building stringers

Discussion in 'Boatbuilding' started by GLD, Apr 16, 2009.

  1. GLD
    Joined: Apr 2009
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    GLD Junior Member

    I have a 86 Celebrity 225 with a cuddy that has a bad spot on one stringer and im new to this but im good with tools so i figured i would give it a go.:confused: Can anyone tell me if you can stager the seems on double layer 3/4" marine plywood to make them longer?
    Is type ABX marine plywood good for stringers? I can't seem to locate CCA marine plywood around my area.
    Any help would be great. I've been reading some of the other posts.
     
  2. thomasda
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    thomasda New Member

    Hi,

    I have a 87 celebrity open bow and did some work on the floor last summer. When you rip out the carpet you might find more that just one spot but maybe not. Anyway, yes you can cut out just the one section if it's the only bad section. You will need to cut out the stringer to a point where it is good. Replacing a stringer is a bear of a job but if it's only rotted say in the one spot you can attach a new section to the good piece of the stringer. Be sure to use fiberglass and glass it in after your repair and it will last for a very long time.

    I used a top grade treated 2 x 4 with a treated plywood wrapper for strenth. Seemed to work pretty well and I keep the boat dry from now on.

    -Doug
     
  3. alan white
    Joined: Mar 2007
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    alan white Senior Member

    GLD,

    Do not use plywood for stringers. Grain orientation is important with long thin pieces. The grain of the wood should follow the length, which goes without saying, but plywood has a large percentage of its grain running across the outer grain direction. As such, that portion of the wood does nothing to create stiffness, which is what a stringer is all about to begin with.
    There are plenty of wood species that will serve your needs. Matching the stiffness of the other stringers will be your best bet; in other words, usie the same wood species that is already there.
    Most important to preventing a future rot problem is to seal the new stringer in epoxy prioer to installing it, and if you are piecing in the section, it should be epo0xy-glued to the existing sound sections in order to completely encapsulate the new wood. This process was likely not used originally, and is likely the cause of the rot you're dealing with.
    In addition to using solid wood and epoxy encapsulation, you should scarf in the the new piece, meaning chiseling a ramp with a length of 10 times the thickness of the stringer thickness where the new piece will be added, and then carefully shaping the end(s) of the new section to match.
    Be careful to remove ALL rotted wood and completely dry out ALL wet but sound wood (unless you replace the whole stringer. Sealing in wet wood is an invitation to repeat the process you are now facing, so take the opportunity to allow the sound wood to dry out.
     
  4. GLD
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    GLD Junior Member

    Ok. Im working on getting the floor (sole) out today if i have enough time. As far as matching the existing wood goes. How do i find out what type of wood was used for the stringers in the first place? I do know that they are atleast 8" high i just dont know if they end at the bulkhead for the cuddy or extend all the way through to the bow. The boat is 22 1/2' long. Or can some one tell me the best wood to use for this. I tried asking a local marina but their service dept. could'nt help me at all.
    Should i put new floatation foam back in after?
     
  5. thomasda
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    thomasda New Member

    My guess is that they extend all the way through to the bow because the stringers provide strength and durability. The type of wood used in celebrity boats, based on my limited knowledge, is that they used #1 red wood which will rot obviously but is very moisture resistant. In my opionion # treated that will match up to the existing is as good if not better for unexposed areas like stringers. I'm not a professional but I used presure treated wood for my stringers and it is as solid as a rock.
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Douglas fir and western larch are common choices in stringer materials. You can use plywood, but then you have to increase the tabbing laminate considerably to compensate for the decreased longitudinal weakness of the plywood.

    The stringers will run as far forward in the boat as is practical, given the height available. They usually stop in the forward quarter of the boat, unless it's a particularly "flat floored" hull form.

    Don't use PT lumber (pressure treated) in your stringers or sole. The resins we use to bond things doesn't stick well to this stuff.

    Yes, you can scab in a repair to a stringer, but you should attach to solid material. A scarf joint is preferred, though sister blocks (on each side) are often employed too. The whole shooting match has to be very well tabbed back into the hull shell to work right.
     
  7. GLD
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    GLD Junior Member

    PAR: Thanks for the reply.
    Out of the douglas fir and western larch What one would be the better to go with?
    On the tabs is their a specific size they should be befor i glass them to the hull? Like 6" long and 3" one the hull and 3" up the stringer every 2'?
    Is woven roven the best thing to use to glass in the new stringer or stringers? I will be using epoxy resin.
    As far as plywood the only thing i can find is TYPE ABX MARINE PLYWOOD Is that a good product to use?
     
  8. rasorinc
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    rasorinc Senior Member

    DOUG FIR AND WESTERN LARCH ARE CONSIDERED THE SAME WOOD. THEIR WEIGHT AND GRAIN ARE ALMOST EQUAL. Sorry for the caps. None of the western mills seperate them==they grow together and have equal properities.
    Usually all listed as D.F. so ask for that rather than western larch. Doug Fir marine plywood also contains peelers of W.L. = = = = = = = = = =Same ABX plywood the X stands for exterior (waterproof glue) the A stands for ` real good side and the B stands for a B grade slightly lesser than A grade. However if it is not labeled Marine it may have voids like missing knots in the interior plys. So ASK QUESTIONS. If it is a CCX grade it has interior voids and not all of them will be filled. DF and WL are good woods for FRAMES AND STRINGERS. strong, holds exoxy well and screws also and has medium rot resistance and bends good, I build all my structural in boats out of this wood. Best to you, Stan I will use CCX- PTS grade plywood PTS = plugged, touched and sanded but I up-size by 1/8" so if it calls out 5/8" I use 3/4"
     
  9. PAR
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    PAR Yacht Designer/Builder

    My understanding is an ABX sheet has no defects in the outer face, a cut and filled (footballs) face on the other side (the B side) and the X means exterior.

