another stringer thread

Discussion in 'Boat Design' started by modunlavy, Aug 22, 2017.

  1. modunlavy
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    modunlavy Junior Member

    I didn't want to hijack the other thread. I did see the reference to Gerr's book and downloaded and read his IBEX presentation from 2010. I didn't really see an answer to a question I have....

    in the picture below, the lower layout is the stringer and bulkhead layout currently in the inboard layout of the boat as built by blackfin. It seems to have held up well as I haven't found any failures or issues with it. I have never seen stringers that weren't linear like this before. Held up well with the inboards, since the engines were forward of it, but not sure about an outboard configuration.

    In the upper layout is a type of layout that would work well for how I am laying out the boat (not exact, but very close). Is the spot where the stringers aren't linear a weak spot, an issue? I really could use that wider space there. I could carry that same width forward further to allow for a wider fuel tank too, but couldn't really go further forward without major reconstruction.

    Any input on this?? Thanks,

    Michael stringer comparison.JPG
     
  2. modunlavy
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    modunlavy Junior Member

    Just saw there is a "Transom Engineering" course at IBEX this year, so I guess I will try to get in that while there and see what I can learn - may answer my questions...
     
  3. TANSL
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    TANSL Senior Member

    The solution for the stringers of the lower figure is more suitable and is completely normal in many boats with inboard engines.
     
  4. modunlavy
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    modunlavy Junior Member

    but how is it for outboards since that is what I am converting too? that is what I am trying determine. If not good for the outboards, is something like I drew with it more squared off into a new bulkhead be better?
     
  5. SamSam
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    SamSam Senior Member

    It does seem like a weak spot, considering outboards vs inboards, but I'm no expert. How many horsepower? Is the deck still on the boat? Just wondering what is meant by 'major reconstruction'.
     
  6. modunlavy
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    modunlavy Junior Member

    twin 300 hp outboards. If I have to go all the way forward, would have to cut the cabin out of the boat - something I am trying to avoid. I guess major is a relative term as most people would probably consider what I am already doing major... lol
     
  7. TANSL
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    TANSL Senior Member

    You should get the stringers to have continuity without breakpoints. The bottom stringers are continued by reinforcements that "climb" through the transom, to the deck, if any, connecting to the deck longitudinals.
     
  8. SamSam
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    SamSam Senior Member

    The thrust of the motors is applied to where they connect to the boat. With inboards it is the engine mounts ahead of that transverse bulkhead point, with outboards it is to the transom. With inboards, the hull astern of the engines isn't doing a whole lot, that area is just sort of being dragged along with not a whole lot of dynamic forces being applied, the weight of the hull being distributed fore and aft of the motors . With outboards on pods, all the thrust plus all the weight of the motors is being leveraged by the full weight of the boat.
    The transom will have to be built up a bunch, as I'm sure it was designed just for inboards. As I said, I'm no expert at all, but I think that the longitudinal strength has to be built up so it is continuous through the bulkhead. I don't think it has to go all the way forward. I'm GUESSING the two sets of stringers are fine by themselves (as far as the width apart of the aft ones and MAYBE the forward ones could be widened, like for a wider tank) but they have to be lapped or somehow made so the applied forces are able to be distributed forward and aft, as the load varies.
    So, I think the transom would have to be strengthened and the stringers somehow made continuous.
    Hopefully, this will bump up and someone with the engineering and practical hands on experience will jump in here.
     
  9. TANSL
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    TANSL Senior Member

    Something similar to the picture.
    Snap15.jpg

    Although there may be other solutions
     
  10. modunlavy
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    modunlavy Junior Member

    TANSL, sorry, but I don't understand that diagram or what you implying - I guess I am still too new to this .....
     
  11. SamSam
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    SamSam Senior Member

    bump
     
  12. Barry
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    Barry Senior Member

    I would leave the stringers as they are. The strength have already been proven in the current configuration. I Googled 30foot Blackfin Bottom Pictures and the hull has chines parallel to the keel in the aft 2/3rds. This is a normal config
    for planing hulls, and the deadrise looks RELATIVELY constant. On another thread about this boat, a contributor suggested that this hull would have been designed for the midship mounted engines. Looking at the hull, at least from the
    pictures that I found, there is nothing significantly different on this hull as for rear mounted inboards or even outboards (barring transom reinforcement and design for the outboard forces and weights)

    I would leave well enough alone with the stringers if the stringer depth is constant from where the old engines sat to the transom.
    Your challenge is to build the transom and engine mount strong enough to carry the engine loading into the existing stringers. They do not have to go all the way to the front.

    I know you have other ideas about this but (and I am not the only contributor who has suggested this) I would leave the tank where it is, mount the engines, take it for a spin to see how it runs. It is easier to trim the nose down with trim tabs and
    engine trim than to trim it up THAN if you have too much weight forward.

    Static Center of gravity is important for sure (and extremely important for the static sitting position in a displacement hull) BUT you need to include the impact of the other forces in a planing hull.
    Ie the thrust line of the props and respective moment which changes the trim of the boat and of course the forces on the hull due to the resistance, water movement resistance and skin friction

    A lot of threads discuss CG as an end all. Thinking that the Static CG is the limit of calculation for the running trim of the boat. What is often lost is that calculations for the CG only deals with vertical loads, ie motors, fuel tanks
    seating, the rest of the material in the boat that then can be represented in a single vertical force at a specific location, the Static CG.

    If that was the case, then the hull would be at the same trim angle at rest, static, as it is running, dynamic.

    What I am saying is that with say 800- 1000 pounds of thrust at applied at a specific points, angles, and the hydrodynamic forces that the moving hull produces (another 800 - 1000 pounds of reaction forces acting at different points and angles to the boat, the dynamic center of gravity is quite different.
    Along with this is the speed as this affects both of the above.

    Due to the non vertical forces acting when the boat is running, and the boat at a constant speed, say 25 knots just to pick a number, the boat is stabilized by one force acting in a direction that is not vertical and a COUPLE.

    I expect that you could Google, "Free Body Diagram Analysis" and find some You tube video that would explain it better than I can with words.

    So in summary, (if it were me doing your restoration)
    1) leave the stringers as they are
    2) strengthen the transom and motor mount and tie this into the two center stringers *****
    3) leave the tank as is, mount the trim tabs that you were going to add, take it for a spin

    You could add a short stringer from the transom to tie into the original cross stringer that was immediately ahead of the engines if you really want to put some stringers in
     
    Last edited: Aug 30, 2017

  13. TANSL
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    TANSL Senior Member

    The reinforcements that you place on the transom should be connected to the bottom longitudinals and to the deck longitudinals. Just that.
     
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