Adding Logitudinal Stringers

Discussion in 'Boatbuilding' started by bristol27, Oct 1, 2011.

  1. bristol27
    Joined: Oct 2011
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    Location: Portland, OR

    bristol27 Junior Member

    Hi Everyone,

    I'm steadily making progress on the full restoration of my Bristol 27, but before I fully build out the interior, I've been considering adding some fore-to-aft stringers in the boat. I may be using the term "stringer" wrong here, but in my mind, I consider a stringer as a forward to aft reinforcement, fiberglassed directly to the hull which will add add strength and resist torsion of the hull. Below is an image of the stringers I plan to add (the stringer locations are in red):


    To further assist, I've broken down my current thinking and questions below. If you'd like to add some input, feel free to address the specific questions I've posited and/or add comments to anything I'm not considering.

    + What material should I use for the stringers?
    -> I've heard that one can use halved PVC, foam, wood or even halved garden hose. My issue with the hose or PVC is that there would be a gap inside that could possibly trap water. Since these stringers will likely be installed below a layer of insulation, there won't be an opportunity to drain any condensated water. Adding wood would seam quite heavy, so maybe foam is the best choice? If so, what type of foam?

    + Should I drill through already installed bulkheads in an effort to span stringers between bulkheads?
    -> I've read the most effective stringers run all the way from forward to aft. However, it would seem to me that adding them between the major bulkheads in the boat would also add a significant amount of strength and I don't really want to drill through bulkheads to run these stringers if it's not necessary.

    + How important is it to add these?
    -> I'm really trying to go all out for strength on this boat. I'd like to go to northern latitudes where ice may be an issue. Obviously I'm not trying to winter over or anything with a fiberglass boat, but I want to build the strongest boat possible.

    A few note about this plan:
    + I won't be able to install stringers between the mast-head bulkheads (the head compartment and icebox area), because I've already completed major construction in these areas; meaning I have insulation already installed and no clear access to the hull to install stringers. That being said, these areas are quite reinforced already so I'm not too-too worried.

    + I would add more stringers, however things like the settee tops, interior bulkheads and shelves will act as stringers as well. So, the stringers have been placed only in areas where there will be large "panels" of fiberglass.

    + If you'd like to get a more full idea of how the interior is constructed at this time, see the images below or visit my project site -






  2. rberrey
    Joined: Oct 2010
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    rberrey Senior Member

    Closed cell foam might be a good option. Rick
  3. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    I am not a sailor and have not built or educated myself about sail boat hulls. In power boat hulls to me a stringer, like an engine stringer, runs in a continues connected line from aft to fore. While they can curve they usually do not end with an offset at a frame
    then continuing on again to another offset at another frame. They can abut a frame and continue on the other side aleigned with connections. I just have not seen in pleasure boats offsets or gaps between frames as you show. I'm sure others with greater experience will chime in to assist you on this.
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I would consider using your furniture as hull partitions (as it appears you've started) and in those areas you feel the extra need, then stringers will work, but longitudinal ring frames may be a better choice. The arrangement you currently have will improve longitudinal stiffness a fair bit, but ring frames in these locations will do a much better job of this.

    I'm not sure why you think you need to triple the longitudinal stiffness of your already well built boat, but there is such a thing as being too stiff.

    To add some more support at the large unsupported panel areas, I normally use Christmas wrapping paper tubes. I cut them in half on the band saw and saturate them with resin as I place them down in position. They're easily trimmed to fit too. Over this apply a few layers of biax (if using epoxy or vinylester) or several layers of mat and roving is polyester. The wrapping paper tube just acts as a form to shape the laminate. This will greatly stiffen the unsupported areas for relatively little cost.

    Yes, it's possible to have them trap moisture, so drill some holes in the low points and condensation can drain out. Foam is a costly option, wood isn't as costly, but more difficult to shape.
  5. bristol27
    Joined: Oct 2011
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    Location: Portland, OR

    bristol27 Junior Member

    Thanks for everyone's input! I've responded to a few comments below

    -> I guess that does get to the root of the question - is it necessary to have full length stringers to add strength or would partial stringers suffice? I guess I would like more information on the difference between the two types of stringers - partial versus full.

