Aluminum tunnel hull floor and transom

Discussion in 'Metal Boat Building' started by Alumination, Nov 4, 2015.

  1. Alumination
    Joined: Oct 2013
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    Alumination Junior Member

    How would you go about fabricating these two areas?

    The boat will be 6' wide, each sponson is 1' wide. The floor needs to span across the 4' wide tunnel area and over each sponson, 6' wide total.
    Would you use rectangular tubing or I-beam?
    Spacing?

    The transom will mount a Volvo SX sterndrive and needs to be roughly 20" wide by 24" tall and 2.25" thick max.
    Again, tubing or I-beams?

    Have there been any threads which discuss this type of fabrication?
     
  2. PAR
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    PAR Yacht Designer/Builder

    What do the plans say? No plans - well you'll have to engineer something appropriate. Do you have an idea of bottom loading on this design?
     
  3. Alumination
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    Alumination Junior Member

    No plans of course.

    By bottom loading would I just add up the weight that would/could be carried on the floor?
    4-5 passengers so 900 to 1000lbs maximum.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    The whole design needs to be sound, not just stay in one piece. I am struggling to understand how 1' wide "sponsons" are going to support a boat with a sterndrive in it, and/or ever plane satisfactorily.
     
  5. PAR
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    PAR Yacht Designer/Builder

    Maybe you can post some drawings of this, so we have a clue as to what you're dealing with.
     
  6. Alumination
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    Alumination Junior Member

    I'll have to work on a drawing I can post.

    6' wide boat, 4' wide tunnel which is 11" tall.
    Floor will be above the tunnel and span the 6' width.
     
  7. gonzo
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    gonzo Senior Member

    Bottom loading can increase to several times the weight of the boat when it slams on a wave. It is a rather complex engineering problem which takes into consideration shape, weight, speed, etc.
     
  8. lance linked
    Joined: Sep 2015
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    lance linked Junior Member

    in general, rectangular tubing is more useful in a dynamically loaded structure in which space for structure is at a premium. if you provide us with a rough drawing I can suggest tubing sizes, assuming appropriate welding or fastening design. for comparison a 3X3X.187 wall tubing can serve a similar function to a 6" wide flange beam in terms of moment of inertia around a center in a 6 foot length, but a 6" beam will support more dead load in a static application. gusseting and load distribution are critical when using reduced size materials.
     
  9. Alumination
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    Alumination Junior Member

    Thank you so very much, this is exactly the type of info I am looking for.

    I am trying to find a program to use to draw the cross section and figure out some of the details.
    Are there any easy to find on-line programs or apps which you'd recommend?
    I may have access to SolidWorks here shortly.
    A co-worker suggested Sketch-Up I believe was the name.
     
  10. Barry
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    Barry Senior Member

    To suggest that a 3x3 tubing equates to a 6 inch wide flange beam is misleading without specifying flange/web thickness in the I beam and wall thickness in the tubing

    The moment of inertia is part of the analysis but the other element is the distance from the neutral axis to the outer fibre for max stress

    Best to compare section modulus
     
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  11. TANSL
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    TANSL Senior Member

    I totally agree with Barry, what it says is essential if one knows how each element of the structure works. The moment of inertia is important when it comes to an element working in compression, which is not the case of the deck beams or beams connecting the two hulls. In most of the reinforcements of a structure thing to consider is the section modulus of about one of the section axis. The moment of inertia (central) about a longitudinal axis It is only taken into account in compression studies. Of course in the webs connecting the two hulls, due to the stresses to which it is subjected, the tubular section is most suitable but, in all other cases, the tubular section adds large amount of material that hardly contributes to the strength of the reinforcement.
    Alumination, SolidWorks can be very useful software for you.
     
  12. TANSL
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    TANSL Senior Member

    It's an interesting claim but I'm not sure that's true. One thing is to keep in mind accelerations due to ship motion and another to say that the load on the bottom is several times the weight of the entire vessel. I wonder how you can reach that figure.
     
  13. PAR
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    PAR Yacht Designer/Builder

    Several times the boat's weight isn't unusual in a high speed pleasure boat. Picture a 2 ton 500 HP 24' powerboat blasting along doing 70 MPH, when it flys of a signifigant wave or wake, just to slam down in between the next two.
     
  14. TANSL
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    TANSL Senior Member

    I had never seen expressing pressures due to slamming in those terms. I know the force of inertia there, how they act not only in the bottom of the boat but also, for example, on the deck, but I think everything should be expressed in fair terms, with clear ideas.
    I insist that say "Bottom loading can Increase to several times the weight" need for better explanation and that's what I ask how you come to that figure?.
    It's very nice release large phrases, as if one were very knowledgeable on the subject, but in reality are just generalities that do not clarify anything.
    It is not easy for the bottom of any vessel to withstand a load of several times the weight of the boat. Now, speaking of pressures due to slamming, very high, it is a very different thing.
    On the other hand, as many people know, the pressure can not be expressed in terms of force and weight of the boat is usually a force.
     

  15. JSL
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    JSL Senior Member

    The forces are G forces and should be taken into account. Even the classification societies allow for it. I have seen light duty boats fail when driven hard in rough weather and carrying a payload. Some call them Rice Krispie boats.... Snap, Crackle, and Pop.
    The structure between the hulls (wet deck, AKA, etc) should allow for bending, sheer, and torsion although on a small boat (under 6m) this may be overkill so to speak.
     
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