Repairing cracks / splitting in foam cored floors.

Discussion in 'Fiberglass and Composite Boat Building' started by Midday Gun, Jul 13, 2021.

  1. Midday Gun
    Joined: Mar 2019
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    Midday Gun Junior Member

    I'm looking for some advice on how to tackle this repair to the floors that stiffen the bottom of my 28' sailing boat around the keel area.

    Post is a bit heavy on info & pics.
    Boat is around 2800kg (ish) total weight, keel weight around 1300kg, lead, built 1985 in polyester.

    A couple of months or so ago I noticed a small crack in the paint in bilge forward of the mast compression post. After monitoring it for a while it looked to have grown, so I decided to take off the paint in this area, and the crack was actually rather large.
    It should be pointed out that due to the way the vessel is built, this crack is in an internal laminate, not the actual bottom of the boat.
    20210614_151155.jpg 20210614_165642.jpg

    If you look at the floor in the right of the picture its obvious that some repair has been attempted before with what looks to be very lightweight finishing cloth.
    Now if it were just this then I would just grind out the crack, taper it out and glass it back up with some biaxial cloth.

    However I started doing some more digging & on two of the floors glass fibre has split away at the bottom radius in the middle areas. Its actually sprung forward over the top of the backing plate for the keel bolts.
    There is also evidence of a repair on the fwd edge of one as its not very true compared to the others.

    20210614_152109.jpg 20210614_152248.jpg 20210614_164925.jpg 20210614_171541.jpg 20210614_172255.jpg

    Two of floors are showing the break on the lower radius like the one above.
    It should be noted that there is no evidence of keel movement or any water leaking into the bilges, old photos from an old for sale add make it look like this problem may have existed for some time.
    The splitting is limited to around the backing plate of the keel bolts & doesn't extend any further up the stringers.

    While the laminate is solid in way of the keel bolts, it can be seen that the area just outside is cored, when the boat was laid up she had the internal liner inserted this included the floors, all the internal structure etc, it was bonded in place & all the voids filled with 'low density PVC foam.
    As the foam is low density, I'm not sure how much structure it is actually adding.

    This is the layup of the hull & internal liner in that area:
    upload_2021-7-13_10-7-46.png
    upload_2021-7-13_10-8-14.png

    upload_2021-7-13_10-9-9.png

    Its not clear from these original design drawings if the floor area area outside of the keel bolts is supposed to have any core in it as shown in the above pictures of mine or not.

    It should also be noted that the original cast iron keel was replaced in 1992 with a lead one, this was designed by the original designer of the yacht & is approx 5% heavier with a lower CoG. The shape and bolt pattern is identical (the keel has a hollow pocket to keep the weight down), the work was carried out by Iron Bros in England who have a good reputation for these things.

    Is this a keel out job, or could I carry out effective repairs in situ? I had the idea that it may be possible to remove the nuts & backing plate from just one floor at a time to carry out the work, although this would mean having to work around the protruding studs.
    The fact that the split is just in the centre seems to suggest that its not in any danger of falling off,
    but its definitely something I want to fix as soon as possible.

    If it needs dropping then of course I'll do it, but then I imagine I'm in for a world of pain when it comes to re-aligning it all on the reassembly.
    Sorry for the essay but I thought to include as much info as possible.

    Any thoughts?
     
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  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    What a wonderfully informative post MG!

    What class of boat is she please?
    Does this class have any history of keel problems?
    The construction drawings shown are beautifully drawn, from the days before CAD systems.

    I would be inclined to try removing the backing plates one at a time in order to work locally on the affected areas. Maybe secure a plastic tube over the stud so that you don't get resin in the threads when you are re-laminating.
     
  3. Midday Gun
    Joined: Mar 2019
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    Midday Gun Junior Member

    I would prefer not to mention the class, there's not many of them around and I wouldn't want the first google search to bring up this! (I'm sure anyone could work it out by looking at my previous posts anyway)
    You're correct, the drawings are lovely, and I was very lucky to be able to obtain them.