    -> Yes, I've built in a number of collision bulkheads as well as "water-tight" areas of the boat should I take a holing.

    -> If you have a moment, can you go into more detail about ring frames and how you might install them in my case? I'd also be happy to do my own reading and research on this if you had some good resources to provide.

    -> Not sure if this is even possible to answer, but when is the point of being too stiff? What happens when a boat is too stiff? If a fiberglass boat was flexible, it could possible tear the tabbing off the hull, so it would almost seem that the stiffer the better in this case. I'm curious about this, because I thought stiff was a good goal!

    -> I could drill holes, but the stiffeners will be installed below my 1" of armacell insulation, so having them drain won't be easy. It wouldn't be impossible, but would require some thought. If I use close cell foam, this might be a better option (especially since I already have the foam on hand).
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Longitudinal ring frames are just like bulkheads, except the center is cut out for access to the space. You can butt and tab them to neighboring bulkheads and furniture partitions, which essentially makes them continuous or at least part of a "grid" system.

    'Glass hulls are fairly flexible, which is why the old 60's and early 70's hulls were so thick (I have an early 70's sailboat of 22' with a 1/2" thick hull shell!). They used heavy laminates and minimum internal stiffeners. Later it was found you could decrease the hull shell thickness, use internal stiffeners, such as stringers and bulkheads, and furniture partitions. This saved weight and materials cost, but added to the labor and complexity of the build. Eventually, labor costs were relieved with structural liners that incorporated stiffeners.

    Breaking out tabbing isn't an uncommon thing, especially in powerboats. As a boat flexes, loads are transmitted to neighboring elements and then they also flex. It's a normal thing and also a good thing. If it was too stiff, you'd break out much more tabbing, as the separate elements wouldn't "give" a little under load and forces would become concentrated, rather then dispersed. How much is too much? Well, this is a very difficult thing to answer. From the very general information you've provided, I'd say you're pushing it, but not there yet.
  7. bristol27
    Joined: Oct 2011
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    bristol27 Junior Member

    I have decided against adding longitudinal stringers at this point in the project. Had it been earlier in the process (like, right after I gutted the entire boat) it might be a different story. However, considering the current access I have to the interior, plus the uncertainty of what I would gain from adding just partial stringers, this doesn't seem like a wise design decision or effort.

    Perhaps in the next boat I build I will implement this type of thinking earlier on, but now onward to cockpit modifications, seahood construction and other fun things :)
  8. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Remember to cut big drain holes in your furniture bulkheads or you will have a dozen small stinky bilges.

    Also while you have the boat gutted perfect the efficiency of your bilge pumping system. This means pouring Chockfast into the low point of bilges to raise the level, then forming a Recess in this chockfast a " cup " footprint.... for the bilge pump intake to lie in. Anchor lockers...engine bilge....shower sump bilge....

  9. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Anyways, try to avoid solutions like the one in your first picture. If arranged in that way, stringers will push against bulkheads to create bending moments which bulkheads were not designed for.

    A picture (made through a FE software) of a simplified structure with nonaligned stringers attached to a transverse bulkhead visually shows the stresses acting on that bulkhead, and the resulting (exaggerated) deformations:


    Stringers should be arranged in such way that stress is transmitted longitudinally and possibly with no discontinuities across the bulkhead, for example by connecting non-aligned stringers with diagonal sections:

    Deformation 2.jpg

    You can see that in the latter case the bulkhead is not subject to bending moments originated by stringers, because they are no more pushing against it in separate points.
    You can also notice from the two previous examples that the second solution also gives much smaller overall deformations (deformation scale, material, thickness and pressure have the same value for both models). So whenever stringers have to be kept non-aligned (for whatever reason, the second solution is definitely to be preferred over the first one.

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