Hard Top Attachment - Will This Work as Planned?

Discussion in 'Fiberglass and Composite Boat Building' started by fly186, Mar 7, 2017.

  1. fly186
    Joined: Nov 2016
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    fly186 Junior Member

    My hard top is finally finished. It was a huge amount of work (8 weekends!), much more than I estimated, but it came out very nice. I've included a few pics below but this post is about my planned method of attaching this hard top to the T-Top tubing.
    First of all, the Key West style top was vacuum bagged using a 1/2" Divinicell H80 core with 10 oz glass on both sides and another layer of 1708 biax on top. This is all epoxy resin (US Composites). The finished product, with many coats of Awlgrip 545 and then Awlcraft 2000, weighs in at about 44 lbs.

    The new hard top was built with a camber that exactly matches the existing T-Top tubing. The boat is a 29 SeaVee that originally had a Weblon top.
    I am planning to use Sikaflex 221 to attach the top to the aluminum pipework. There will be NO fasteners holding the top down. The T-Top tubing is VERY rigid and well made and this top is about 1/4 the weight of a production hard top. The 3/16" gap between the top and the aluminum tubing will be filled with the Sikaflex (gray). The goal is to not allow any contact between the paint on the underside of the hard top and the T-Top tubing.
    There will be approximately 60 linear feet of adhesive with a bead of 1/4" wide.

    Does anyone have serious concerns with this attachment method or first-hand knowledge with Sikaflex 221? I'm especially interested in ideas that will help to ensure the best possible bond between the painted underside of the top and the aluminum. Aside from getting those surfaces totally clean is there anything else I should do?

    Thanks in advance.

    Some Pics..

    20170305_141426.jpg
    20170302_151943.jpg
    20170225_181526.jpg
     
    Last edited: Mar 7, 2017
  2. fly186
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    fly186 Junior Member

    65 views as of this morning but no comments? I'm surprised. I figured I would be getting a lot of responses saying that not using fasteners is a really bad idea.
    Any feedback on this approach would be appreciated.
     
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Well, I've just discovered this thread and yeah I have several issues. Sikaflex 221 is a sealant, not an adhesive, though it does have some adhesive properties, it's no way capable of holding the top on at speed or in gusty condisions. There's two basic reasons for this, first is you're bonding to paint, not the top's structure and second, the stuff just isn't very strong, particularly in tension, which it will be at speed and with wind loads. Wind loads are pretty easy to figure out, just multiply by the cantilever and you get the amount the minimum peel strength will need to resit. Add a reasonable safety factor and goo's in a tube quickly drop off the list. Simply put, Sikaflex 221 has about 250 PSI tensile strength.

    Bonding something that needs to be strong to a painted surface means you're relying on the band to substrate bond, to hold it on. You look to have done a very nice job of your hardtop, do you really want to lose it, the first time out?

    Yep, mechanical fasteners are the typical method and for logical reasons. A 1/4" stainless bolt will offer thousands of pounds of tensile strength in comparison to Sikaflex 221 or other polyurethanes, bonded to paint.

    If you want to isolate the frame from the top, the bead of goo idea can still be employed (I'd use a gasket) with small stand offs for the hard fasteners, possible made from HDPE or UHMWPE. Possibly some machine screws in the top and "nutserts" in the frame and you're good to go.
     
  4. fly186
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    fly186 Junior Member

    First, thank you for the feedback - it is appreciated. This mounting issue has been gnawing at me since I started to build this thing. Let me list a few reasons why I'm not sold on the idea of fasteners:

    1) Galvanic corrosion between stainless fasteners screwed into the
    anodized aluminum top. In some areas this would not be an issue since
    there are 1/4" plates that I could use for the anchors. In other areas,
    unless I have a local shop weld on some tabs, I'll be tapping a 1/4"
    hole into a 1" OD pipe in places and this could lead to future corrosion
    and cracking.

    2) Large (ugly) fender washers or the like would need to be used on the top
    side to spread the fastener loads to avoid cracking the lamination/paint.

    3) The top is horizontal of course so while there are wind loads to consider,
    it's not exactly a sail since the cross-section area is very small.


    The Sikaflex 221 spec sheet here does say this product is a "Sealant/Adhesive" and also includes the following:

    "Sikaflex®-221 bonds well to a wide variety of substrates and is suitable for
    making permanent elastic seals of high adhesive strength.
    Suitable substrate materials are wood, metals, metal primers and
    paint coatings (2-c systems), ceramic materials and plastics."

    Using the stated 260 lb PSI tensile strength and about 60 linear feet of 1/4 wide beads of the adhesive, I calculate approximately 3900 lbs of tensile strength. That's pretty strong. Even at 50% of this we are at almost 2000 lbs.
    I know this isn't 5200, but I hate 5200.

    I may compromise in the end and add two forward mounting fasteners. In the aft part of the top the outrigger bases are actually serving as large fasteners since they contact the top (with a starboard shim) and are bolted to aluminum plate that is part of the welded t-top structure.
    So 4 attachment points and then Sikaflex 221 all around should work.

    Additional thoughts?
     
  5. JSL
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    JSL Senior Member

    wind loads are not the only forces you have to deal with. Motion due to sea-state will put loads in all directions on all axis and could cause real headaches... literally.
     
  6. missinginaction
    Joined: Aug 2007
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    missinginaction Senior Member

    First, nice job on the hard top.