    Not history of keel problems within the class, its a non extreme keel design. I'm fairly certain that this has been this way for some time & its shown no indication of getting worse. (I used a very rudimentary method of permanent marker on each split to see if it expanded)
    upload_2021-7-13_14-13-45.png
    upload_2021-7-13_14-14-44.png
     
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  4. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    There is not much you can do but grind and relaminate. You should use biax and/or uni fabrics for the repair, not the original WR. Grind the liner down to the original hull laminate and bond the new laminations to it. The backing plate seems to be very close to the floor/hull transition, you should make a better radius there and grind the plate edge to fit, so it's edge does not cut into the laminate (wich I suspect happened here).

    If the keelbolts are actual bolts, you can remove them two at the time in the area where you are working, plug the holes with some foam (as to not get resin into the hole), then redrill after the laminate is cured. If the keelbolts are cast in place into the ballast, it's a keel out job. Realignment should be easy enough, you plug the holes you work with foam and redrill them later from below, you don't loose spacing.

    There is a series on youtube about a new HR having the same job done professionally with polyester resin, if you are interested I can put a link to it.
     
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  5. fallguy
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    fallguy Senior Member

    So, I will only comment on that which I well understand and not the repair et al. I am a builder and that is all.

    1. A question first. The floors are foam core wrapped in 600g woven roving?, but how are they tabbed to the hull? Or is the wr spec the tabbing as well? This is terrible tabbing if so because there is effectively only 4 layers of strands going over the seam versus 8 layers you'd get with some biax, but what is worse is there is no transition or very little, so it almost appears as though only the final layer of csm tabs the floors to the hull? The drawing also says solid timber under mast post, so this needs some clarification.

    2. The seam, as Rumars mentioned, between the floors and the hull at the backing looks like it was not sealed and perhaps has rotted some, but I could be off. I am not too familiar with placing floor timbers over backing plates, but I can't comprehend not sealing them if so.

    3. I really think what you call splitting is crushed laminate. The backing plates are dreadfully small and the studs are very near the edges.

    4. All that said, the only place that looks absolutely dreadful is the crack. And the repair for it could be relatively simple. I'd grind away all the gc, vee the crack only a tiny bit and clean/blow it well, then I'd use epoxy and bond 3 pieces of 600g biax from the hull up onto the floor sides. I'd also seal the margins between the floors and the backings after it is well dried. Make sure to fill the vee with thickened resins and make nice big fillets at the floor/hull transistion. I'd shoot for the entire section past the for n aft foam risers..
     
  6. fallguy
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    fallguy Senior Member

    Also, if you remove the keel for a major repair job, the holes can be filled with duct seal or clay and easily rebored later. I use duct seal or plastillina modeling clay to close holes I don't want epoxy getting into.

    Further, the floor timbers are probably wood, so you need to make sure they haven't rotted...or they are wet. If they are wet, you'll want to dry them out. You can use halogen lamps, but they will burn the boat down once the water exits and you must attend them every second they are on. I nearly started a boat on fire once this way when I walked to the cooler for a soda or went to put air in a low tire or some foolish one minute task. Fortunately, I smelled the scorching, so please never leave the light on if you are not sotting there watching steam exit the vent holes you make. The second the thing is dry, the boat will catch fire.
     
  7. fallguy
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    fallguy Senior Member

    I don't care to argue for the qualitative decision on whether to drop the keel and reinforce the entire section....just so you know...it is above my pay grade

    D13C60BC-7650-4E1A-9BDD-4B1A636AEDDF.jpeg
     
  8. Midday Gun
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    Midday Gun Junior Member

    I've seen the series referred to, if its the relatively recent one filmed in Sweden repairing a charter yacht? Not a HR though if memory serves me.

