Possible Design Flaw Causing Cracks?

Discussion in 'Fiberglass and Composite Boat Building' started by crankit, Jan 16, 2014.

  1. crankit
    Joined: Oct 2007
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    crankit Junior Member

    I've found a few stress cracks during my boat restoration of my Caribbean Crusader and they have me a little worried. I'm guessing it's because of the front section of the boat being relatively rigid while the rest is more flexible, seems like a design flaw.

    Any ideas how I go about this one? Do I need more longitudinal support with extra stringers or should I just grind back and add a whole lot of fiberglass?

    At first I thought I caused it during my restoration but after checking some earlier photos before the stringers came out it seem like the cracks had been there a while.

    The red lines in the pic is where the crack runs.

    [​IMG]
    [​IMG]
    [​IMG]
    https://imagizer.imageshack.us/v2/588x882q90/543/vmdg.jpg
     
  2. TANSL
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    TANSL Senior Member

    Perhaps can be of help some brackets and longitudinals like this
     

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  3. daiquiri
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    daiquiri Engineering and Design

    This is a typical crack pattern produced by a hard spot - which is a point of contact between the skin and the internal reinforcements (stringers or bulkheads). In your case it appears to be produced by the bulkhead pushing against the bottom of the hull.

    When the boat is in seaway, the bottom of the hull slams against waves, the hull skin flexes and slightly (but continuously) changes it's shape. If this elastic movement is restrained in a too brutal way, such as by a very rigid bulkhead directly pushing against the skin, very high localized deflections and stresses are produced along the line of the contact between the bulkhead and the hull. When this localized stress exceeds the elastic limits of the laminate, cracks are produced. The gelcoat is usually the first one to fail and crack, because it is generally less elastic than the underlying laminate. So the cracks on the gelcoat are a first indicator of the problem.

    To avoid the creation of hard spots, you have to avoid a direct contact of the bulkhead and the hull, by inserting a layer of soft material along the whole edge of the bulkhead or the stringer. A few millimeters of foam or epoxy putty between the bulkhead and the hull bottom is a good way to go. I have also seen thin bands of neoprene or balsa wood used for the same purpose.

    Check this page to see how to properly mount the bulkhead against the skin of the hull: http://boatbuildercentral.com/howto/repair.php#.UtgRFxB5Ph4

    Cheers
     
  4. crankit
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    crankit Junior Member

    Thanks TANSL
    Just found a pic of another crusader, same model but a later year and it looks like they have a little more support around that area in the later versions.

    Aren't I still going to have to grind out the crack and put in fiberglass both inside and out? The actual laminate seems to have the crack and not just the gelcoat.

    [​IMG]
     
  5. crankit
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    crankit Junior Member

    Something I was considering was putting a small snip of felt under there to keep the bulkhead and stringers off the deck when I place them into the thickened epoxy putty.
     
  6. crankit
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    crankit Junior Member

    Here's another pic of the area
    [​IMG]
    [​IMG]

    Bit of a dilemma here, Initially I was thinking of going under the cabin bed/deck since it seems fairly solid without rot, if I apply fiberglass over the cracks then I'll probably have to cut out the whole section at a guess.
     
  7. PAR
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    PAR Yacht Designer/Builder

    You don't have to cut out the whole area, just repair the cracks. Grind away the laminate around the crack, feathering it out at least 4" on either side of the crack centerline. Grind at least 1/3 of the way through the laminate on each side (inside and out). Fill the concave areas with more materiel and resin and fair, I'd recommend a mat/biax (1708 or similar) as the intial layer of material, mat side against the hull. Then progress with alternating mat/fabric (cloth or roving), until you've bulked up to the required thickness.

    With the cracked laminate repaired, you can decrease the likelihood of a new stress riser, by using a heavy bead of polyurethane (3M-5200 works well) on the contact line where the bulkhead(s) land. You can use foam or other materials too. Make the goo ooze out into a fillet on each side of the bulkhead, then tab it in with enough fabric to hold it down, insuring the you stagger the tabbing overlaps, as to not make additional stress concentrations. The stringers and other structural elements need this same treatment or they too will develop stress cracks.
     