    Second, consider that you're not bonding to a square support, it's a tube. The actual surface area that you'd have for the bond would be pretty small. You're also depending on the paint to hold the top on the tubing. I know that Sika says it will work, but I'm suspicious. There's a big difference between laminating a couple of pieces of wood together and what you're looking at here.

    Why not consider stainless steel, through bolted (not tapped) and if you're concerned about galvanic corrosion use nylon sleeves between the bolts and the tubes.
     
  7. rasorinc
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    rasorinc Senior Member

  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You may not realize that top has considerable mass and this generates momentum. Vertical accelerations and several other considerations need to be accepted, before you make a fastener type decision. Had the frame and top raw material contact surfaces, you could consider a goo, other wise, you're married to mechanical attachment. Attaching stainless to aluminum doesn't have to be a guaranteed corrosion concern. Fasteners can be isolated and thread contact areas can be sealed, after which it's simply a maintenance issue (keep it clean, dry, healthy bedding, etc.)
     
  9. fly186
    Joined: Nov 2016
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    fly186 Junior Member

    Thanks again for the feedback.
    I'll do a final fit check this weekend with the finished top and take some pictures and add them here. As I mentioned in a previous post, I am leaning towards installing 2 forward attachment points along with the rear attachments that will be the outrigger bases. For the forward points I'll probably use some 3" tower pads to spread the load on the top and use a few 1/4"-20 stainless fasteners that will let me snug the top down to the frame. I see these attachments mainly as "insurance".
     
  10. PAR
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    PAR Yacht Designer/Builder

    Now that you've got a great finish and all, can you consider bonding some aluminum brackets to the top? This way you'll have aluminum on aluminum attachments with a bond to the top. You can rivet or bolt the brackets to the tower. Of course, you'll need to grind off the paint in the contact areas to get at the laminate, but you can be neat about this, so to not disturb much paint, possibly having all the "sins" under the bracket, so it doesn't require a repaint or touch up. If you go this route, make the bases that are bonded to the top as large as you can, surface area wise.
     
  11. fly186
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    fly186 Junior Member

    PAR, I'm not sure I understand your last post but if I decide to add additional attachment points to the T-Top tubing itself I'll take it to a local T-Top fabricator and have them weld some small "tabs" to the tubing in a few places. As for taking off the paint, I'm not sure why I would do that. I would just place the pads on top, bed them in some 5200, drill, and then through bolt with 1/4" stainless steel bolts to the tabs below.
    If I wanted to go really nuts I would put pads above and below the laminate and insert aluminum spacers between the pads so it won't be crushed. As you said, I would go with slightly wider pads to spread the loads and again bed the top and bottom pads in 5200. I'm talking about pads like this:
    http://tacomarine.com/category/cat_round_pads/Round-Pads
     
  12. Sparky568
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    Sparky568 Junior Member

    If your thinking about welding some mounting plates to the pipe and you really, really, really don't want penetrating hardware through your brand new top (nice job BTW) why not counter sink and bolt some 1/4" G10 on top of the welded plates and epoxy the whole thing down?
     
  13. fly186
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    fly186 Junior Member

    I'm testing some Sikaflex 221 attachments right now with scraps of anodized tubing and laminated panels. I also plan to add some pads on the top that will be through bolted to some aluminum plate on the sides of the console that are near the edges of the top. In addition, I may add an additional pad up front and a stainless U bolt to fasten to the front edge of the T-Top tubing and a similar setup in the rear corners.
     
  14. philSweet
    Joined: May 2008
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    philSweet Senior Member

    Sorry, but I don't think you have a clue. That stuff isn't nearly strong enough, and even if it was, the paint isn't. Through bolt the lid about every 4 inches with 1/4 ss bolts. I'd suggest using an ss bonded sealing washer on top. Begin by drilling and filling 5/8" holes so that the fasteners all have a compression annulus. Then, reinforce the edge with 3-4 more layers of 1708 or similar making a decent C channel. Now you will have a 55# hardtop which, with 5# of bolts and acorn nuts, will stay attached and probably in one piece. The bolts will require a local laminate thickness of about 65% of their diameter. This can feather out starting about 2 diameters from the bolt Cl or just beyond the washer. So 4 or 5 layers of 17-08 flanging the bolt on both sides of the core.

    There is a reason the production hard tops weigh 4 times what yours does - they quit making them heavier when they quit ripping off the boats. Consider driving down the interstate with a 40 mph headwind or cross wind. These things need to handle hurricane force winds as normal everyday occurrences for 20 years. Unless there are metal bows underneath, your top will probably oil can out on the water. I think we used to use 1 1/4 Nidacore for this, with metal or wood ribs and a solid perimeter.

    I think this is why 65 people didn't say anything about your proposal.
     
    Last edited: Mar 16, 2017

  15. otseg
    Joined: Jan 2007
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    otseg Junior Member

    I build a 70 mph Center Console Catamaran and I hate fastenings too. There are six for each engine, and a few for hinges and hardware. Our T top solution was bonded sockets for anodized Aluminum Tubing legs. My first customer liked the traditional look, or the legs would have been composite too.

    If you had a canvas top previously, The new top seems very robust to me.

    There are some heroic caulking/adhesives out there. Teak decking sells some,
    and the glass windshield guys that bond the glass to a frame steered me towards an industrial grade of silicone used to bond glass to metal frames on buildings.

    My guts tell me you need more than a 1/4" bead. I would tape it off neatly and caulk each side so you had at least 3/4" of bond area. Philsweet is right about the wind speed trailering.

    I tried a hi bond foam tape to secure a poling platform and it only made it about 15 miles.
     

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