    @Rumars, the keel bolts are J-bolts due to the keel being lead. So I would have to work around them, I was concerned with being able to adequately prep the area around them as fitting a sander in there would be awkward with the bolts present. I think you may be right about the laminate being cut, its possible slightly larger backing plates were fitted when the keel was replaced leading to this.

    @fallguy
    1. Its not clear from the drawings, but given the era of the boat circ 1985, I don't believe biax was available, or if available, it certainly wasn't common. Nearly all boats from that era seem to be built of CSM / WR with uni directional on top to enhance the stiffness.
    Since the boat was built with an inner liner which was held in place with special tooling & bonded, voids filled with low density pvc foam its not clear in which areas things were manually glassed and which areas they were bonded.
    The transition to a foam core outside the keel area does seem to suggest that at least in that area they were manually glassed.

    2. I'm not sure what you mean by this?
    The only area with wood is underneath the fwd most floor which has the mast compression post. Last year I drilled out the holes that the mast compression post screws go into & backfilled with epoxy before predrilling in an effort to prevent water getting in. The wood shavings seemed a touch damp but otherwise sound, no rot. This is the only floor with timber, & there is no timber over any backing plates.
    My suspicion is that the rebate you see on for the mast post was not standard, a previous owner lost the rig (the compression post folded) and he had a much more sturdy one built, I suspect it came in over size & they rebated the floor, but this would take away the uni-directional rovings.

    3. The laminate under & around the plates appears sound, the only splitting is at the transition from bilge to floor, I suspect as Rumars points out that the plate may have sat on the radius of the transition.
    The size of the backing plates I would have thought adequate, they're larger than standard for this boat & looking at other vessels of this size and keel weight from around the era they seem to be if anything quite generous.

    4. This area has evidently been repaired poorly before, as can be seen from the edge of the finishing cloth on the floor. The crack however appears to be in the inner liner, unlike the keel bolt section further aft I think there is a layer of foam between there are the bilge.

    Sadly the builder is long gone, so no one left to consult.
    At the moment this is feeling like a keel off job to me, though I suppose on a nearly 40 year old boat we can expect things like this.
     
  9. Rumars
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    Rumars Senior Member

    You are right, it wasn't a HR, but that's the one I was referring to, it's indicative for what's expecting you.
    If they are J-bolts, drop the keel and use the occasion to inspect the keelbolts, it's probably time for it anyway. I hope yo don't need to replace any of them.
    It's not really impossible to work with them in place, you must cut out the entire center section and do the scarfing well away from them, where you can grind freely. Use an appropiate punch to cut holes in the fiberglass beforehand, then drop the fitted piece over the bolts.
     
  10. fallguy
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    fallguy Senior Member

    I see, so the only wood floor is only the loaded one.

    I'd avoid removing the keel unless you want to inspect the bolts a/o get a better understanding of the extent of that crack. For a b/w boat, it may be wise.

    where you have the screwdriver jammed in...how would you expect any difference? A polyester bond to that sheer metal surface is not very strong. And there is no bond length either, so the least bit of movement would result in that opening. The bond is what 100mm long by 2-3mm wide only. A total of 300mm square. 1/2" square, at 500psi means that joint only can take 250#, but what does that mean? It means the hull has sagged some on the centerline over 40 years of gravity or that she was rested on the keel? It is not going back home, if you will. But you can stop her from moving further with amendments, but if you want to change the bond strength at the screwdriver you fillet and use epoxy

    say the fillet is 10mm wide, now you have about 3 times the area and epoxy has 4 times the bond strength and so the bond to the backing plate is 12 times stronger...or capable of. 3000# my math right

    make sure to get to the old glass and get rid of the gelcoat, use epoxy resins

    I will see if @rxcomposite will coin in and offer any advice on a layup, but I'd add 2 layers of db600, one turned degrees..the boat seems to have moved across its width and so another temptation might be to get some tows across that sag which lends itself back to a uni...so another thought would be a biax and an uni where the fibers go port to star, but sort of beyond my paygrade....

    continuing in a conversational manner only, the weight of the keel at full heel must also be considered, so you really need to consider how the glass is laid and my original idea of db sounds bad to me now...I think you need some uni (knitted perhaps) or a triax to get long fibers/strength port to star and the long fibers would be up..
     
