Re-coring with balsa of fiberglass

Discussion in 'Fiberglass and Composite Boat Building' started by 67-LS1, Mar 12, 2023.

  1. 67-LS1
    Joined: Aug 2003
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    67-LS1 Senior Member

    I’ve started a stringer/floorboard replacement project on a 1984 Century Coronado (fiberglass) and have now realized that the balsa core in the hull bottom is also bad in a few areas. Once I strip it all out, do I need to go back with Balsa? Or can I add some additional level of glass and forgo the balsa?
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

  3. 67-LS1
    Joined: Aug 2003
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    67-LS1 Senior Member

    I’ll followup with some pics once I get the engine out, hopefully later this week. I’m loath to ask my buddy with a tow truck to lift the engine/trans out in the rain.
    But I can feel some soft spots so I know I’m going to have to address it.
    And yes, my boat is just like the one you linked to but Blue.
    PS - I just got home from Barbados. We stayed at The Crane and had a wonderful time.
     
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  4. fallguy
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    fallguy Senior Member

    Most of the time, a balsa core failure is the end of a hull.

    If the core rot is limited and the entire core not saturated a/o rotten; it can be repaired, but it is so much work that is is generally not economically viable.

    The standard way to fix a total redo is to go to solid frp.
     
    Last edited: Mar 12, 2023
  5. 67-LS1
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    67-LS1 Senior Member

    I’m invested zero dollars in the boat and will be doing it all myself so economics aren’t as important.
    Is there a correlation between x layers of glass/balsa/x laters of glass vs solid glass Matt?
     
  6. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Svens in Alameda will have the necessary balsa.

    Removing and reinstalling the interior is the second greatest hindrance to core replacement.

    Finding all the rotten balsa is the trickiest. Drill just through the interior lamination then probe the balsa. It should be firm dry and not funky smelling. Plan on a few thousand probes.
     
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  7. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Stiffness comes from thickness, so if you remove the core you still have to retain the thickness, meaning a lot of solid laminate and its associated weight. The usual replacement for balsa (provided you don't want balsa again) is foam, or sometimes another wood or plywood. Given it's the bottom of a planing powerboat if you decide for foam use something high density, the no calculations brutal approach beeing coosa.
    For cheapest option some cedar fencing stock will do nicely.
     
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  8. fallguy
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    fallguy Senior Member

    The economics of time and value affect all of us.

    If you have a boat that requires a total core removal, depending on many factors; it can be hundreds or thousands of hours.

    If you spend 1000 hours working on the repair; then you have to recognize the time and the cost of materials against the market value of the boat.

    I had a beautiful old 1974 Starfire 24' with major balsa core rot. I calculated 2000 hours for a full repair, and several thousand dollars materials for a boat with a final value of $10k max. The economics were not there. So sad, the stringers were solid mahogany and not a spec of rot..

    The boat also has to be well supported for recoring if a large area.

    This is why boats go to the dump.

    And I am genuinely tryin to help..best of luck..
     
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  9. 67-LS1
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    67-LS1 Senior Member

    I know the boat will never be worth it I figured for my labor but it’s a learning experience for me too. I have a 66 Chevelle that I updated to a 4 cam direct injected V6 with the ECM, electronics 6 speed auto, etc. I’ll never get back what I have in it but I love it and it’s currently my most economical car at 30-32 mpg.
    I’ll make final call when I see the full extent and I’ll also price balsa at Sven’s. I’m in Alameda often.
     
  10. wet feet
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    wet feet Senior Member

    If you really do start the project,it will be a very big and messy undertaking and it will smell pretty nasty too.Given the weakening that it will cause,it might be best to do it in stages,adding the inner laminate to a re-cored section before moving on to the next zone and you may need to re-chock the hull at times to be sure that support is derived from a sound area of the surface.You are likely to have to cut away parts of the bulkheads to ensure fibre continuity in the area they bear upon and these strips will need to be bonded back.It is theoretically possible to do the job but I will add the warning that I have known one or two people who have begun similar projects but none who have completed them.I will be very happy if you are the exception.
     
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  11. rxcomposite
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    rxcomposite Senior Member

    Balsa has high specific strength and comes in three grades. Low density, medium density, and high density end grain balsa.. It is comparable in shear strength to Divinycell H80. H100, and H130. End grain Balsa is vastly superior in compressive strength, hence it is used in decks or floorings.

    The cored construction is very stiff and all the supporting structure is designed around it. The spacings, the frames, the stiffeners. Replacing it with a single skin laminate will require some re engineering as solid laminate has low stiffness. If you make it thick to compensate, it will be very heavy.

    Try replacing only the few rotten core.
     
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  12. fallguy
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    fallguy Senior Member

    Just to be clear, wet core also requires removal. It is really not possible to dry it.

    RX offers great wisdom on core choices.

    The real difficulty comes in dealing with stringers and transverse elements. It is probably possible to underfill the stringers with bonding paste versus removing it all. I don't know whether this creates a structural deficiency, but if the rot is extensive; removal of the other structures is where this job can become hellish.

    Most of the time, the core rot is caused by ingress from above when people screw something like a bilge pump down into the core without xtra efforts like decoring and filling the hole with something that won't allow ingress. So, be careful to determine sources or the issue repeats. You can also use this in part to chase rot areas, etc.
     
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  13. 67-LS1
    Joined: Aug 2003
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    67-LS1 Senior Member

    This project started because the stringers and floors are mush so that all coming out anyway.
    Once I get the engine out I’ll be able to get the boat off the trailer and onto a frame I’m building to support it,
    Then I’ll be able get all of my accurate measurements and strip out the floors and stringers. I’ll get a better chance to determine the extent of the core failure at that time.
     
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