repair to foam core f/glass hull

Discussion in 'Fiberglass and Composite Boat Building' started by Nakufeel, Oct 20, 2014.

  1. Nakufeel
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    Nakufeel Junior Member

    Help needed on repair to foam core f/glass hull

    Dear members,

    i've recently acquired an old (early 1980's) 18ft Jurgens Craft ski boat and have a few queries about some signs of age and how to restore her. Im not sure if she is a polyester or epoxy resin hull but she has what looks like a foam mat core. All the ribs (across the hull) are plywood.
    Most worrying are
    1) The underside has two cracks across the stringers/spray rails (both on port side) that appear hairline but drip for a few days each time the boat is taken out of the water. Ive exposed them and they certainly appear deep. Buoyancy chambers above these cracks used to flood slightly each time she went in the water so i have since cut the old deck out and found that not only parts of the deck but also sections of the plywood formwork (again, in the area of the cracks) are showing signs of rot. From underneath the cracks are about 2" long. From inside they line up with two of the ribs where the interior glass has peeled up a little. Any ideas on how to best fix this? Its tricky sanding in the corners where the plywood ribs cut across the stringers, despite having a handheld linishing machine with various flap and fleece wheels.
    2) the stingers/spray rails seem to be moulded (glass and foam sandwich) into the main hull but internally they are in bad shape and in a few spots I've found weak spots which I've since exposed and found that the void beneath is full of water saturated foam. Pls see photo of boats interior (i believe its attached!). This foam seems to be a separate piece and is not bonded to the main hull below it but seems to be something used to help create a raised profile internally, along the length of the stringers. Any ideas what type of foam this might be? Or how far i need to expose this and how to repair? the stingers run full length of the boat so I'm worried about the integrity throughout!
    Anyone out there who has done work on old f/glass boats please lend a thought here. she's a beauty, just got to get her floating again.
    dom
     
    Last edited: Oct 23, 2014
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    It would be unusual for your boat to be a foam cored and it's extremely unlikely it's epoxy, given it's age and general arrangement.

    It's tough to tell from the single photo, but some cheap foam may have been used as a male mold for reinforcements, but it appears your stringers and athwart stiffeners are "tabbed" plywood covered in fabric.

    There are hundreds of previous posts about these types of repairs, which simply boil down to cutting and grinding back to good laminate and/or wood, bonding/tabbing in new stuff, smoothing and painting. Spend an hour or two, looking through some of these old threads, so you can get a feel for what you're up against, you might be able to figure it out solely from these, though more questions will likely arise and we'll still be here.
     
  3. Nakufeel
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    Nakufeel Junior Member

    Hi
    Thanks for your reply. I will certainly take a look at other related posts.
    Perhaps my terminology was off before... Sorry am not sure of precise names but all the formwork/dividing members in the boat (both across and along the hull) are indeed plywood that has been glasses in around the corners. When I refer to stringers/spray rails I mean the small f/glass profiles that run from bow to stern of the boat...on the underside they are triangular profiles but internally the glass has been moulded over foam strips, which are saturated. In the photo you will see two good sized holes in this in the foreground.
    Is it possible to upload more than 1 photo? I struggled yesterday just to attach one picture. I have a few more which show the areas of concern much more clearly.
    Regarding the core...I can see a very regularly perforated white material between the inner and outer skins of f/glass....what else could this be other than foam?

    I'll get reading my end.

    Thanks again
     
  4. Nakufeel
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    Nakufeel Junior Member

    Dear PAR

    I have been reading.

    It seems the cracks are stress cracks due to a hard spot where the gunwale meets the hull. The gunwales are sitting flush on the hull and looks like they were never tabbed in leaving a space or using a foam/balsa type cushion. Yesterday I removed two sections of rotted out plywood stringers and further exposed the rotted areas of the skid rails internally. I now plan to push out the saturated foam without cutting away too much of the glass.
    Re the stress cracks though... Is it recommended to cut out the gunwales and re-tab them in using a spacer/cushion of sorts so that the hull can flex or just add additional stringers through the problematic gunwale to stiffen the hull?

    A side question of resins and f/glass.... Can I use epoxy resin and chopped strand mat to fill out the cracked areas once I've prepared them? It's likely a polyester hull, can I use epoxy on a polyester hull in these structural areas?

    Dom
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Mat really hasn't any structural value, but you can use it as a bulking agent. You'll use cloth or knitted fabrics for these types of repairs. Yes, epoxy is my first choice for novice repairs.