  11. Midday Gun
    Joined: Mar 2019
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    Midday Gun Junior Member

    I think maybe I've not explained the problem very well.

    There is no bond to the backing plates & there never was, the backing plates are slipped over the studs and then the nuts put on, if I undid the nuts then I could remove the backing plate,
    The screwdriver through was to illustrate that the fibreglass has broke away in the inner radius.
    With the boat on the hard there is no evidence of the hull sagging over the keel, if it was grounding damage I would expect to see the problem at the aft edge of the keel.

    upload_2021-7-14_15-53-51.png

    The two arrows show the areas in question where the laminate has split. (Ignore swing keel detail it was just an option that mine didn't come with.)
    Essentially in the very middle of the floor its become detached from the bilge at the fwd edge. When I put the screwdriver through its not going through the hull, its just gone through the front of the floor.
     
  12. Midday Gun
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    Midday Gun Junior Member

    So the boat is out & I've started on this repair.

    My plan is to start at the fwd end and make my way aft, replacing structure on the way & ensuring the boat is well supported all the while. Never removing too much structure at once.

    The cracks shown earlier from around the mast compression support were indeed caused by compression of the floor, the core was completely saturated with water & compressing.
    It was also ill fitting, made out of several pieces of wood that were not bonded together & bogged into the bilge with bits of fibreglass filler. There were also significant gaps between it & the glass of the floor it was supposed to be the core of.

    The FWD area with a crack which is supposed to have low density foam inside was basically just air, no foam at all, so no strength at all.

    20211114_171905.jpg

    Lot of resin pooling here, & again you can see in the area that looks to be cored, there's nothing at all, its just hollow. Well apart from the water that has accumulated inside the liner of the years & migrated down to sit in there.

    20211114_184418.jpg



    My current intention is to bond in a new solid oak foot into the hull on thickened epoxy.
    This can then be glassed over into the bilge & I'll use the stubs I've left of the old floors in order to tie it into the old structure.

    I'll probably end up cutting out the sections of the bilge that have no core and glassing them up properly, the floors need to be tied into the hull & at the moment they can just flex on their outboard ends as the core material is missing.

    Question for the fiberglass guys here:
    Is the this area a sufficient overlap to glass the saloon settee bases into the bilge / hull?
    (If I take the glass any higher then it would be visible inside the cabin & need a lot more fairing work, gelcoat / paint etc)
    20210916_145559.jpg
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    I would think it is near impossible for say a marine surveyor to appraise the situation without being there, unless they are familiar with the actual make of boat from first hand experience. If it is revealing surprises to you as you delve further, then it will be a mystery to anyone who is not au fait with the boat, which you are reluctant to name for some reason, it would seem to me that if it is a common happening with these boats, the cure can take advantage of the experience of others who have done rectification work for the same issue.
     
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  14. Midday Gun
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    Midday Gun Junior Member

    Its a Humphries designed MGC 27.
    There aren't many of them around, and the builders are long since out of business.

    I've heard that MG Yachts were known for not the best build quality, this improved a lot when the construction went to Northshore, but my boat is one of the earlier ones. I've spoken to a couple of other owners, but none have had these issues.

    To be honest after a few months considering it, I've got a pretty good idea of how to repair it now, & make it better than new, its just a lot of work. I mainly post the pictures & updates so that if anyone sees an omission or a really bad idea then they can point it out to me. And also to give some closure, nothing more frustrating then trawling the net & not finding out if someone managed to fix their problem.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Do you think a heavy grounding may have caused it ?
     
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