Longitudinal stringers...

Discussion in 'Sailboats' started by Pylasteki, Jun 3, 2008.

  1. Pylasteki
    Joined: Apr 2008
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    Pylasteki Junior Member

    Hi Guys,

    I've been pondering longitudinal stringers. It seems like a lot of boats have the bulkheads installed first, and the stringers installed later, like on my sailboat.

    Does this make a hinge point at the bulkhead? Or does an adequately fiberglassed bulkhead take the load as if there was no break?

    Thanks guys... this has been bugging me.

    Zach
     
  2. CTMD
    Joined: Dec 2007
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    CTMD Naval Architect

    Your assumption is correct except that most boats are actually built stringers first (or at least partially). When you are engineering the structure of a boat you have to consider these joints and as a result a continuous stringer will normally be lighter. For example (because i happen to have the relevant spreadsheet open at present) on light aluminium craft, GL will let you have a 33% reduction of section modulas for a continuous stringer when compared to an un-bracketed intercoastal stringer.
     
  3. Landlubber
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    Landlubber Senior Member

    CTMD,

    Correct, the stringers get installed whilst the boat is still in the mould, as do the bulkheads and other serious stuff. The bulkheads are simply cut around the shapes, as are the floors, and foam pads inserted then glassed over to form one great big hunk of reinforced plastic.
     
  4. sharpii2
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    sharpii2 Senior Member

    Hi, Zach.

    Are you saying the stringers in your boat begin and end between the bulkheads?

    If so, this is the first time I've heard of it being done like this.
     
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  5. gonzo
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    gonzo Senior Member

    Many boats have stringers between bulkheads. Often they are not aligned form one side to the other. There is nothing wrong with it as long as the design is correct.
     
  6. kengrome
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    kengrome Senior Member

    I have never seen such a boat and I've seen lots of boats ... but apparently not the kind you've been seeing.

    Why on Earth would anyone design boats to have longitudinal stringers as structural members and then let the builders chop them into short pieces at every bulkhead? Kind of defeats the purpose doesn't it?
     
  7. Pylasteki
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    Pylasteki Junior Member

    On my 1961 Pearson Triton the only hat shaped stringers stop on each side of the bulkheads. One starts on the aft end of the chain locker bulkhead, and stops at the main bulkhead under the mast. Then there is the head/hanging locker which has no stringer. Then on the aft end of those two bulkheads it runs to the galley and stops.

    After further looking and head scratching, I think they used it to pull wire and as a support for the shelving. Weird. Maybe they were not intended to be structural?

    I'm thinking it can't hurt to continue it up to the stem and aft to the transom while I have the interior apart, if only to reduce the possible hinge points and stresses! (Other half of me says it made it this long without...)
     
  8. gonzo
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    gonzo Senior Member

    Many thousands of powerboats have stringers between the transom and the first bulkhead. Also, thousands of sailboats have short stringers to support the engine and transmission. Sailboats also use short stringers under the mast to spread the load.
    Kengrome: what makes you say "Why on Earth would anyone design boats to have longitudinal stringers as structural members and then let the builders chop them into short pieces at every bulkhead? Kind of defeats the purpose doesn't it?" These boats are designed with short stringers.
     
  9. diwebb
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    diwebb Senior Member

    It all depends on what the stringer is for. If the stringer is designed as a structural support for the skin of the vessel then it needs to be continuous the full length of the boat. This allows the actual skin to be made thinner as it spans between the stringers. The bulkheads then support the stringers to maintain the shape of the boat. Short stringers are usually designed to spread a point load such as a mast or engine, or are not part of the hull's structure but are attachment points for the interior joinerwork.
     
  10. Pylasteki
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    Pylasteki Junior Member

    To bring this one back up...

    Been pondering stringers once more.

    When full length stringers are installed, do they taper down at the ends? I'm trying to visualize how they would meet at the stem and transom!

    Zach
     
  11. Landlubber
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    Landlubber Senior Member

    Yes, they could taper or they could end at the transom or combine into the stem.
     
  12. Boston

    Boston Previous Member

    well this may be kinda old school but my two cents

    we always used stringers and they always were full length
    the taper is relative to the stringer cross section and the distance between stringers
    not the length

    sometimes they follow the sheer line sometimes they dont
    depends on who built the boat

    way I learned it you never but joint anything
    so the stinger sits in a stopped dado under the rabbit line

    first real piece that goes in the molds is the bulkheads which get cut to fit the keel and stringers which pass through them
    the stringers are definitely solid
     
  13. PortTacker
    Joined: Nov 2008
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    PortTacker Junior Member

    There's all sort of "stringers."
    I typically think of "full length" stringers as in ply-on-frame construction. They are indeed structural.
    On most fiberglass boats, stringers aren't needed for the load structure, they are often there if at all to reinforce expanses of hull to keep them from "oil-canning." Used this way, there is generally no need or advantage to them running unbroken. Add where needed. The 'furniture' often performs much of this function, but the forward sections often need a few extra.
    Often, all they really do is keep the hull from flexing when its removed from the mold, and help it hold it's shape until the deck mold is secured.
    Under the sole there is often a 'grid' to spread keel loads - these can be considered stringers.
    In powerboats there is usually a massive bunch of stringers under the floor, full length is definitely stronger - to help the hull skins withstand the severe impact loads.
    Also, there are engine mount stringers.

    Engineering can be tricky though. If you simply overbuild and accept the weight penalty, proably not so tough. But if you want high strength to weight, consequences of poor design can surface in the oddest ways. Example: I know a guy had a 25 year old fairly fast boat who removed his vee berth - he didn't use it and needed the space for stowage. First time in rough weather the skins 'oil canned' badly. So he added some stringers. Seemed simple. After a couple days of pounding, the bulkheads began to delaminate from the hull. Apparently his stringers redirected the loads to a new spot. After replacing most of the vee berth structure, he has had no more issues.
     
  14. Boston

    Boston Previous Member

    I see your point
    jsut that my experience is limited to plank on frame

    have seen that oil canning you are talking about
    its funny as all hell looking
    always makes you wonder if the hole thing is about to stove in or something
    B
     

  15. ancient kayaker
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    ancient kayaker aka Terry Haines

    Boston, big difference between wood construction and glass I suspect.

    However, even in wood construction short battens are sometimes used between frames, usually to double the seams after planking.

    Maybe doing that in a molded boat allows you to take the boat out of the mold quicker so you can start the next one. Or perhaps the skill requirement is lower than for continuous stringers due to simpler bulkhead and short stringers. Reason for short vs long stringers is not necessarily structural.
     
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