Crack at a seam

Discussion in 'Wooden Boat Building and Restoration' started by missinginaction, Jul 7, 2016.

  1. missinginaction
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    missinginaction Senior Member

    As some of you may remember I've restored a 1973 Silverton sedan as a hobby boat over the past few years.
    I noticed this spring that I'd developed a crack at a seam in the deck. I'll post a photo with this thread.
    The crack developed at a seam where the foredeck meets the side deck adjacent to the forward roof pillar(s). I have these cracks on each side (port and starboard) at the same spot.
    Underneath the seams there is an inside bulkhead and between the bulkhead and the bottom of the deck, I installed generous amounts of Mahogany (about 1 inch thick and 6" fore and aft of the seam in order to spread the load out and eliminate the bulkhead creating a hard spot. Nevertheless, some 7 years after I built the deck I have this crack.
    There is no evidence of water infiltration and I believe I'l just sand down the crack, apply a piece of 3" fiberglass tape, taper the edges and repaint it. Maybe it'll last another 7 years.
    Here is a link to a couple of photo's that show the seam in the building stage, years ago. A couple of photos I took today also appear below. Any repair suggestions would be appreciated.

    http://www.boatdesign.net/forums/at...-73-silverton-restoration-photos-img_0280.jpg
    http://www.boatdesign.net/forums/at...-73-silverton-restoration-photos-img_0313.jpg
     

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  2. PAR
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    PAR Yacht Designer/Builder

    Odds are you can grind out the areas and fill as you've described, but this is a classic example of careful panel arrangements. It's difficult to see with these close ups, but I'm pretty sure the windshield frame and pilothouse opening in the deck terminates in this area. Had the plywood been extended several inches further down the side deck, before the joint, this issue wouldn't have occurred. This is one of those things you learn in engineering or simply from experience.

    [​IMG]

    This is a perfect example of where NOT to place a plywood seam. In spite of this designer's intentions, I can guarantee this area will develop a crack along the seam. Even worse, is a bulkhead that lives directly under this seam.

    [​IMG]

    This is another example of the same thing and a true guide, to the lack of engineering or building experience. I'm not cracking on you Bart, but I see this a lot now, with all the novice designers and free software packages, making small craft design simplified.

    The panel joints need to be placed away from these naturally point loaded locations. On side decks of that general size, I'd try to space the joint at least a foot away.

    The attached file is one of my designs and shows some of the panel layouts, on sheets of plywood. The 2nd and 3rd lower left has the foredeck and side/aft deck panels. Note the cutouts for the cabin and aft deck are nowhere near the joints. This arrangement doesn't crack out.
     

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  3. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed. Trying to tell/teach someone one about load paths and structural redundancy is not shown on free software packages. Thus, it ends up being a hard lesson learnt, but sadly well know to others.
     
  4. missinginaction
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    missinginaction Senior Member

    Yeah, I know more now than when I built the decks back around 2009. When I did the restore I just duplicated the original work with some modification. It seemed to me to be a little "busy" around there with the roof pillar, that seam and all. That's why I placed a big backing plate under the deck there, to spread to the load. Maybe I bought myself some time in doing that.

    Thanks for the advice gentleman. It's appreciated as always.

    MIA
     
  5. PAR
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    PAR Yacht Designer/Builder

    When you grind it out, plow down pretty deep, so you can lay in a few layers of 'glass, preferably biax, with a structural filler (milled fibers, silica, cotton flock, etc.), before topping it off with some fairing compound.
     
  6. missinginaction
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    missinginaction Senior Member

    Thanks Paul. Sounds sort of like a dutchman, but tapered more gently. I've got plenty of leftover pieces of fiberglass cloth and tape plus all of the fillers. I gave it a good look today, the raised section of deck is barely up, easily less than a millimeter. It doesn't appear that the fiberglass covering the plywood decking is cracked, just the paint above it. Nevertheless, it'll need some attention.
     
  7. PAR
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    PAR Yacht Designer/Builder

    If it was me, I'd grind down at least 1/2 way through the plywood deck surface, maybe as much as 3/4's, depending on how pissed I was. I'd bulk this up, (after an initial neat wetout) with biax and thickened filler, leaving it slightly shy of the surrounding surface, so I'd have room for a topcoat of fairing compound. I'd also make the tapered areas as generous as practical. So, if the deck is 3/4" thick, I'd grind down to 1/2" deep and taper this back 6" on each side of the crack (8:1 taper), before dropping in the fabrics.
     
  8. missinginaction
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    missinginaction Senior Member

    Thanks Paul. I wouldn't have gone that deep, so your advice gives me some confidence. Looking at it I think my belt sander with some 40 grit to start might make a nice tapered section. I can see where you're going with this, spreading the load out on top of the seam.

    Thanks again,

    Bart
     
  9. PAR
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    PAR Yacht Designer/Builder

    It's not spreading the load so much, as offering a much larger transition (old to new) from the butt jointed panels (old and my assumption) to the areas around them being repaired (new). The idea is to make the plywood panel think it's a continuous sheet, with the deep, well tapered and taped repair.

    I remember your original posts here and how many tried to scare you away from this project (probably me included), but you've come far 'Grasshopper . . .
     
  10. missinginaction
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    missinginaction Senior Member

    Everything you see in those photos I posted is new. The deck forward of the butt joint is 1 1/8" thick, it supports the front of the cabin. Behind the joint the side decks are 5/8". All new materials but at the time I was doing this (2007-2009) I was learning and generally copied what Silverton originally did. One modification that I did make was strengthening the beam that supports the cabin sides and the small side decks aft of the cabin. I found that the original beam had failed (to light, I think) and allowed things to flex.
    You'll notice that the crack I have doesn't extend to the rub rail or right up to the cabin sides. There are substantial supports there that must be preventing any movement.
    For the time being I'm just going to do a small repair to keep any water out. In September, after she's hauled I'll do as you described and solve that problem.
    Thanks for the compliment BTW. One thing I realized in doing this is that the successful projects are ones where the builder has enough resources to buy proper materials (and tools). Just as important is determination. Weather it's a boat or a job or just about anything in life, you need focus and determination. Never give up!
    Yesterday I took a little trip down towards Albany, through a couple of locks. The guys running the locks (lockkeepers? lockmasters?) hadn't seen my little boat before. We talked for awhile as we were getting ready to lock through (yesterday was my first time locking through single handed, I'm a rookie at that too). On the return trip in the afternoon there was a lot of radio chatter as more boats were locking through. It probably doesn't mean anything but I smiled when I realized that I was the only one that the lockkeeper called "captain".
    In a few weeks I'm off on my first trip to central/western New York. I wouldn't be doing this without the assistance I got for you and many others here.

    Best,

    Bart
     
  11. PAR
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    PAR Yacht Designer/Builder

    Don't let it go to your head Bart, he called you captain, because he didn't know you well enough to call you skipper yet. You came into this project a lot better prepared than most do and you took the normal pitfalls on the chin, coming out at the bell for the next round. It takes more than determination for a project like this and it's your character more than anything else, that permitted it to come off so well.
     
  12. missinginaction
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    missinginaction Senior Member

    I've still got a lot to learn Paul.
     

  13. PAR
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    PAR Yacht Designer/Builder

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