    You'll have to provide more pictures, so we can figure out how your boat is built. I suspect what you're calling the gunnel is actually the deck cap or liner. These typically aren't bonded or placed on a cushion of some sort, but are physically fastened with rivets or screws, sometimes bolts over a bedding. The age of your boat would make me think it could be butyl rubber, though it could also be polyurethane caulk.

    Stress cracks should be explored to see if the damage is superficial or has mitigated into the laminate. Just grind back any area around the cracks and see if it goes below the gel coat. If it does, you'll need to repair the laminate. If it doesn't typically you can just patch it up as a cosmetic repair.
     
  6. Nakufeel
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    Nakufeel Junior Member

    Hi again

    Can I email photos to you? I don't seem able to upload more onto the forum.

    Cheers
    Dom
     
  7. Nakufeel
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    Nakufeel Junior Member

    I've removed my old photo and uploaded another photo of the inside. Hopefully this will help.

    At this stage I'm wondering if, in an ideal world, i should sand back the whole underside of the hull before doing much more internally as water seems to have infiltrated the areas shown on the photo despite no visible cracks in the gelcoat below.

    But I'm operating with basic tools and do not have the capacity to turn the boat over where she is so i would like some advice on how best to continue from inside the hull?

    Should i be considering cutting out the athwart plywood members and sanding everything back down to the hull and starting again?

    thanks again for help and guidance. its greatly appreciated!

    dom
     

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

    The first thing you should consider is blocking the boat up properly, so the hull doesn't distort as you remove structural elements, such as the foam filled tube like things. These are there to provide support to the hull shell, between unsupported areas. 'Glass is pretty flexible, so between stingers and bulkheads, it'll move and deflect a lot, so they mold in these little stringers to stiffen up the panels.

    To support the hull you'll need to make some supports that hold the centerline and the bilges in relative position. Again, without knowing what you boat looks like (exterior profile of the keel, chines, etc. it's tough to say, but most boats like this have a dead straight centerline from about midship and aft. This straight line needs to be maintained. The chines and forward areas also need to be kept in position. Some plywood forms will do, though any scrap wood can be cobbled together to work, so long as it prevents the boat from moving as you walk around inside and grind off stuff.

    Wholesale grinding is just going to make you itch more. I don't cut or grind anything unless it needs it. As to where it's leaking and how the core is getting wet, well without a personal inspection, this is very difficult to tell, but there are "the usual suspects", such as along lift strakes, which may be added after the hull is molded, any hull shell penetrations and other obvious places to look.

    The athwart plywood pieces, if wet and rotted should be cut out whole sale. You treat this just like cancer - you cut it back until you're absolutely sure there's no rot, then scab on new pieces to replace what you've hacked out. In many cases (like this) you might find it easier just to remove all of the offending plywood parts and start over. I go by a percentage. If more than 20% to 30% of the surface has the potential for rot in it upon further inspection, I remove the whole piece. If on the other hand, just a few inches of the bottom of a bulkhead is wet and rotting, I might just cut this off, fit a new piece and move on to the next pain in my butt (or yours in this case).
     
  9. Nakufeel
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    Nakufeel Junior Member

    hi,

    great. thanks for advice on supporting the hull.

    itching is my new mistress! i take her to bed with me and we wake up arm in arm each morning!

    By Lift Strakes i presume you are referring to what i was calling the stringers/spray rails, i.e the longitudinal profiles that run along the underside of the hull from around midship towards the bow? From cutting into the tubes on the inside, and sanding away the paint on the underside, it looks like these were moulded together with the hull, not added later. So far these look in good condition internally where i have cut away the foam filled GRP tubes and removed the wet foam.*

    Im still not sure how the foam filled tubes became saturated as they have clearly been added after the hull was moulded so the interior GRP skin of the hull should be a barrier between the core and the void in these tubes. Unless the cracks in the "lift strakes" go clear through the outer GRP skin, the core, and the inner skin, into these tubes.

    Agreed, wholesale grinding is a chore. Today I'm crawling underneath to grind away the paint on the "lift strakes" at each point they cross under the interior "bulwarks" to see if more cracks are lurking under the paint. I'll hopefully be able to deduce whether the water is coming into the foam filled tubes through these cracks or if its a core issue.

