Newb question - Dry rot

Discussion in 'Fiberglass and Composite Boat Building' started by mrmarkos, Mar 31, 2009.

  1. mrmarkos
    Joined: Mar 2009
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    Location: Sydney, Australia

    mrmarkos Junior Member

    Hi,

    I have a fibreglass boat, that has a shelf that runs most of the length of the boat about midway between the deck and the gunwales. The outer edge of this shelf is now rotting away.

    The shelf is about 6 inches wide and the rot is in the first 1-2 inches of the shelf. Since the boat is an open runabout, this shelf (and the one on the other side) provide pretty much the only storage space on the boat, so I don't want to just cut the rot out and seal the end with epoxy / fibreglass - I want to do a proper repair. What would be the right way to do this?

    I know I have to cut the rot out, then grind back the fibreglass from the remaining shelf, but then how do I rebuild the width of the shelf and make sure it's strong?

    Thanks.
     
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  2. Lt. Holden
    Joined: Sep 2007
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    Location: Western Massachusetts

    Lt. Holden Senior Member

    A picture would be most helpful. Lacking that; if I read you correctly, it is the top corner of the shelf closest to the centerline of the boat that you are talking about?

    If so, I would cut all of the rotted portion out until you reach sound material. Then I would make an L-shaped piece (out of marine or exterior ply) to sit on top of the remaining (partial) shelf with the vertical part of the "L" against the vertical face. I would install a triangular shaped gusset inside the corner of the "L" to reinforce it. Then tab it and glass it in place.

    Since this shelf is a key support structure; as a precaution, I would stabilize the gunwales with a brace and/or ratchet strap while doing the work.
     
  3. mrmarkos
    Joined: Mar 2009
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    Location: Sydney, Australia

    mrmarkos Junior Member

    Yes, it is the edge of the shelf closest to the centreline of the boat. Is this shelf a part of the support structure?

    What to you mean by "tab it"?

    I will post some photo's as soon as possible - it's just been raining all day and I haven't been able to get to it.

    Thanks
     
  4. Lt. Holden
    Joined: Sep 2007
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    Location: Western Massachusetts

    Lt. Holden Senior Member

    Tabbing would entail the use of fiberglass or other tape that would be laid on top of the new piece and run up vertically inside the hull and be glassed in place to secure it. The same would be done down the vertical surface of the new piece and be tabbed to the deck.

    Once you post pix I can probably mark one up to demonstrate what I mean. Depending on your configuration you may also be able to use some countersunk screws through the new piece into the remaining original shelf.

    I will post a pic of my Dorsett Sea Hawk Jet Runabout which has the same shelf as yours (I think). Since it is a runabout hull with only a closed foredeck for support, I believe the box structure of the shelf adds rigidity to the hull sides (someone please correct me if I am wrong in my assumption).
     
  5. Lt. Holden
    Joined: Sep 2007
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    Location: Western Massachusetts

    Lt. Holden Senior Member

    The shelf is to the left of the helm. As you can see it reinforces the juncture between the deck/bottom and the chine/hull side.
     

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  6. mrmarkos
    Joined: Mar 2009
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    Location: Sydney, Australia

    mrmarkos Junior Member

    I finally got a chance to take some photos. The shelf on your boat serves a different purpose to mine. Mine does not appear to be structural. If the white bit wasn't attached it would just be a floating plank of wood running along the side of the boat.

    02042009069.jpg


    Here's a picture of the actual rot. The whit vinyl around the wall of the shelf has something soft and mushy inside (that I suspect was once unlaminated wood) and is obviously holding water. This water is seeping in via the screws holding it in place.
    View attachment 30506
     
  7. Lt. Holden
    Joined: Sep 2007
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    Location: Western Massachusetts

    Lt. Holden Senior Member

    What is behind the white vertical vinyl-covered piece? In order to form a shelf doesn't there have to be a horizontal piece tied into the hull? It also appears that the damage is at the deck juncture, not the shelf top?
     
  8. mrmarkos
    Joined: Mar 2009
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    Location: Sydney, Australia

    mrmarkos Junior Member

    Here are some better pictures of what I'm dealing with.
     

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  9. Landlubber
    Joined: Jun 2007
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    Location: Brisbane

    Landlubber Senior Member

    No good playing with it, chop it all off, make up a new one from plywood, grind back the flocoat to bare glass about 6" away from the edges and glass it all back up as mentioned above. A final coat af flocoat again, and she will be like new....oh and do support the whole boat with jacks or lumber supports just to hold her stable, the shelf does not really hold the thing together, but it certainly is a part of the show.
     
  10. mrmarkos
    Joined: Mar 2009
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    mrmarkos Junior Member

    One more pic from on top.
     

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  11. mrmarkos
    Joined: Mar 2009
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    Location: Sydney, Australia

    mrmarkos Junior Member

    How many layers of fibreglass would I need to give it the strength it requires?
     

  12. Landlubber
    Joined: Jun 2007
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    Location: Brisbane

    Landlubber Senior Member

    Using 300gsm csm, you will need three or four, the topsides where you grind away the flowcoat will be built up staggered, say 25mm overlaps, the plywood is just covered totally. Try not to just soak it with resin, but use as little as you can to wet it out, I could get technical, but to no real avail, just roll the wet csm out as best you can, the end result will be as or better than new.
     
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