Repairing and/or drying GRP covered stringers and bulkheads

Discussion in 'Fiberglass and Composite Boat Building' started by Martin Upton, Apr 8, 2020.

  1. gonzo
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    gonzo Senior Member

    It is incorrect to state that all stringer cores are not structural.
     
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  2. missinginaction
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    missinginaction Senior Member

    How so Gonz? Since I live in New York I have a few minutes on my hands. Actually, I have a lot of minutes on my hands right now. So, I'll take a couple of them to quote a paragraph from Dave Gerr's book The Elements of Boat Strength. This is in chapter 5 Fiberglass: Internal Structure, page 43, where he's discussing stringers and engine beds. He specifies foam cores with wooden inserts at the engine beds then discusses laminate schedules, finally he writes this:

    "NOTE: Some builders have installed hollow or partially hollow-core engine beds/stringers. They reason that the core is really just a former and does no work, so they can save weight. Although many fine craft are constructed this way, hollow stringers worry me, because any small cracks or leaks into the hollow stringer will turn it into a rather long water tank. Not only could this add considerable weight, but - should it freeze - it also will burst open the stringer and destroy it's strength. It is accurate, however, that the core is a nonstructural former. You can use any low cost foam that is convenient, as long as it is compatable with the resin and cannot absorb water. (Solid-wood core at the engine mounts is always required." The emphasis is mine.

    Gerr also discusses stringer/floor construction in his book "The Nature of Boats".

    Engine beds need to be able to hold fasteners of some type so I see the need for solid wood in that area. Gerr gave me the confidence to build my stringers and floors the way I did. I suppose in certain situations the core may indeed be structural, but aside from the engine mounts I can't think of any.

    I just wanted to raise the issue in an attempt to save the OP some unnecessary work.

    Take it easy and have a nice Easter.

    MIA
     
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  3. gonzo
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    gonzo Senior Member

    If the wood is only used as a form, then the statement is correct. However, stating that every structural member that has a fiberglass covering is unnecessary is incorrect. Even though it is possible to build hollow structural members, the only way to know is to either have the original plans with calculations, or to reverse engineer the calculations. I have built many boats with stringers and other structural parts that were laminated with fiberglass. The wood was a necessary structural element.
     
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  4. Blueknarr
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    Blueknarr Senior Member


    I have seen the full range of core/srathing . From hollow FG to minimal FG over wood. One vessel I worked on had 2x8 stringers with bairly enough glass to hold them to the hull. The void on the outboard and under the sole was filled with expansion foam. The foam extruded thru the FG Matt into the space reserved for the fuel tank. The one layer of Matt was never going to be structurally sufficient.

    Just because wood is sheathed in FG does NOT make it a spacer only core.
     
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  5. missinginaction
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    missinginaction Senior Member

    Sure Gonzo. I understand that every part can't be hollowed out like a stringer. That said the OP is talking about stringers and floors.

    Bluenarr, I understand what you're describing. That's careless building though. Not necessarily bad engineering. My boat was a perfect example of that. Whoever designed it did a great job, at least I think so. But the build quality left a lot to be desired. That's why I spent so much time and effort on it.

    My foam cored stringers have been in the boat for 11 years and in service for 6 years. They're fine. The fiberglass scantlings are probably a little heavy as I used Gerr's formulas as a guide and understand from some professional builders that Gerr tends to go on the heavy side. From what I could see, the stringers that the OP showed looked pretty good to me and the boat didn't show evidence of any structural failure that I could see anyway. He'll be the judge of what needs doing or not doing.

    Have a good day guys....MIA
     
    Last edited: Apr 10, 2020
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  6. Martin Upton
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    Martin Upton Junior Member

    Hey guys thanks for the robust discussion. Missinginaction I did wonder that about the core however I have had such ranging advice from people and I do rather concur with gonzo that to think it would depend on the design.

