Building a stitch & glue boat in Fiji

Discussion in 'Boatbuilding' started by Saqa, Oct 20, 2013.

  1. Saqa
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    Saqa Senior Member

    I wish I had corel draw! Would making sketching damn easy!!!
     
  2. Saqa
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    Saqa Senior Member

    Micheal
    Is keel same as centreline? The centre seam? The edge of each fabric is 2" from the glue line. I can pull up the fabric another inch, I left that much margin when cutting

    Also I thot that sheathing the side panels was the recommendation. There is about 4kg of fabric on the hull in that pic. I assume that would mean 4kg of resin too after application. Would really appreciate a solid recommendation on cutting off the sides to save what I guess would be about 4kg of overall weight vs what benefit if any leaving as is may provide. I wouldnt claim to know either way as I really dont know. I have heard of things like reinforcing the sides from impact damage, ply checking (dont know what that really means)
     
  3. michael pierzga
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    michael pierzga Senior Member

    Centreline and keel are the same. Sheathing overlaps the keel by 4 to 6 inches each ply.


    The sides of the boat need little additional reinforcing. They will already have a sheer clamp rub rail , several frames inside plus the cockpit sole...

    Save the valuable biax fabric and epoxy and use elsewhere.
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    If you log onto the CorelDraw web site you can down load a previous version as a demo for free.

    The keel is a boat part, the centerline is a reference point, so there are some differences.

    Using double bias (biax) over the exterior of the hull will make fairing it very difficult, compared to a finishing cloth, such as 200 GSM. Besides, in most cases, you'll need more then just one layer of fabric over the seams. If you will sheath the hull, the last layer that goes down should be the full sheathing.

    Simply put, you'll have to fair the boat at least twice. The first time will be after the seams have all their tape on them. You fair these areas to make applying the sheathing smooth. This is commonly referred to as localized fairing, because you're just smoothing out seams and screw holes. Once the whole hull is sheathed, then you go back and fair the whole hull. This is the time you'll do whole panels, rather then just seams.
     
  5. Saqa
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    Saqa Senior Member

    I am not taping though, only sheathing as per my previous line of questioning on whether the tape and sheath can be the same piece of fabric that spans the hull plates. And I gathered that 450gsm double bias is plenty for that purpose....so has been what I had gleaned to date :/
     
  6. michael pierzga
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    michael pierzga Senior Member

    Careful about skipping the tape on joints. Tape adds addition fibers to the joint and makes it stronger....the loads on a hull concentrate loads at the joints.

    Also consider physical damage like chafe..drag your boat over the rocks a few times and you can easily cut thru fibers.


    Concentrate you precious biax fibers on the joints and avoid covering the topsides. ...its wastefull.



    You can never have to much fiber across a joint, but you cetainly can have not enough.

    FIBER ACROSS A JOINT is what its all about...

    Building up fiber on the inside of the hull at chines, transom, centreline is easiest because you must not fair in the surface...it s not visable

    Building up fiber on the exterior is about chafe protection....choose your reinforcment points
     
  7. Saqa
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    Saqa Senior Member

    From the previous page

     
  8. Saqa
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    Saqa Senior Member

    Hey another application for that damanu plank, ripping strips to use as strakes and keel over the seams on top of the glass. That should sort out the chafe protection issue right? Damanu is quite flexible in narrow strip about a cm or so thick. I cant see any probs in laying it as 'rub rails' under the boat over the three long seams. I can also plane it sharp on the edges.....one thing I been reading about, planing pads. Wonder if I can whack a 2m bit of damanu about 4" wide as the keel
     
  9. michael pierzga
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    michael pierzga Senior Member

    Sure...wood strips..skids on the keel are perfect fot chafe , strike protection.

    They get beat up just ter them off and renew
     
  10. groper
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    groper Senior Member

    Its all about what you have on hand and the compromises you have to make to get it done. PAR`s advise is spot on - Normally for a ply boat like this, you would just tape the joins with the 450gsm double bias and then do the full sheath in a 200gsm woven. But from what your saying, you only have 600gsm woven and 450gsm dbias available to you. So what are you going to do?

