Replace with wood vs fiberglass stringers

Discussion in 'Fiberglass and Composite Boat Building' started by 67-LS1, Sep 6, 2022.

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

    I know your supposed to replace stringers to match the originals. I believe this is to maintain the original structural strength in the original areas the designer had in mind.
    My question is, is there a way to compare the strength and stiffness/flexibility of a given wooden/fiberglass encapsulated stringer to a fabricated fiberglass hollow beam glassed into the hull? In other words could an engineer specify a fiberglass layup schedule to have the same characteristics of a given wooden stringer.
    Not that it should make a difference but this is regarding a 22’ 1980 Century Coronado.
    Thanks
     
  2. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Yes.

    A reasonable repair just laying over existing and allowing the wood to rot is possible...
     
  3. tpenfield
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    tpenfield Senior Member

    An engineer could specify a layup schedule that would/could replace a wood core stringer system. However, it is fair to say that a fiberglass over foam stringer system may give different characteristics to the boat. Not necessarily bad, but different.

    If I were doing a full cut & gut restoration, I would probably go with a foam core and fiberglass stringer grid, probably keeping the grid and dimensions the same as original, or close to it. IMO it would be best to use higher density foam as the core and make sure to have adequate glassing of the stringers, since the fiberglass is the strength and the foam is the shape.
     
  4. Tops
    Joined: Aug 2021
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    Tops Senior Member

    I went through some of this trying to figure out what to do on the waterlogged keel supports on my 21'.
    I ended up working with the people at Vectorply via the Vectorlam software at Vectorlam - Vectorply https://vectorply.com/vectorlam-4/ which allows to you to compare lamination schedules and if you are willing it can do beams and plates.
    In the end I decided to go back to marine ply with an 1808+epoxy schedule that is close in key properties to the marine ply and CSM and polyester that I removed. It seemed like foam was going to cost more and take more glassing to have similar properties, plus I had already bought some of the materials and was gifted others so I wanted a 'recipe' using what I had on hand. Had I not ran the software I would have shorted myself on a layer of glass and not placed the fibers in more optimal directions. Now I just need to install it...
     
  5. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    Most older production boats weren't engineered in the same way they are now. They were sketched out on a napkin late at night after far too many drinks, then built the next day by relatively unskilled labor with the cheapest possible materials. If it broke they added more chop, if it didn't they left it as is.

    It resulted in boats being mostly over built for there intended use. This is why most owners don't know the stringers or transom wood are pulp until they really dig into it.

    Replacing what was there with the same stuff ensures you'll likely get the same 30-50 year life span. There are better materials and methods, but are you actually looking for another 50+ years.

    Switching from wood cores to foam, or no core will yield different properties, but probably not a different result. The boat frequently survived the last 10 years with totally rotten stringers, and may have gone another 10 if they hadn't been detected.

    Going to foam cores, epoxy and improving the design looks much better on a spreadsheet, but may do nothing in terms of performance or longevity.

    Attention to detail in the rebuild will typically yield the greatest benefits.
     
    Last edited: Sep 7, 2022
  6. 67-LS1
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    67-LS1 Junior Member

    Interesting points brought up. So is the wood in older stringers nothing more than a form for the fiberglass and 100% of the strength is in the fiberglass?
    And if so, what about where the engine beds are. Mine sit on top but I’ve also seen some though bolted through the stringer.
     
  7. bajansailor
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    bajansailor Marine Surveyor

    If the fibreglass has a good bond to the wood, and the wood is in good condition (no rot) and is bedded down well on the hull, and nothing has sheared anywhere, then (all else being equal) the wood cored stringer should be 'stronger' than a fibreglass stringer with the same lay-up, but no core.
    However if the wood is rotting in places, and the fibreglass is delaminating away from the wood, then the wood is not doing very much any more.
     
  8. fallguy
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    fallguy Senior Member

    No no no!

    All I said is sometimes you can make the old rotten wood into a former and build an all glass stringer. I did not say the wood does nothing. Extensive laminate differences for all glass vs wood n glass.
     
  9. 67-LS1
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    67-LS1 Junior Member

    Sorry. How you explain it here is how I understood it. I guess I could have asked if a laminate schedule could be designed to compensate for the fact that the wood is rotted and adding zero to the equation.

    Now I’m going to say a brand name that is sure to elicit some opinions.

    Seacast.

    I see good and bad opinions on the web but mainly to do with transoms. Or really skinny stringers. Their web page is down and their Facebook page reads like it was written by the same chucklehead that try’s to convince me he’s a Nigerian prince.

    Anyone have any experience with Seacast?
     
  10. ondarvr
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    ondarvr Senior Member

    Seacast is a resin filled with glass fibers and other things, it works, but could be better.

    In the 80s I made a similar blend but used a catalyst with much better characteristics for this application.

    I'm not sure why someone would use it to rebuild stringers, but for a transom it may save a great deal of time and effort if done correctly.
     

  11. 67-LS1
    Joined: Aug 2003
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    67-LS1 Junior Member

    I’ve watched some videos of repairs being made with Seacast and replacing the wood as it was originally. I can see how it may save some time on a transom but not on stringers. By the time you clean out the stringer fiberglass he’ll of all the rotten wood, I might as well just replace it with new, put the glass top of the stringer back on and glass it up. It would surely get another 20 years which could be more than me.
    Thanks for the info.
     
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