    Being who I am however (a bit of a perfectionist) I like to ‘know’ things are right and I’m now committed (having already taken the outside of the stringers off and removed the rotted wood) Having pulled it apart I can now see a small amount of thinning of the glass on the inside of the stringer so I’ll be fixing that.
    Oh btw what I am calling a bulkhead is not the floor but the vertical structures that are perpendicular to the stringers that run from the hull to the underside of the floor.

    When inspecting glass on the inside of the stringers there appears to be some mold in spots that I can see on the glass. Note the other side of that glass is unaccessable given that it is under the moulded floor which is also filled with foam.

    Questions :
    1. What could I use to kill off the mold?
    2. Any other suggestions as to a waterproof alternative for stringers and bulkheads?

    missinginaction that’s scary news out of New York atm .. stay safe
    Cheers
     
    Last edited: Apr 10, 2020
  7. missinginaction
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    missinginaction Senior Member

    Hi Martin,

    A couple of things. Boat terminology can be confusing. What boat builders call a "floor" is that vertical piece that runs perpendicular to the stringers, port to starboard. Why isn't it called a bulkhead? I don't know. What you would think of as a floor (what you walk on) is called a sole. Go figure. That's how it's always been explained to me.

    If you've already got it apart it is what it is. As for waterproof core material, you probably know I'll recommend foam. Extruded Polystyrene closed cell. But you don't seem convinced so there is another way. Use a good marine grade plywood or exterior grade plywood made with waterproof glues. You can check the quality of the plywood by getting a sample piece. Put it in a pot of water and boil it for awhile. Maybe 1/2 hour or so, longer if you like. It should show no signs of delamination. Now put it in the freezer overnight. Let it thaw out and give it a good look. Try to pry it apart. It should look like new. That's your test for waterproof, boil proof glues.

    Once you have some decent wood cut it and fit it as best you can in the space where your old stringer material was. Now get your epoxy resin out. Coat the stringer. Not once, not twice but three times, three times thoroughly on all six sides. Pay particular attention to the edges and the end grain. This will entomb the wood and keep water from getting in. Those stringers and floors live in a hostile, moist environment. I would not recommend polyester resin. Poly is not waterproof. Moisture will permeate poly. Use epoxy.
    Once you have your new form in place you can drape new fiberglass over it. I'd use 17 oz. per square yard fiberglass cloth or it's metric equivalent since you live in the civilized metric world. You can find a lot of posts about making up stringers and doing the glass work. Make sure that you tab them in well.

    Thanks for the kind words about the whole COVID-19 thing going on here. We live a couple of hours by car north of The City. Most of the deaths and issues in New York are down there. but it's a terrible situation as you may have seen. The morgues are overwhelmed and they're using refrigerated tractor trailer boxes as makeshift storage for the bodies. People can't have a funeral or memorial service, can't pay their respects. It's a weird disease. Many relatively young people have died. The statistics say older people are much more likely to get a severe case. Yet some people who are over 100 have recovered. My wife's aunt is in her nineties and in a nursing home. She's diabetic and has some other issues. She tested positive. The doctors say she's doing pretty well with it and they haven't even told her that she has it so as not to upset her. She thinks she just has a bad cold or something.

    I see that Australia has strict rules in place for social distancing and keeping at people at home. You're doing the right thing. Also closing the boarders is a good idea. Most of the cases in New York are said to have come in with people traveling here from Europe. By the time we realized how bad it was it had spread too far. I don't know what you hear in Australia but the economy here is really plunging. I'm older and retired but I really feel for younger people who have bills to pay. Not a good situation.


    Jeff, who is the moderator here asks us not to discuss politics so I won't. But if I could.........I could go on and on and on.......PM me if you'd like.

    Take care and good luck with your project.
     
