate; a pinhole or 12 will kill the boat fast. This means forget thin wovens.
     
  7. Tops
    Joined: Aug 2021
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    Tops Senior Member

    What is the general arrangement / size of the boat? What works for a dinghy might not work for a larger boat.

    I would suggest using marine industry grade materials. The reduced cost of substandard materials will wash out with extra fillers, laminate layers, and associated labor.

    I agree with the comment from Fallguy that thin woven over substandard ply is a recipe for failure...been there done that on a small boat and that hull was rarely immersed during most of its useful life. My other transgression on that one was unpainted epoxy exposed to sun and weather.
     
  8. Tom Peach
    Joined: Oct 2020
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    Tom Peach Junior Member

    Ok so a little more info on the boat I am building, it is a 12m x 4m wide so thats about 39' x 13' ft I think, then it sits on a 1 metre section of 3" x 2" done like a house on 400mm 16" to the centre of the joists, sorry but I forget what they call them, and then there is 12m at the top of the boat and 11m undeneath and then we have a section that is at 45 degress

    ____________________________________
    | /
    | / <--- This piece is obviously at 45 degrees
    |_________________________________/

    OK so I am not going to use osb3 I am going to use 3/4" (18mm) on the bottom and 1/2" (12mm) on the sides
    The question I now have is does it need to be marine plywood or would I get away with normal plywood
    And of course I am using epoxy not polyester
    but why did one of you say I mustn't use CSM with epoxy, I have seen this done loads
     
  9. wet feet
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    wet feet Senior Member

    Its your boat and your time and money,do it any way you want.Good luck.
     
  10. Tops
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    Tops Senior Member

    See post #3. Some CSM's uses a binder that needs styrene (from polyester or vinylester resins) to dissolve properly. It can be had compatible with epoxy as well.
     
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  11. rwatson
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    rwatson Senior Member

    Wise decision, not to use OSB

    Re the Type of Plywood, there really isn't a "normal" grade.
    You could get non-structural, soft wood for interior only, right up to structural grade, with waterproof glues.
    The better you can afford, the longer the life.

    In either case, there is a strong case for at least a light scrim glass and epoxy layer on the inside of the plywood as well, because fresh water in the bilges is really bad for plywood of any type.
    Marine ply is supposed to feature a lot better grain structure, as well as better quality species and waterproof glues. Impact and load bearing abilities are something to consider.
    I would even suggest you at least use Marine Ply for the Bottom and Lower parts of the hull, for this reason.

    You could also do your calculations, and see if you reduced the thickness of the ply, you could cover the hull with say 400 double bias glass and epoxy, and come out close on costs.

    Since the dollars seem critical, you could also use Marine Ply, and only glass the hull to just above the expected waterline, and use a quality coating for the non immersed parts of the craft.
     
  12. rberrey
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    rberrey Senior Member

    Form ply HDO here in the states is made with a water proof glue , Baltic birch form ply will not rot unless there are exposed edges , cut edge with nothing sealing it . I have some cut pieces of Russian birch form ply that have been laying on the ground exposed to the weather for 20+ years with no rot except on the cut edge on one that has ground contact . Finn Form about is the same product as Russian birch form ply but cost out the ying yang here , there may be a marine grade Baltic birch in the EU . You will not bend it , so bulkheads and flat work on the hull . There may also be some from China now .
     
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  13. wet feet
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    wet feet Senior Member

    You must have a remarkably dry climate,here in the UK you can leave birch ply outside for about five weeks before it starts growing blue mould spots.I have doubts about how much notice the OP will be taking of opinions that aren't the same as his own but would suggest he seeks an insurance quote based on hull material,since the insurer will be liable for the environmental clean up costs.No insurance leaves the boat owner liable for those costs.
     
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  14. rberrey
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    rberrey Senior Member

    One of the wettest climates in the U.S is the Gulf Coast of Fl and Al . We are not talking about plain birch ply , it will rot quick , no marine glue or coating as on birch form ply . I did check online for marine grade birch ply and found it on sale for $129.99 in N.C for a 3/4 sheet . He should be able to find it somewhere in the E.U .
     

  15. wet feet
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    wet feet Senior Member

    This shows the specification for marine plywood :BS 1088 - Wikipedia https://en.wikipedia.org/wiki/BS_1088 . The relevant part is this

    In materials, BS 1088 is the British Standard specification for marine plywood that applies to plywood produced with untreated tropical hardwood veneers that have a set level of resistance to fungal attack. The plies are bonded with Weather Boil Proof (WBP) glue.[1]

    Although this is a British Standard, the finished product does not have to be made in Britain, just manufactured to meet the standard. The standard is associated with Lloyd's Register since it performs testing of products to this standard. It does not follow that it is a structural plywood.


    While birch might be bonded with the correct adhesive,it really doesn't comply with the resistance to fungal attack part.There are plenty of other tropical hardwood veneers that are bonded to the correct specification and stamped WBP to indicate the glue quality.Those would be the minimum I would consider for this project.
     
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