DIY hard top for bridge

Discussion in 'Fiberglass and Composite Boat Building' started by skipperguy, Jan 13, 2013.

  1. skipperguy
    Joined: Jan 2013
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    Location: florida

    skipperguy New Member

    I have a tubular aluminum bridge top with Sunbrella lashed in place. Would like to replace it with semi-rigid fiberglass top. 1/8" to 3/16" thick would be ideal - supports are every 20" or so. Overall size is appr. 11 ft X 13 ft.
    Need light weight - can anyone suggest construction method and materials. I would Awlgrip paint it when done.
     
  2. groper
    Joined: Jun 2011
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    Location: australia

    groper Senior Member

    grab yourself some corecell, divinycell or airex structural foam core sheets from your composites supplier, 5lb density. Laminate it with 1 layer of circa ~20-25oz (depending on what they stock) triaxial or quadraxial e-glass reinforcement, on each side of the foam. Use epoxy resin to wet it out - not polyester or vinylester resin unless you want to add an additional layer of CSM to each side (add weight and use more resin)

    Before you laminate the 2nd side, mark out the positions of the fastening points/bolts etc. You have to remove the foam core oversize in these areas and backfill it with structural bog so the foam core cannot be compressed in these clamped areas which will lead to delamination down the road. I use a router to remove the foam. So you also need to get yourself some milled fibres and cabosil to make up a structural bog/putty with the epoxy. Fill the areas to be bolted, sand it flush then laminate over it. Once cured drill out the holes for the bolts and your good to paint it as you see fit, preferably using a 2 part polyurethane paint for a long lasting durable finish that provides the epoxy with UV protection.

    The thickness of the foam really depends on how stiff you need it to be, at a guess with 20inch supports, 3/8-1/2in thickness foam would be a good starting point, the costs of the foam go up quickly if you really dont need the extra thickness. anything over 5/8 would be overkill. If you follow the above, the finished panel weight will be exactly 150lbs based on the 11X13ft surface area. You will also require 75lbs of epoxy to do the job assuming no waste based on my spreadsheet which calculate things quite precisely.

    Good luck.
     
  3. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    You can get 1/8" fiberglass panels at Home Depot. They are 4X8 though. You could join them and paint over.
     
  4. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I just bend some 1/8" or 1/4" plywood over the whole shebang and move on. Sheath it if you must, but a good paint job will probably do as well that high up on the boat. Scarf the seams or use the Payson butt joint or even butt blocks for the no muss, no fuss method. You can spend a fortune or be done this weekend, you'll call.
     
  5. michael pierzga
    Joined: Dec 2008
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    Location: spain

    michael pierzga Senior Member

    A local company makes hardtops for center console type boats. Each one is custom. They are all foam core glass epoxy.
    They build these cambered tops on a simple mold. I don't know the density and type of foam. Thickness is about 40mm, with a half round type profile on the perimeter. The tops are very lightweight and not for walking on. An important detail is to imbed high density foam inserts or aluminium bar stock at attachment points.
     
  6. Yellowjacket
    Joined: May 2009
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    Location: Landlocked...

    Yellowjacket Senior Member

    The size (11 x 13) is going to require piecing and framing for plywood, and a foam cored structure would be lighter and won't need any framing. I'd use Airex, and a 3/4 of an inch thick core is going to be a lot stiffer than a half inch, and the weight increase is neglegable.

    I'd make a frame work that I could attach the core to set the shape, and I'd make the shape such that the side edges rolled down with some curvature (big radius, like 5 inches) and use 60 degrees or more of arc, or in other words, roll the side edges for at least three inches of roll off to give you stiffness. And I'd put some crown in the mid section in the fore/aft view to give it stiffness too. I'd also design in with a bit of curvature in the side view, which will also make it a lot stiffer. The Airex can be heated with a heat gun to get it to fit the framework and it works pretty easily. Screw the Airex to the framework by drilling thru the framework and putting the screws into the Airex from the framework side.

    Figure out where the hardpoints are going to be and oversize drill those spots and fill them with a slug of filled epoxy.

    I'd also grind down an area around the attach points and put at least two or three layers of glass on those areas. And radius all of the edges all the way around.

    I'd use probably one layer of 6 oz cloth outside, take it off of the frame, and then lay one or two on the inside, and overlap it over the edges to give it a solid edge.

    Finish the surface and done.
     
  7. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Price up some Airex and see where you stand. My understanding is there exists a frame of some sort, so a scarfed or butt joined plywood roof could just spring over it. You could also strip plank the thing too. Buy some pretty 2x12's sixteen foot long and rip the edge off, on a table saw with the fence set at 3/4" of an inch. Lay these on the frame, edge gluing full length as you go, sheath and paint.
     
  8. Yellowjacket
    Joined: May 2009
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    Location: Landlocked...

    Yellowjacket Senior Member

    In racing the mantra is cheap, fast, reliable, pick any two, but you can't have all three..

    In boating it's light, cheap, or long lasting, pick any two....
     

  9. Frosty

    Frosty Previous Member

    I did similar. I wanted a light weight sun shade for the bridge but I settled for something a little smaller than yours. I bought the skin off a boat yard that makes fishing boats and made a sun shade that fitted with tubing over the helm.

    I fitted the skin with ribs made from foam and glassed in looking a little bit like the Uk flag.

    It is very strong but I could not walk on it. I fastened the rear to the radar stancheon and made 3 inch fibre glass poles for the front that come down vertical into the fly bridge at either side of the console. kinda like the Africat has.

    I really dont think a sheet of ply wood has the effect I would be wanting but then some are happy with this kind of thing. As you seem to have a nice boat a little care would be beneficial.
     
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