Best Design For an Alum or Steel Landing Craft?

Discussion in 'Boat Design' started by RAWRF, Dec 27, 2006.

  1. RAWRF
    Joined: Dec 2006
    Posts: 72
    Likes: 1, Points: 0, Legacy Rep: 14
    Location: Eagle, Alaska

    RAWRF Junior Member

    I am going to build my own 52 x 16 Landing Craft soon, similar to a Munson or Sea Ark Transporter, and I have come up with a honeycomb design of 12 inch square aluminum tubing 5-8 mm thick sandwiched between layers of sheet aluminum for a 25-26 inch deep hull with 30 to 36 inch sides. Has anyone seen anything like this, or would I be better off using a more standard method of construction like sheets surrounding a framework? The design I came up with only weighs about 7000 pounds without engines or cabin, but the aluminum costs are pretty high. Please don't give yourselves hernias laughing at my simpleton drawings. The pic is a cross section of the construction viewed from from front or rear. Any suggestions would be great.
     

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  2. kmorin
    Joined: Apr 2005
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    Location: Alaska

    kmorin Senior Member

    Extrusion Costs vs Sheet and Forming

    RAWRF,

    large closed shape extrusions are usually pretty expensive compared to sheet materials. The forming process for closed shapes is more expensive than flat sheets and the alloys most commonly available are less cost effective for hull longs.

    Instead of buying the 12" boxes just have the sheets ordered in 2' x 6" formed angles- sheared and press braked into L's or C's. The sheet materials will behave better in this application than extrusions.

    The deck between the bottom and working deck is of little use to your project, and the lower sides mean not only do you loose freeboard (load carrying capacity) but also have to rely more on the longs for strength where 4' or 5' tall 1/4" plate sides would give more deck capacity, stronger hull and less work to build.

    Using the words "standard method of construction" to describe the newer planing aluminum landing craft might be stretching things as these craft are hardly 30 years old. There are many versions but most use long, tall formed beams to separate the bottom and decks and economically all the builders have looked closely at what will do the most work for the most competitive costs.

    All-up weight isn't very important as these hollow boxes would have to be plated with 1" material to be 'too' heavy.

    Everyone has to learn to draw, boats or whatever else, you may need to practice more to become as skilled as someone who has already practiced; but everyone can learn.

    As large a boat as you're planning will cost plenty just in materials. If you haven't designed before, and haven't built in welded aluminum it may be a wise course to closely copy what other builders are doing? If they've done it before and are still doing that same work- well, it follows that someone is buying what they're building and that's proof of the effectiveness of those designs.

    The main reason I'd question the box shape extrusions is the doubling and tripling the horizontal members in the stacks of boxes? That much material would be better used in the vertical to hold more weight to resist lengthwise bending and twisting of the hull.

    Cheers,
    Kmorin
     
  3. RAWRF
    Joined: Dec 2006
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    Location: Eagle, Alaska

    RAWRF Junior Member

    Thanks for the reply, I agree with you, mostly because of the price of the box sections. Ease and speed of construction is a big concern, I am a good welder, but it isn't something I care to do a whole lot of. The box sections eliminate a lot of the welding compared to L's and C's, and create lots of bulkheads that won't flood in case of a rupture, but they are just too pricey. What do you think of my design with just one layer of boxed sections, made from L's or C's welded together? This craft will be carrying heavy equipment, so maybe a framework of beams is the way to go. Thanks again.
     

  4. kach22i
    Joined: Feb 2005
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    Location: Michigan

    kach22i Architect

    Build a model first using scaling laws, think of it as a mock up to get you thinking of the process of building as well as design.

    I'm no expert, but at least with models your mistakes are small.:D
     
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