    Regarding the core. I have taken out one of the two brass plugs that penetrate the hull just to the side of the keel. You will see one of them in the photo of the interior just by where i have written "Sanded down and wet foam removed. I presume these were for the original vacuum bagging tubes when the hull was made? I need to sand around this plug hole a little but it is not telling me much of the core material as the side walls of the hole are solid. Is it possible that the first 5" or so either side of the keel is a solid GRP layup and then from there out, and up the chines, it becomes a double skin with core? As i mentioned, everywhere i have sanded back paint i can see a whitish material inside that has a very regular pattern of small pinprick holes through it (presumably to assist bonding between inner and outer skin?).

    The screws in the keel are another hull penetrating feature and, as i said before, some of them are free-spinning in their holes. I took one out just now and it was certainly wet. Im wondering if i should cut some inspection holes internally above the keel or if that is a silly idea?

    have a good day.

    thanks again,

    dom
     
  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The bronze plugs you see aren't left over from vacuum bagging operations. These are bilge drains and are opened to let water drain from the hull if it gets flooded for some reason.

    There shouldn't be any screws in the hull below the LWL, unless it's holding some hardware in place. If there are screws, remove them, clean out the holes, fill with epoxy, then if you are holding something on (bilge drain, raw water pickup, etc.) redrill for new fasteners in the restored holes. It would be a good idea to remove every bit of plastic or metal that might be attached to the hull. This will let you inspect the hull shell, which I'm still not convinced is foam cored. I think they used foam as a male mold over which they laid fabric, making a localized reinforcement. A foam cored hull will have an inner and out skin, bonded to the foam. Keels would often be a solid laminate, as would other areas, like the deck cap flange, the chine crease, etc.

    The white, pin hole laden stuff is cloth, in the laminate. If you pour some acetone or MEK on the white stuff, it should change color and look like the other laminate. If it doesn't, the laminate is probably fractured in this area and needs to be repaired.
     
  11. Nakufeel
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    Nakufeel Junior Member

    hi,

    i hope you had an enjoyable weekend.

    understood about the brass plugs. With regards other fasteners below the waterline.... my boat has a stainless steel flatbar reinforcement running the length of the keel which is held on by screws. Many of these are just spinning in their holes and have clearly been siliconed in place. Would you suggest the same treatment?... remove, fill holes with epoxy and then refit?

    Im now in agreement with you that the hull is not foam cored, it seems to be just one skin of laminate with, as you say, a cloth in the middle. Without a core is it possible to have a "core" issue with water creeping into the foam filled tubes or is it now clear that this water must be coming in through cracks?

    Regarding the underside, having sanded away at the intersection of each longitudinal "strake" and athwart "bulwark" i have found more micro fractures as well as another good sized crack which i have now sanded back and consequentially exposed a decent sized hole. There are now three definite breaches in the hull that i know of. Im now curious to flip the boat and sand back all the paint to check the condition of the hull before i put too much time and energy into the inside. Now that i have cut the deck out and removed a number of plywood members (stringers, bulwarks...) from below, i am nervous that i have compromised the integrity of the hull. Is there a clever way to flip her over without causing damage? i don't think she needs to flip 180 degrees, just rolling her enough one way then the other, so that i can remove the keel screws and do some sanding would be enough. Im looking on youtube but haven't found anything too useful yet. I can upload a picture of the whole boat to give you a better idea of her shape. She is currently on her trailer. I can get lots of guys to lift no problem.

    anyhow, i would appreciate you thoughts on all this.

    Thanks again,

    dom
     

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

    I'm not sure about the flat bar stock, but typically these would be a sacrificial keel strip, on the outside of the hull. Yes, remove the strip and the fasteners, restore the holes and reinstall the strip over polyurethane bedding, with new screws.

    Yep, you've run into an issue we all must face, how to flip a less than solid hull, while keeping it fairly intact, shape wise. The usual approach is to reinforce the inside of the hull with some forms (plywood, 2x4's, etc.). Often these reinforcements also serve to become to stand for the hull as well, once it's rolled over, offer space under it, so you can get inside and do stuff.

    Conversely, you could just roll her on her side a bit, though this still requires the same type of reinforcement, plus a way of holding her up on her side. I prefer to work in the "down hand" position, which makes sanding and laminating much easier, that overhead or vertical work. If she's on her side, you'll be doing vertical work, but this is slightly better than crawling under the hull and working around the trailer.

    Free beer can get a boat rolled over in minutes and is an old, time honored boat builder's technique.
     
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