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Thread: Square tube aluminum load ratings

  1. #1
    Associate Engineer
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    Apr 2013
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    Square tube aluminum load ratings

    Good morning,

    I am building a catamaran dolly, using square tube aluminum T6061. The weight of the catamaran will be distributed on four points, indicated by the shaded tubes at the end of the cross-bars in the attached diagram. The catamaran weighs around 400 lbs, and has a beam of 7'8" between the hulls.

    I'm not sure how to calculate this, but my questions are these:

    1) What is the minimum size of square tube I can use to support the load of the catamaran, given the attached diagram. The gauge of aluminum tube I have to work with can be either 0.12, 0.188 or 0.25". The size of square tube aluminum is either 1", 1.25", 1.5" or 2".
    2) The axle diameter in the wheel rims is 3/4". Can I use a solid aluminum 3/4" bar as an axle, or would I have to go steel? I don't think 3/4" aluminum tube would be strong enough. Rim diameter is 8".
    3) Given that the dolly will be in and out of the water, I need to think about metal corrosion, especially since I plan to bolt the frame together. Must I use aluminum plates, bolts and nuts to bolt together, or can I use stainless steel, perhaps with plastic washers?
    4) What would be the best way to mount the axles to the supports? I do not have access to an aluminum welder.

    thanks in advance!

    Cameron


  2. #2
    Technical Fellow
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    Feb 2011
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    Hi Cameron and welcome. A bit of a big ask there.

    Why aluminum? Stiffness is about pound for pound with steel in load carrying situations. Steel costs less too.

    Corrosion, well, aluminum has the advantage up to a point. Stainless steel bolts will cause galvanic corrosion and the trailer will fall apart. Steel can more easily be welded by the home builder and should never fall apart.

    Axles have bearings and seals to keep grease in and water out so there needs to be some machining done. Best bet is to go to a website that sells trailer parts like axles and hubs. Far cheaper than getting your own machining done.

    Finally to answer a little more specifically, for 400-pounds you could almost use well-aged Pasta as a building material. That is just 200-lbs a side so when you decide on a material, ask the questions again. The load you are looking to calculate is from the resting point of one hull at the end of the support and the point at which it crosses a longitudinal member, or looking at your drawing, about the 18" mark. That is the bending moment you need to define.

    Then use one of the calculators on this website kindly provided by Kelly, and all will be revealed.

  3. #3
    Associate Engineer
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    Apr 2013
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    Hi, thanks for the quick response. Why aluminum? Simple, weight. I have a steel trailer for the boat, and it weighs nearly as much as the boat itself. With the boat loaded on the trailer, the combined weight is more than one person can manage, especially over sand and in and out of the water. So the trailer is for road-based transportation, where I can hook it up to my truck.

    However, the use for this dolly is on the beach, moving the boat across the sand, to launch in and out of the wate. If I can keep this dolly under 100 lbs (which I can do if it is aluminum), I will be able to move it around

    The rims that I have for the tires have plastic bushings, which I think will suffice for the simple rolling (100 metres or so) to the water and back, so I don't think I'll need bearings and such.

    I think I understand your answer - the longitudinal member at the outside of the tire should provide enough support to prevent the cross bar from bending inside that. Hmmm...interesting. I had initially planned at bolting the crossmembers at right angles to the longitudinal supports, but now I am thinking that I will place them on top, and bolt from the top down (instead of from front to back). Think I get it, and I'll look for Kelly's calculator's. Thanks!

  4. #4
    Technical Fellow
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    Hi Cameron, you missed the "pound for pound" thing. You can build it out of much thinner steel tube than aluminum as steel is better than double the strength as aluminum. If you are building it to calculated load specifications, then you will finish up with a dolly of about the same weight in either steel or aluminum.

    Having said that, just build it out of what ever you can get locally that suits what you want. The 400-lbs is nothing and at a guess, probably 1" x 1/8" aluminum should do the job with bags to spare. Just a guess mind you and based on the twin rails design of your sketch. That makes it 100-lb per rail per contact point and as I said, Pasta would almost do.

  5. #5
    Associate Engineer
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    Jan 2015
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    I can see Cameron33's point in using aluminum since this is a marine application. Properly configured attaching the components should not be a problem. While bimetallic corrosion plays a large role without planning. I believe that fasteners might be the answer, such as quarter turn. Using zinc coated fasteners might provide him the level of safety he needs, although it would not be my choice. We are not building a bridge here, simply a short lifespan (5 to 15 year) and possibly low utilization dolly. The reduced contact of coated fasteners in distributing the weight might even call for plastic fasteners, larger and more of them. Plus, using tubulars provides additional engineering strength to weight advantages. The boat weight is fairly nominal. If budget is no object, why not graphite tubulars :-)

    Your design is simple and elegant. I assume the boat CG is placed over the two rubber coated contact poles extending from the wheels. The trailer CG should be well forward of this point, and I don't see the mechanical weight distribution being such. How do you plan to weight it? PinkertonD suggests using steel. Why not a plastic coated steel tubular? Several companies sell marine and oil industry coated pipes you might evaluate. Just from the what its worth dept. this might give you the forward weight you need, while keeping the aft portion in aluminum/plastic or wood. I used to work in the oil industry and trust me, strong coated metal pipes are available in large variety. Perhaps, you have a undocumented hitch mechinism that weights sufficient to provide the loaded weight forward of wheels CG.

    Another consideration is wood. Wood is light and certainly strong enough for a 400 pound catamaran. Personally, I love working with wood. A laminated wood structure behind the steel forward tubular/rod would be ideal IMHO. And, I would add a couple of spring leaves for the wheels. I am sure you would glue/and coat the wood with several coats of waterproof primer and paint.

    The forces at play are complex. Road travel involves so many variables. Heck ignore everything I said and just buy a suited trailer for your catamaran. And be sure it has a modest suspension so the wheels to road don't pulverize your catamaran. I bet it would cost less than this project. I know projects are fun though.

    Please take into account I just was doing a quick brainstorm to give you several ideas, to use or not use. I am sure some don't fit your goals. Also, in all likelihood you have already built your trailer. But, perhaps not. I hope I was of some help.

    jouleblu

  6. #6
    Technical Fellow jboggs's Avatar
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    jouleblu,
    Welcome to the forum. One thing we all eventually learn here is to check the dates on posts before responding to them. You just wrote a very thorough and well thought out response to an inquiry that is almost two years old. I'm sure the frame is built and working (or not.) Anyway, welcome!

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