Structural Lamination Help Needed

Discussion in 'Fiberglass and Composite Boat Building' started by jangr, Jan 31, 2019.

  1. jangr
    Joined: Nov 2018
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    Location: Chesapeake Bay

    jangr Junior Member

    I gave my old thread a week without input, so I'm guessing that between the old title and too many questions in one post, there's noting but crickets :) So I'll try to keep it short and simple here. I'm building a drive hull for a 24' pontoon, which will need to support 1000 lbs of battery, PMAC motor, and controller in an in-board configuration. The drive hull will have a 2" laminate flange through-bolted to a 2"x2"x1/4" welded aluminum frame. After much helpful feedback from many of you, I'm using (free) donor hull-sides to reduce my material and labor costs. A few pictures might help...

    Drive hull sketch, with approximate battery pack, motor, and drive train dimensions...
    Drive Hull v2.jpg
    Hull-sides temporarily mounted on strong back...
    7.jpg

    Pine mock-up of (to-be) coosa keel arrangement...
    8.jpeg

    I'll be working in a commercial shop that does extensive repairs and refits to wood and glass workboats up to 60 feet. So I'll have some help, though I'm pretty sure they've never put something together like this before! My main concerns are what materials I should use (and where) to get a stiff-enough shell and flange to reliably carry a 1,000 load.

    The shop has a ton of experience with various polyester, vinyl ester, epoxy, fabric, mat, and roving. They tend to use poly/vinyl for topsides to chine, and epoxy below the waterline, on both wood and glass boats. I'm thinking to do the same (generally), but am unsure how much material I need to build up to get the result I need.

    My thought was to attach the hull-sides to the keel with silica-reinforced epoxy adhesive and epoxy fillet, and then overlay with 3-4 sheets of progressively wider biaxial fabric, outboard to the first bottom strake. I would then shape 1 or 2 pieces of epoxy soaked luan to cover the air-gap forward of the keel up through the stem, attach with a filet, then repeating the biaxial layup for the keel, stepping each layer of fabric between the stem and keel to keep each joint covered with continuous glass.

    Next I would use a 'face' mold for the transom, with a filet, 3 layers of tabbing, and 3 or 4 more layers of biaxial cloth. The final interior laminate would be 2-3 layers of cloth running across the beam but set back 3" from the top of the gunwales.

    Once everything sets up, I'd demount the hull and flip it upside down, shave off extraneous hull-side material, and repeat the keel, stem, and transom layup identically as above. With the hull upside down on a concrete floor, covered with plastic sheeting, I'll lay up three layers of fabric to form the exterior part of the flange. Then flip the hull upright to lay up another three layers of flange matched up to the beam-to beam gap on the interior. This should give me a nice flat flange that's true to the hull. I'm not terribly concerned about rebates on the exterior flange or transom, I'm happy to fair the result to a smooth 'bulge' in both areas. The exterior will be roughed up for 545, awl fair, and topcoat to finish.

    My basic question is how many layers, and what fabric (or blend of fabrics) to use, for each component of the hull - keel, stem, hull-sides, and flange. Putting aside the stinger arrangement (for now), while I read the more posts on that topic, I'd like to be sure I've got a solid foundation before framing out coosa, tabbing, and laminate work for this last piece of the puzzle.

    Many thanks in advance. I've seen some really great technical/structural advice on the forum, hoping some of you can provide same for this project.

    Cheers,

    Jack
     
  2. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    The layers should run progressively narrower, not wider, to avoid bumps and resulting weak spots from air entrainment or future sanding.
    I’d set up the wood keel piece so that it could be removed after glassing the halves together on top, maybe shape it to more easily glass over without fussing with fillets. Packaging tape works well for isolation from resin, or just move the halves together and glass the joint.
    Wood likes to soak up water, so a solid glass keel is to be desired here. You will be drilling into it for your shaft log, so build up some extra layers in the area of that penetration.
    Use of epoxy here is of no advantage, as the rest of the hull is most certainly polyester, and when all’s said and done, the unit will be no stronger than its weakest point!
     
  3. jangr
    Joined: Nov 2018
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    Location: Chesapeake Bay

    jangr Junior Member

    Thanks Kapn. The wood keel is just a mock-up to confirm the bevel angle, the actual keel will be 3/4” coosa. I’ll also run a quarter-round router bit over the keel corners inside and out, only filet where the flat hull sides meet the keel abruptly.

