Stringers mounted on balsa?

Discussion in 'Fiberglass and Composite Boat Building' started by Bigtalljv, Feb 11, 2022.

  1. Bigtalljv
    Joined: Aug 2019
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    Location: California

    Bigtalljv Junior Member

    Hi,

    I’ve finally started on my project boat and in doing the initial demolition I’ve discovered the stringers were set on top of a layer of balsa, it’s about 8” wide under the stringer. It’s a mid-70s boat. What as the rationale for that method? Do I need to duplicate it if I am replacing all the stringers?

    Sorry I dont have great pictures

    thanks
    Jason
     

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  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Is this a follow on (same boat?) to your hull extension thread here? -
    Outboard bracket Vs hull extension https://www.boatdesign.net/threads/outboard-bracket-vs-hull-extension.62704/

    Re how the stringers were set on a layer of balsa, has this balsa all gone mushy now on the stringer(s) chopped out so far?
    What is the condition like of the timber inside the stringer(s)?
    If you are replacing the stringers you could just use timber or foam to give the shape, and then apply extra laminations to give the strength in the form of a 'top hat' section.
    Make sure that you keep the hull well supported when cutting out stringers, to avoid distortion.

    Do you have any additional photos of the boat or the relevant working area(s) that you can post?
     
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    That is to avoid creating a "hard spot" on the hull. It spreads out the forces and prevents cracking.
     
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  4. Bigtalljv
    Joined: Aug 2019
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    Location: California

    Bigtalljv Junior Member

    Thanks guys. Yes, this is the Radon I posted about a while back. I’m away from the boat right now so I can’t get any more pictures.

    Is the balsa method archaic or is that still a common method of construction? I’ve only seen people bed stringers in thickened epoxy in my very limited internet experience.

    The wood in the stringers looks ok as I move forward, the stringer wood and balsa were wet back at the transom and around the old engine mount holes. I suspect that it will be hit and miss depending on how well they sealed the limber holes as I move forward. There is a 1/4” of glass on the outside of the stringers. The hull is 1/2” (or more) of solid glass.
     
  5. bajansailor
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    bajansailor Marine Surveyor

    I don't think it is / was very common to use balsa - whatever they use, it does not do much other than allow the tabbing / overlaminations to spread onto the hull over a wider area, as noted by Gonzo above.
    You could do the same now (and probably more effectively) with a large radius fillet on each side of the stringer to create the radius.
    With 1/4" / 6 mm of solid glass over the core of the stringer then that should be pretty strong, even if the core is rotten, assuming that the bond to the hull is good.

    Please do post some more photos when you are able to.
     
  6. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Gonzo gets an A+.

    A way to avoid hardspot and maybe get cheaper on the stringer height.

    Padding is common, but best done with foam core. Suppose the stringer is 1" wide, a pad made of 12mm core 2" wide with 45s on each side can work.

    Hard to tell how high they made this padding from pics.
     
  7. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    As other's have said you don't need to use balsa. Everything you look at represents what the builder had available at the time. We've progressed a lot in 50 years or so. Once you determine what needs to be replaced, you can move forward. The idea is to make as smooth a radius as possible where the stringer meets the inside of the hull. Fiberglass cloth doesn't like going around sharp corners. Making a radius with a solid epoxy fillet is expensive and unnecessary. That's why they used balsa, to save money.

    Today you can use closed cell foam to make the fillet and even the stringer itself. You'll figure out the angle that the stringer needs to be cut. Once that's done you'll need to figure the angles for the foam fillets. These can be easily cut on a table saw. Knowing a little trigonometry/geometry doesn't hurt.