  8. crankit
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    crankit Junior Member

    PAR, Do you think it's advisable to add some extra support with girders fiberglassed in along the chines?

    At the moment I'm using boat cote epoxy which is what I would like to stick to with further repairs to this boat. I think you actually recommended it to me a few months back after I did work on the transom:p

    I've used 17ounce biaxial with the boat cote epoxy on the transom, just wondering If it will be also be fine for the stress cracks?
     
  9. PAR
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    PAR Yacht Designer/Builder

    Welcome back. This is that Bertram you've been working on?

    Skip the mat with the epoxy layups, you don't need it. I don't think you need extra support and reinforcement such as extra stringers and athwart stiffeners, like bulkheads and partitions.

    What mass produced boat fail at is the details. They tend to use low skilled labor and shoddy work results, so tabbing and bulkhead/stringer placement so they don't create hard points abound. You have the advantage of paying far less for your labor, so you can afford to do it right. Eliminate the hard points, so stress concentrations can't form, insure good, well staggered tabbing and it'll be stronger, stiffer for the same relative weight, which is a good thing and how the boat should have been built in the first place. I'd use biax to fill the bulk of the ground out areas around the cracks, but a light layer of mat on the outside to prevent fabric print through. Other then this, there's not reason for mat.
     
  10. daiquiri
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    daiquiri Engineering and Design

    PAR, mat as a first layer against the ground out area sounded like a good idea. It soaks with resin and makes a good interface with biaxial cloth. Why did you change the idea and say it is not necessary?
     
  11. TANSL
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    TANSL Senior Member

    I think there has been a problem in longitudinal resistance, worse, perhaps, by the bad connection of the bulkhead to the hull. But a longitudinal problem can only be solved by increasing the longitudinal reinforcement.
    The diagram is very primitive but may help to explain what I mean
     

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  12. Petros
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    Petros Senior Member

    looks to me that all of the strength is in the keel and gunwales (where there is no cracks), so the chine area is free to flex and "oil can", allowing it to develop the cracks right where the bulkhead line is located. consider that when under way in a chop this area must flex in and out, fatiguing the unsupported skin.

    If it were my hull I would repair the cracks as PAR suggests, and than add continuous reinforced stringers at the chine line and at the existing stringer than runs well past the bulkhead (notch the bulkhead where the stringer passes through). Taper the thickness of the stringer as it goes forward to spread the support such that no hard point or hinge line will form on the skin.

    This is the way aircraft structures are made, and it will serve well here as well. the reinforced stringer will not allow the hull to "oil can" over the bulkhead line and develop the crack there.

    If you really want to make it strong, you might consider adding auxiliary stringers in between the existing stringers so the skin is well supported in this area. It might not be necessary, but depending on how you intend to use the boat (rough seas and higher speeds for example) it would not be that much more effort to really support the skin well in this area.

    good luck.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    He is using epoxy for the repair ? That would make mat unnecessary.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    I doubt the cracking arises from a serious design flaw, this is an old boat and would have cracked up long ago if that were the case. It is a proven deep-vee boat sold in large numbers, would be subject to a lot of hard use offshore, and when the internal framing gives up the ghost through rot etc, more flex, and more cracking is to be expected. What does the gelcoat look like in this area ? I have seen plenty of glass boats that cracked the gelcoat around bulkhead hard spots.
     

  15. PAR
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    PAR Yacht Designer/Builder

    This is likely just a poorly placed bulkhead, that rested on the hull shell instead of hovering over it, held in place by it's tabbing. This was a common tactic of boats built in this era. No cushion, just a floating bulkhead, tabbed to the hull shell.

    Gel coat looks to be cracked clean through and the laminate also looks to have cracked as well.

    Yes, I initially didn't remember this poster and assumed a polyester laminate, but changed the recommendation after realizing he was using epoxy.
     
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