    Only reason for epoxy is it’s all I’ve ever used and I’m comfortable with it. I’ve never had an epoxy laminate I did fail over the years so it feels safe. I guess working with polyester would be similar except for laying over a tacky surface within a set amount of time, but maybe that’s not as tricky as it sounds.
     
  4. kapnD
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    kapnD Senior Member

    81492C21-F017-4C24-AC44-A936CC955B54.jpeg I’d do something like this, but wider than the pic allows, and of course, similar layup on the bottom after it’s flipped.
    Keel piece tall enough to support the battery shelf.
    It’s important to glass the halves together first to unitize the hull, then add the internal stiffener-keel.
     
  5. jangr
    Joined: Nov 2018
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    jangr Junior Member

    Thanks for the illustration Kapn, a couple of points though..,

    I was hoping to unitize and register the hull sides with the keel milled to a consistent bevel. The stiffness is an added plus, killing 2 birds with one stone. And the flat surface allows for a simple ‘speed’ strut mounted to a backing plate.

    Also running an upright keel piece will get in the way of the shaft log, which enters the hull in the middle of the forward battery pack. A lot of jiggering around with battery form factor, to get the pack amidship as much as possible, led to this layout constraint. And the platform will be two piece, so it and the batteries can be removed to service the bearing and seal.

    I’m thinking an integrated keel/hull joint as designed will be stronger and simpler than just glass alone. Am I wrong on that score?
     
  6. kapnD
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    kapnD Senior Member

    I don’t see why you would want any core in the keel area.
    Solid glass with a glassed in fiberglass stern tube integrates all very stoutly, no bolts or core wetting ever.
    Cut the wood piece down until it’s level with the inside of the bottom, then you’ll have solid glass to glass, inline, instead of glass to filler to glass, and you’ll still have your flat spot in the keel. Leave open the area of the shaft penetration, do that once the motor/shaft is correctly aligned with strut.
    When you flip it, you’ll be grinding again, so remove the wood, trim up the old glass so the new bottom glass lays nice and flat, and bonds with the inside laminate.
    The old glass needs a heavy grinding and acetone scrub prior to any layup.
    I’m guessing the hull is around 2’ wide, and fastened solid along the top flange, so it won’t require much additional longitudinal stiffeners to keep it inline, however the drive will put some torque and thrust pressure on it, so it might be good to have at least partial bulkheads at motor and strut locations.
    You could suspend the batteries from the deck beams with allthread hung battery trays, taking a great deal of stress off the hull.
     
  7. kapnD
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    kapnD Senior Member

  8. jangr
    Joined: Nov 2018
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    jangr Junior Member

    As ever, you've provided sound advice Kapn. Thank you.

    I like your suggestion to retain the wood on hardback, but with a bevel that leaves the laminate flush instead of embedded. The wood then becomes a mold face, and a packing tape release is easily enough. Too bad I dropped $200 on a full sheet of 3/4 coosa FRP :(. Tho much of that material was going to be used for stringers and battery platform anyway. Question - how many layers, of what type/weight of fabric/tape, should I use on inside/outside keel laminate? I'm only starting with about 1/16" laminate, which is pretty flimsy. Then, how much more (2x?) around the strut and shaft log sections?

    Suspending the battery pack is also interesting, although my biggest concern overall is lateral acceleration, e.g. 800lb battery mass on a 24" moment arm (assuming stringer mounted). It doesn't take much lateral acceleration to put some major tension/compression on the flange. Crossing the 2 foot wake of a 50' gin palace could amount to 10 deg/sec2 on that moment arm. Even distributed across 2 flanges, 10 feet long, seems like the flanges will need to be pretty rugged. A threaded rod suspension for the batteries has a shorter moment arm, but the load would only be distributed across the 4' x 2' platform. Less load on the hull, but maybe just as hard to keep from swinging side to side as the platform pitches?

    I don't have the math chops to figure this out, but it seems that the flanges are the main point of failure in this build. I had thought that a 1/2" flange layup, roughly equal to the tensile strength of 1/4" aluminum sheet (@70% glass to resin) would be sufficient. But that's just guesswork, would be great if someone here could confirm/deny.