    One option is to use the 450gsm dbias over the whole thing which gets the taping and sheathing done in 1 hit, but you pay a weight penalty and also a little more effort fairing it because the 450gsm takes more filling than a 200gsm woven to get a flat finish. The sheathing is only there for a bit of added toughness and to help waterproofing, the plywood itself has enough strength in it and doesnt need any reinforcement except for the joints - hence the normal method of fibreglass taping which adds strength only where its needed.

    Like i said earlier, i cant really think of a use for the 600gsm woven besides local toughening in places - which if you decide to use timber wear strips is also redundant...

    And i wouldnt build 1 tiny bit more until you have a complete idea of what you want from this boat. Sounds like your thinking of lengthening it, making it bigger, bigger outboard motor etc this changes everything and the structural design will need to change with it. All the advise youve received thus far is for a 4m dingy powered by a 15hp... Not a 5.7m with a 150hp on the back...completely different beast.

    Its far easier to do multiple drawings or designs, then build it once, as opposed to building it several times over... as the old saying goes, 1 hour on the drawing board saves more than 10hours in the workshop...
     
  11. Saqa
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    Saqa Senior Member

    Hey groper, thanks for the clear explanation mate, laid out like that, I found easy to comprehend as regarding my build

    Btw bigger engine is 15hp vs the 8hp current. Max available to me is 40hp short shaft that I have been offered for a very good price

    Its the reason why I haven't gone ahead and assembled the boat with all my preconceived ideas and misconceptions

    I think one of my original questions was if I can use a polyester fabric to sheath. Its the stuff I used on the "flying esky" Styrofoam build, doesnt soak much resin and no dramas about fairing if receiving surface already is and the recommendation was to use something structural. And also not have bare panel to prevent checking (dont know what that really means)
     
  12. Saqa
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    Saqa Senior Member

    Yeah and you are spot on, 450 double bias and 600 roving is all I have. Its now to decide whether to just tape the seams with the 450 and coat the hull with epoxy vs what I currently have draped over the hull in that last pic.....Thats 8kg fg/resin weight if I read the 50/50 ratio thing right vs 4kg of just resin so a 4kg weight penalty?
     
  13. Saqa
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    Saqa Senior Member

    Been reading up on similar sized boats and came across this thread on a forum

    http://bowdidgemarinedesigns.com/Forum/viewtopic.php?f=10&t=556

    From what I can gather they seem to be discussing the same thing, sheath only acting as tape on the seams where they overlap, using the same 450gsm double bias fabric

    Will one layer of sheath on the outside and one layer of tape inside the hull give me a join close to the strength of the ply? Groper, I couldn't get my head around your calculations in one of your earlier replies on that
     
  14. groper
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    groper Senior Member

    Well if you sheath in 450gsm dbias, and tape the inside joints, then you have 450gsm on both sides of the joint. So if the design calls for 1 layer of 450gsm tape each side, then its the same thing.

    Im not familiar with structural design for such a small boat, but at a guess id say 450gsm each side of a 9mm ply joint is fine - I think PAR laid out the general scantling equivalents somewhere earlier in this thread didnt he?

    Polyester doesnt stick to any type of typical fibreglass resin i know of - so you cant use it, anywhere! i use polyester fabric as peel ply, so you tear it off a cured laminate and its ready for secondary bonding as it leaves a texture and removes any amine blush with it. This wouldnt be possible if the resin stuck to it. I also use polyethylene and polypropylene as release materials for molding - because the resins simply will not stick to it at all. This goes back to the advise PAR gave you regarding that exotic twine you were thinking of using as ties, it simply wont stick so it just cant be used no matter how strong the fibres themselves are, without adherence to the matrix, its not a reinforced plastic.
     

  15. Saqa
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    Saqa Senior Member

    Hey maybe the fabric has been IDed incorrectly. I been told by a local garment factory its Nylon Polyester but they were not the supplier, found a roll in a clearance centre and bought it as it looked the goods to make stretch over pvc frame to make molds. Can stretch to fit almost over any shape and wicks epoxy. When cured over polystyrene block I cant hole it with a hammer, just bounces. Take the claw side with a full whack to break through. I think I have enough to do the outside hull and it will use about 5L of epoxy from past use experience

    Anyway, I like the rugged appeal of that Tropic 14 and if they using 450gsm for sheath then might work on my boat as well

    Thanks heaps guys, have learnt a lot from you all
     
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