    I'll post a few pictures below. I used Owens Corning Foamular 250 for my stringers. You can get it at Lowes, Home Depot and many other places. This is closed cell foam so it doesn't absorb water should you get a leak someday. Since you're an outboard, you won't need the wood blocks that I installed. Those blocks gave the stringers strength in compression as I had to mount engine beds for an inboard. Based on the photo, it looks like you could use 1" thick foam rather than the 2" thick product I used. I used 1708 Biaxle Stitchmat for my layup. Others will tell you to use roven woving or cloth. It probably doesn't matter either way but you may use more resin with the 1708. I don't know the size of the boat or the power so I can't speak to the thickness of the layup.

    BTW, those stringers were built in 2008. The engine photo was taken in 2021. They're as solid today as they were in '08.

    Good luck with your project.


    IMG_0778.JPG IMG_0242.JPG IMG_0243.JPG IMG_20210517_142007111.jpg
     
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  8. Bigtalljv
    Joined: Aug 2019
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    Location: California

    Bigtalljv Junior Member

    thanks! That’s the definitive answer I was looking for and I’m going to run with it. I am familiar with the bedding in epoxy and the radius fillets for the glass. I have a few hundred yards of 1708 ready to go. I was expecting to use the epoxy putty method and I’m not worried about added cost, it’s what I am familiar with unless there is a darn good reason not to.

    What’s the thought on replacing sections of stringers vs the whole things? Replacing all of them is a massive job but would enable me to know the condition of every inch and if I’m doing it I do it once? Replacing sections would be easier and I’ll have splices in the core no matter what I do?

    Second issue in in my limited internet experience I have seen the stringers are usually are tall enough to support the sole. Obviously these are not here in the engine bay but I also know they are the same height moving forward as there is a horizontal fuel tank just forward if the engine bay and then a large below deck hold in front of that.

    If I replace them completely do I make them sole height?

    if I replace sections do I add height to them?

    I’m inclined to replace the stringers totally. For consistency sake, the whole length will be the same rather than rather than creating stringers of various construction and to know they are in perfect condition. If I’m stupid enough to undertake rebuilding a boat I should do it “right” and completely, that’s kind of my style.

    Jason
     
  9. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Make sure the hull is well supported from below with no pressure points and do one side at a time.

    You can use 1708 and epoxy or the esters. Either will work, the bond strength of epoxy doesn't care about the csm bonds. Won't conform to too crazy.

    Adding height is pretty hack and can result in pinpointing; so I'd avoid it.
     
  10. missinginaction
    Joined: Aug 2007
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    missinginaction Senior Member

    Is this an outboard runabout? Before we go much further we need to know exactly what we're working on.

    In general I'd say that you do not have to replace the entire stringer. I did mine because over the years the engine beds collapsed. But your stringers aren't supporting better than half a ton of metal. Consider how the outboard is mounted and how the forces are transmitted through the transom and into the boat. Again, generally, I'd want to replicate what the original builder did but use newer, better materials.

    You asked about the floor or sole. That doesn't need to rest directly on the stringers. You also have what I call transverse bulkheads (side to side) creating a grid pattern with the stringers. The transverse bulkheads are sometimes higher than the stringers. If you need more height you can add what i call sole stringers to bring the floor a little higher still. Again following the original builders lead will usually be the best course of action.

    I'll post a photo or two from when I did my boat years ago. Look carefully and you'll see how the original builder (Silverton) used "sole stringers" to raise and support the cabin sole. Those sole stringers were just made out of pressure treated southern yellow pine. No epoxy or special coating and they lasted until I did the restoration in the early 2000's. They were right under the cabin sole and never even got wet.

    This may be more elaborate than what you need to do but I'm just trying to give you the idea.

    [​IMG]

    [​IMG]
     
  11. Bigtalljv
    Joined: Aug 2019
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    Location: California

    Bigtalljv Junior Member

    Thanks guys. I tend to over-do, it’s either perfect or it’s not done…

    I can get some pictures tomorrow

    thanks
    Jason
     

  12. Bigtalljv
    Joined: Aug 2019
    Posts: 55
    Likes: 9, Points: 8
    Location: California

    Bigtalljv Junior Member

    Attached Files:

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