    The problem with all construction grades is the interior construction and total ply count.

    The interior defects are left unfilled, which can lead to rot and potential for catastrophic failure under load (coupled with voids). Since they're inside the panel, you generally can't see them (you can, but you have to know how to spot them).

    The veneer count is another major issue. An example of a 1/2" sheet that is exterior rated construction grade will have 3 layers of wood, an A face, a cross grain veneer with unfilled defects and voids and a "repaired" B face. A marine BS 6566 sheet will have few to no voids, the internal defects will be cut out and filled, plus (a big plus) the veneer count will be 5, not three. A marine BS 1088 sheet is better quality then that.

    I don't use plywood for stringers, mostly because I don't like to do the extra 'glass work involved with it.

    You see (again) plywood doesn't have the longitudinal strength of regular solid lumber (only about 2/3's). For this reason, the laminate has to bear a large portion, if not all the longitudinal loading. In this case the plywood is used as a form and isn't load bearing. You could use cottage cheese for this if you can make it stand up long enough to lay 'glass over it. Since the laminate is holding the strains, it's a good bit thicker (more layers) and you're "goo factor" goes up.

    When I do stringers or other rebuilds jobs, the first thing I do is figure out what they did. The thickness of the soles, the dimensions and species of the stringers, the thickness of the tabbing, etc. Then I look at what failed and possibly why (it's usually obvious).

    With this information I have two choices, make it like it was and hope I'll be long since dead when it's time to redo it again, or address the issues.

    Most of the time, moisture gets in through a fastener hole and wanders around the spaces below the sole. Since it can't get out, it soaks into whatever it likes. The wooden elements swell and shrink repeatedly, with moisture content changes, which eventually rots the wood and/or sheers the tabbing off the separate wooden pieces.

    Sometimes I'll see sparse tabbing, poor lumber coverage with resin, mat, etc. all indications it was a crappy layup when the boat was built.

    These things are easy to fix, particularly with good epoxy procedures.

    If the tabbing was still solid when you cut it off to clean out the stringer areas, then use the same total thickness of epoxy and cloth. It tabbing looks like it got ripped, delaminated or other issue, then beef it up.

    As I mentioned in my email, I don't use roving, preferring biaxial fabrics for their strength and ease when draping over stuff. You're better off with multiple layers of lighter weight fabric then a single layer of heavy stuff. Getting heavy roving to curl over the top of a stringer will test the abilities of anyone.

    You should be able to see how far back into the hull shell the tabbing reaches on each stringer. This is your guide. I generally don't bother, just grind back several inches around everything and tab it in with several inches of fabric. You can't screw up if the tabs are too long, but it can be a disaster if they're too short.

    The stringers should be completely covered from side to side with the fabric. Don't make a flange and hope it'll hold, it probably will not.

    Again it would be helpful to see (pictures) what your issues are instead of making general guesses at your rebuild.
     
  10. rasorinc
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    rasorinc Senior Member

    Maybe I did not clarify about frames and stringers--never use plywood just use good vertical grain lumber which is what Par says. any frame, structural parts should be solid lumber or in the case of a keel you put a layer of ply. sized right on the top of solid lumber and on the sides of motor stringers to prevent splitting. Hoverever you can use several layers of ply to form a stem and there is a formula for that,
     
  11. GLD
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    GLD Junior Member

    Thanks again PAR,
    Im going to look into the biaxial. Is thier any brand that is prefered over the rest? I see alot of brands out and don't want to use a bad one i only plan to do this boat one time the right way.
    I spent some time today hunting down some kiln ried DF and found some not to far from me so thats out of the way. Now just need to find a good brand to use for the resin and glass.
    The flaws i fond so far is that the factory failed to cover the entire stringers with mat and resin and it did not fold over the tops of the stringers. Als i noticed the 2 layers of stringers were not glues together. Should'nt they be? I would think so. If so what type of epoxy glue?
    I will try to get some picks up when i finish getting the floatation foam out of both sides. The stringer on the port side was a disaster. That was the one that looked bad by the motor cover. Well as i started removing the flotation foam i found the entire stringer was junk. So now i am starting to remove the foam on the starboard side.
    It looks like there was a space on the bottom of the stringers so they did not sit on the hull but were up like 1/4" to 3/8" from the looks of the fiberglass. I will find out when i dig in alittle more.
    Thanks again for everyones help so far i will try to get some pics of the mess up soon.
     

  12. GLD
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    GLD Junior Member

    Here are some pics of the boat. I have removed the sole and stringers now and the runouts or bulkheads from the stringers to the sides of the hull. http://s696.photobucket.com/albums/vv327/jdsavage72/ I still have to remove the fuel tank and the rest of the bulk heads. After that im going to grind the existing tabs down.
     
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