Twin hull sternwheeled landing craft

Discussion in 'Boat Design' started by rfleet1066, Feb 11, 2012.

  1. Wavewacker
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    Wavewacker Senior Member

    20' to the center of the tubes. I suggested an overhead structure as an arch or even an "A" frame stance will be stronger than a horizontal beam. I bet I get some scoldings about that as I am not an engineer. You will also have horizontal beams for decking and they can be much lighter if supplemented by a higher support structure. This will also allow you to have a lower bridge deck between those huge tubes. If this is to have an LC feature, you don't want a 30' ramp, it needs to be as short as possible, so the lower, the better considering the wave slap.

    I can't see windage being an issue with this thing, again, not an engineer, but common sence and experience of being on the water in sheltered waters tells me that a 25 mph wind is not going to blow you sideways and how often do you have steady 25+ winds in sheltered areas?

    And, the number of posts does not add any credibility to the post at all, I suggest you consider mine as evidence to that!

    If you have any commercial asperations at all, you need to get an engineer involved now. You could have built space craft before and unless you are qualified to certify the craft, you'll never get it insured sufficiently required for any commercial use. And you sure won't get any business financing using that craft without certs. Do your due diligence in reverse to see what is required now.....
     
  2. mydauphin
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    mydauphin Senior Member

    I am not a nautical engineer either, thou I play one on TV; Think in terms of boxes with cross supports, and on a boat it has to be in 3d. If I understand your idea correctly at the end of the day you will have a flat surface that you can move stuff around in. Even bridge will be movable. So you need a sturdy deck , on top of pontoons and then structure above can be move able. This actually a pretty hard structure to build, it is similar to a car chassis. The problem is twist. I would build it out of aluminum c-channels then cover it in fiberglass. I would put it between pontoon rather than on top of it to lower center of gravity, unless you really needed the clearance. The lower the CG the better it is going to behave in general.

    Don't underestimate the windage because unless it is fully loaded your boat has no keel and very little to keep in place if you build a box on it. For this reason if I was building a house on it it would be turtle/dome like.

    Keep it low, wide and aerodynamic from every direction. Something like the stealth ship.

    If you send me the hulls in something like sketchup, I can draw up some designs.
     
  3. rfleet1066
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    rfleet1066 rfleet1066

    Great points.

    I wouldn't know a sketchup if it bit me on the nose. But I will look it up.

    The hulls are 61' long, 60" outside diameter .375"thick bulkheads every 120"
    The tapered bow cone accounts for the first five feet of hull length and tapers from 60" to 10" The hulls weigh 15600 lbs each.

    I changed the hull position in the yard yesterday and expanded to 25' beam to see how it looks. Like some ***** moving the furniture. But, visually I think we found the sweet spot.

    A rigid connection and deck structure is next. If I want to keep the CG to a minimum, then there must be overhead structure. Conversely, a 30" space frame structure would provide rigidity but with the cost of a higher CG and lower clearance to the water.

    In either case rigidity is neither cheap nor lightweight if the chosen medium is steel.

    Ryland
     
  4. rwatson
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    rwatson Senior Member

    I do hope you mean that "engineering calculations and design of rigid connection and deck structure is next" :)
     
  5. rfleet1066
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    rfleet1066 rfleet1066

    Fair enough. It would appear that there is sublime value to participating here. If for no other reason than to reel in my inductive reasoning. The penalty for amatuer over-engineering, while perhaps functionally viable, will no doubt include a lowered payload capacity. I've always trusted my instincts and added more steel to be sure. I can see where this can compromise results in a design that must bow to gravity, which is not just a good idea, but the law.

    Board members have been more than open and gracious if not brutal in giving support. I hope to be able to give back to this community at some point.

    Meanwhile, every comment is noted, logged, and appreciated.

    Ryland
    "Expert Since 10 AM"
     
    Last edited: Feb 26, 2012
  6. mydauphin
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    mydauphin Senior Member

    Are you hull fiberglass? then a steel structure is too heavy. Aluminum is not that much more expensive in that you will be able to lift and cut it easier. Aluminum flexes more but then you can brace it and still be ahead of the weight by half. It also helps keep your cg lower. Sketchup is a free program to draw in 2d from goggle. It is simple to use.
     