    Thanks again,

    Jack
     
  9. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    I use 1708 biaxial as my go-to for most boat work, but I hardly ever use epoxy, mostly vinylester or polyester resins.
    I’d go three layers on each side of the hull joint, progressively narrower strips, and 3 or 4 extra over the strut.
    I don’t know what kind of torque/hp you’ll get, but don’t underbuild it, a few small strips of extra glass won’t sink the boat or the budget, and much easier now than later.
    You could double up the C channel @ battery hangers, and insert vertical plywood on two or three or all sides drawn up tight under the deck joists to discourage racking.
    The flanges could be backed with aluminum angle, or even doubled (inside and out) to form a T.
    1/2” is Likely huge overkill, especially if a backing is used, I don’t know how to calculate it either.
    Avoid a sharp 90degree angle at the flange, a slight radius will allow the fibers to bend smoothly, I try to match the radius to my smallest disc roller.
    I’ve seen many sharp angles in fiberglass laminate break, if you have to use a sharp corner roller, it’s too tight!
     
  10. jangr
    Joined: Nov 2018
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    Location: Chesapeake Bay

    jangr Junior Member

    Thanks, Kapn.

    I can probably find out how much tensile strength 3x2 layers of 17 oz biaxial provides. After laminating the keel and stem interior, I’ll also build up the hull with a couple of layers of cloth, which also cover the keel and step sections again(?) It’s pretty floppy at just 1/16”

    I wasn’t going to touch the flanges until I’m done with the interior and I flip it onto the floor. My though was to run three progress layers on the outside, with sheeting taped to the floor. Use a roller to create the radius in the first tape let it set up a bit, and then lay a couple more tapes. This will give me an ‘L’ shape nice and true with a consistent radius. Then flip it back upright, grind off the excess old laminate, and repeat from the inside. Does that sound right?
     
  11. kapnD
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    kapnD Senior Member

    Sounds good, The flange laminate can encase the hullside.
    Just to clarify the idea of hanging the batteries from the deck beams, what I’m suggesting there is pretty much rigid battery boxes bolted up tight under the deck beams, not even connected to the new hull, but hanging above it.
    My reasoning on this was partly inspired by how thin the existing glass is.
    As to strengthening the hull/deck beam joint, running a couple of bulkheads high enough to tie to the beams will help take the load off the flanges.
     

  12. jangr
    Joined: Nov 2018
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    Location: Chesapeake Bay

    jangr Junior Member

    Spent a lot of time cleaning off debris and gelcoat to get fresh surface everywhere for the new glass. Before and after pics...

    Interior
    file-12.jpeg

    Exterior
    file1-6.jpeg

    Laminating on Monday, a continuous layup for the whole interior, then flip it over to glass the exterior stem, keel, and set the initial flange with plastic sheet on the concrete floor. The hull sides are mated on the hard back, braced every 12". Despite very thin glass, its a pretty solid base to run a stainless roller on, very little flex. A couple of questions have come up...

    - I was a cautious with the 40 grit on an angle grinder, but I left quite a few gouges, especially on the narrower bits. On the exterior flange, transom wrap, and bottom, I followed up with 80 grit on a DA to smooth out the grind marks and level the surface somewhat. I didn't do the same on the interior. My thought was to run 1 or two layers of mat throughout the interior, since it's a nice continuous surface gunwale to gunwale, letting the mat/resin will help level out the grind marks somewhat before laying up with1208/1708. On the exterior, which is level, I'll skip the mat and go directly to biax. Does that sound reasonable?

    - From the level keel section up through the top of the stem, I've got a gap that tapers from 5" flat to 0" and very acute (interior). From other posts I've noted bow forms blocked back to a flat surface on the interior, so the glass can run easily, followed by a foam nose with a rebate to laminate on the exterior. In this case the material to block the stem back gets buried in laminate from both sides. I'm thinking of using FRP 'wall board' for blocking, which is thin, flexible, polyester-based, and cheap. Although one side is pebbled, I'd grind some texture into both sides to get good adhesion and to knock down the pebbles a bit. Is this material useful as intended? I'm also considering the same material to layup the transom, using double sided tape to mount the wall board to what I had intended as a mold face (3/4" marine ply). Seems like I'd save alot of fussing around with the transom if I bury a piece of FRP inside to set the surface up. Thoughts?

    Thanks!

    Jack
     
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