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  8. Martin Upton
    Joined: Feb 2020
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    Martin Upton Junior Member

    Hey missinginaction,

    Re boat: I think I’ll run with a structural composite like this Thermo-Light Board Product Range | Constructive Composites https://www.ccaboards.com.au/thermo-light-board-product-range.html.
    There’s still some damp areas in the boat that I can’t get to without riping the entire thing apart so I’d rather have this new stuff So it’s integrity wont be compromised.
    Day 2 today , getting into the boring stuff. Fortunately it’s Easter so the shops are closed so I can’t race out and but new materials so I can get into the fun stuff (rebuilding). This will force me to take the time to clean and align everything properly:)

    Re COVID: I feel for u guys. It’s terrible. I feel so lucky and blessed to live in the country that I do (Australia) atm. I think the authorities here have responded in a very balanced and proactive manner. There are a lot of competing interests to balance.
    Our economy has ground to a halt too . Personally I have a reasonable size business with a bunch of staff that was locked down a few weeks ago (so now I have to work harder just for no pay) . The restart will be interesting.
    On a positive note I was looking at the stars tonight and the sky is so clear. :)
    Thanks for your advice missinginaction.
    cheers
     
    Last edited: Apr 11, 2020
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  9. ondarvr
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    ondarvr Senior Member

    I like hollow stringers, the idea of them filling with water is not a concern. Any and all structures in the hull should have a generous number of limber holes to direct the water where you want it to go. This completely eliminates retained water.

    While polyester resin does absorb more water than epoxy, both are considered waterproof (although nothing is truly wsteproof) .

    The polyester is not waterproof comes from people looking at rotten wood in Polyester hulls and not really understanding what they're looking at.

    When wood is correctly encased in a polyester laminate the chances of rot are minimal, the correctly part is where 90% of production boats come up short.

    This results in people assuming it must be the polyester at fault because it's so common.

    Use the same techniques and attention to detail and an epoxy coated wood stringer would fail in the exact same way and time frame.
     
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  10. ondarvr
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    ondarvr Senior Member

    You could use just about anything to create the desired shape (core) as long as the correct amount of glass is used over it. Whether you use the potential strength of the core is up to you.

    Glass filled foam cores add a small amount of potential strength, but the cost is high for what you get. And just about any other core than wood supplies almost no strength, these cores are typically lighter than wood though. Since These cores don't add strength, the laminate is engineered to handle the entire load. These cores tend to cost more than wood too.

    If I was going to use a core in a stringer, which I would rather not do, I would seriously consider using wood and not figure it's strength into to the laminate schedule.

    Wood is cheap and easy to get in just about any size and variety you need. It's also very easy to work with using average and commonly available tools.

    Here's the other thing about doing it this way.

    Because you used a laminate that was designed to work independently of the cores strength, it will be thicker.

    This thicker laminate, and the typically much better attention to detail the boat owner applies to the project, means the wood doesn't rot.

    Now you have an even stronger structure, that costs far less.

    If I was going to use a core that didn't rot, I'd look at PVC trim boards. They cost more than wood, but are still very easy to buy and shape, you can even heat them and shape them as needed.
     
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  11. ondarvr
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    ondarvr Senior Member

    Kill the mold with bleach, mix it strong.
     
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  12. Martin Upton
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    Martin Upton Junior Member

    Ah thankyou ondarvr. That was what was bouncing in my mind too ... pool chlorine.

    those trim boards you mentioned is that like the artificial timber we have here in Australia... non structural?
    Ty
     
  13. fallguy
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    fallguy Senior Member

    Most of that mold is just on surfaces, right?

    wire brush with a mask; bleach only works on non-wood surfaces; wood surfaces are not generally bleached as the bleach tends to have spurious results; wire brush the entire surface well; it creates an excellent substrate for the next bond; if you blow through; don't sweat it; it just means the old stringer was structural.

    I really like high density composite cores. Corelite, Aquaplas, Coosa. You basically put 1/16" vee trowel of thickened epoxy on each side and bond them together....new core to old laminate; use a 26 pound density minimum; to ship it, you can get it cutdown to avoid freight $$$

    the bigger question is the transom...typically, the transom would be repaired first
     
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  14. ondarvr
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    ondarvr Senior Member

    The mold he's talking about is on the glass.
     
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  15. ondarvr
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    ondarvr Senior Member

    The trim boards are typically white and made from expanded PVC so they're lighter than solid PVC.

    It's the type of product used around windows and similar non structural applications.
     
    Last edited: Apr 12, 2020
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