  7. rfleet1066
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    rfleet1066 rfleet1066

    I will look for that program.

    Be advised, steel is my chosen medium. The hulls are steel. Torsional ridigity is a concern that can not be ignored. I like the suggestions that include overhead structural reinforcement. Good news- bad news. I provides the structure for an upper deck. The war department (wife) wants a big upper deck for sunning. When making a structure to incorporate these two pontoons, there are some rules (not guidelines) that are painfully persistant.

    It becomes painfully obvious that while instinctive injuneering may provide a working solution, I may sacrifice capacity in the interest of overbuilding out of ignorance. There's a happy medium that resides in the uniform distibution oferrors.

    Ryland
     
  8. rfleet1066
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    rfleet1066 rfleet1066

    I will look for that program.

    Be advised, steel is my chosen medium. The hulls are steel. Torsional ridigity is a concern that can not be ignored. I like the suggestions that include overhead structural reinforcement. Good news- bad news. I provides the structure for an upper deck. The war department (wife) wants a big upper deck for sunning. When making a structure to incorporate these two pontoons, there are some rules (not guidelines) that are painfully persistant.

    It becomes painfully obvious that while instinctive injuneering may provide a working solution, I may sacrifice capacity in the interest of overbuilding out of ignorance. There's a happy medium that resides in the uniform distibution of errors.

    Ryland
     
  9. rwatson
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    rwatson Senior Member

    I know the feeling myself, its hard to let the experts have a go.

    When I started my project, my instructions to the NA were :
    "Just give me the lines - I am pretty good at figuring out wooden structures"

    6 years later, I am expecting a bill for $1500 of software used in finite element analysis, and another $4K for the engineering of a 28 ft wooden boat ...

    Its not just the ability to have the hull approved for charter, re-sale, insurance etc - its also the comfort of putting all my hard work into a structure that has half a chance of actually working for a reasonable time, and not wasting time and materials on "maybe I should put this here" hull structure.

    Going on my half assed shed renovations, the boat would have stood a good chance of being 25% overweight (not good on a trailer sailer) , with another 25% of unneeded material, running up 20% more time scratching my head and figuring out what would work.

    sigh .... as Clint would say "a mans gotta know his limitations" :(
     
  10. waikikin
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    waikikin Senior Member

    & therefore saving the the $ invested in material & labour................. & applied over more than one build even more! All the best from Jeff.
     
  11. rfleet1066
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    rfleet1066 rfleet1066

    Trapazoidal connectors

    This project continues. I have designed trapazoidal connectors that are flanged in the middle to disconnect for transport. My boat yard is in the middle of the woods with a narrow road and 3 miles from the river. The vessel, when complete will be disassembled, each hull and deck section rotated 90 degrees, placed on a dolley and taken to the water with a farm tractor, then reassembled.

    What are the thoughts on using structural members as fuel and water tanks? I will have 14 of these to choose from and might find it handy to be used for weight distribution.

    Ryland
     
  12. rwatson
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    rwatson Senior Member

    Well, you could post the "designs" and give us a fighting chance of knowing what you are talking about !
     
  13. rfleet1066
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    rfleet1066 rfleet1066

    Right you are, Mate

    Agreed. I should have the new components by week's end and will certainly post photos then. Thes are being fabricated at a sub-contact shop, as my facility is not large enough to handle the workpiece size.

    Thanks for the interest.

    Ryland
     
  14. rwatson
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    rwatson Senior Member

    Photos ? Where are the line drawings with material specifications ? 3d Cad files would be easier.
     

  15. rfleet1066
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    rfleet1066 rfleet1066

    I've got those

    I have the drawings but not a clue as how to post them here. I'm a worker bee with a dream and few computer skills. You may see my website, however,
    www.tanstaafl.biz
    and find that ignorance does not generally prevail here.

    Ryland
     
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