Weight of welds on aluminium

Discussion in 'Boat Design' started by S17665, Apr 19, 2017.

  1. TANSL
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    TANSL Senior Member

    Dear Barry, based on the data in my study, you are drawing conclusions, and giving very concrete advices which are totally incorrect. As I said before, many of the data I have invented myself.
  2. Barry
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    Barry Senior Member

    Was the information on the boat that you provided in your thread, yours or the OPS.
    As some of the measurements for spacing was the same as the OP's, I assumed that this was his boat? Is this not the case?
  3. TANSL
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    TANSL Senior Member

    The only data provided by the OP are those of post # 11. Everything else I have supposed. The only thing I have intended is to show the results for a boat that is much more demanding than that of Satan, to prove what I say in post # 13, that the boat is oversized. Perhaps I have led to an incorrect interpretation; I am sorry.
  4. S17665
    Joined: Nov 2006
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    S17665 Designer

    Sorry for delay..




    8800 mm. Loa
    2900 mm. Beam
    2x250 Hp Outboards XL

    Spacing between bulkhead 800 mm

    Transom and central keel 10 mm
    Keelsons and longitudinal stiffeners of hull 5 mm
    Bulkhead 5 mm
    Deck square tubes (longitudinal stiffeners ) 50x50 x 3 mm thk.

    My first estimation that give a perfet hydrostatic is 5 mm bottom 4 mm side 3,5 mm Deck and 3 mm console. I have used square tube to help on slot welds and plug procedure.Yes the deadrise is 20 deg. Fuel tank 740l fresh water 150l.

    My estimation are the same then TANSL but i would avoid the empty feeling walking on deck with 3 mm .
  5. TANSL
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    TANSL Senior Member

    I think the central keel could be much thinner, even assuming a plus for the possible abrasion on a beach. The mirror can also be thinner and, if necessary, a pair of reinforcement consoles are placed.
    Instead of deck square tube why not "C" ("U") profiles?. Inverted "L"?
    Transversal frames, towards the side, could have lightening to reduce weight.
    You could use the spray rails as reinforcements, thus eliminating some of the bottom longitudinal.
    Side lengitudinals could also be reduced, 50x4 flat bar would be more than enough.
    All of the above is just an opinion.
  6. JSL
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    JSL Senior Member

    Good looking boat.
    If you used inverted T or L for the deck long's you could save a few pounds.
  7. S17665
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    S17665 Designer

    Thanks for the precious support so we can keep the scantling of post 19 ( that is perfect regarding CG calculation and freeboard ) and the increase of 1 mm for hull , side and deck is only ballast .Only my doubt was the choice of several company that uses more then 6 mm for bottom starting with 7 meters loa. I know that is related to arrangement of underneath structures and arrangement and i wonder if is due welding distorsion or grounding.

  8. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Just a few observations

    Your frame and stringer system is extensive and if weight saving is a focal point, there are better ways to do this.

    Normally, or at least often, frames that are welded to the bottom of the hull and the sides are not needed. When there is a lot of inner living quarters, bulkheads can be combined with frames.

    While there are many who will say that vertical welding of the sides to frames will not cause distortion, it can.

    Even for a one off boat, we would build a steel male plug, inverted. You can use the drawings for the frames as the guidelines. A 30 foot plug would probably take a couple of guys maybe 2 days to build, depending on what tooling you have available.

    When the plug is done, you can fit the aluminum bottom and sides. Flip the shell, pull the plug, level it and begin to fit stringers

    As it appears you have a lot of depth in the stringers out to the chine, I would consider having the stringer cut with some lightening holes. Note. an engineer should locate these as too many times I have seen novices locate the center of the lightening holes with their center halfway between the top of the angle stringer and the bottom edge. This is not the optimum location as the hull when welded to the stringer will form part of the stringer beam. Ie for stress calculations.

    After the stringers are laid out and tack welded, you connect between the stringers with cross supports, similar to the frames but they do not touch the hull.

    The side stringers, we called them trays can be formed, gunnel formed and tacked in.

    The frames can stand off .


    Well you are really trying to keep the longitudinal lines fair.
    running stringers that are straight as a shear can make them, ensures straightness.

    Your picture shows frames mounted, then many many short
    stringers to go in between the frames. Harder to keep fair but more important, you will be creating stress concentrations all the way along the stringer which could crack over time.

    You should be able to see a significant overall weight savings using this process.

    I have never seen a disjointed stringer system that you have drawn in aluminum boats up to 50 feet.

    I gree on the comment, use something else besides tubing.

    When you weld in tubing against a sheet, you are adding in a lot of useless material.

    ie in a beam, without to much complication. The most efficient or at least an extremely efficient strength to weight profile puts a vertical web (upright) between an upper horizontal flange and a horizontal lower flange.

    The upper and lower flange carry the bulk of the tensile and compressive loads.

    As a stand alone beam, square tubing is great. The vertical sides act like the web. But when you weld it to say a horizontal deck, the horizontal deck forms another horizontal flange on top of the top of the tubing. So extra weight when it will never really be required to carry much load.

    (an edit. While square tubing is great as a stand alone beam, it carries with it a weight disadvantage as compared to say an "I"beam. Normally an I beam, or channel for that matter, has a thinner web than the flanges because the stresses in the web are less than the stresses in the upper and lower flanges. with tubing, the web thickness if you take both vertical thickness and add them together and call this the web, will be double the thickness of the upper and lower flanges.)

    Ie take a piece of angle, or T-bar, weld it upside down to the horizontal deck.

    Say your spacing is 12 inches between deck stringers,

    Considering the deck material, say you have 12 inch centers,
    you take half the distance from stringer 3 to stringer 4, for discussion sake. Then add half the distance from stringer 3 to stringer 2. This will be then 12 inches by what ever the thickness is of your deck and this will be in essence the top flange of your beam. The vertical part of the angle is the web, and the bottom part of the angle is the lower flange.

    In essence, you will have removed about half of the weight of the tubing that you would have used if you used tubing welded to the deck.

    The last 36 foot inboard that we built, we welded only two frames into the hull that touched the side. A engine room decoupling bulkhead and the bulkhead to hold the anchor rode. All other "frames" cross supports, if you will, were not welded to the hull, only to the stringers and side trays

    There are a myriad of aluminum boat builders in the US, mainly the pacific northwest, and Canada, that use this method. You can google them and some actually have some pictures of the construction process.

    Another problem with setting up frames for a one off, is that if the designer was not really aluminum (or plywood for that matter) aware, you might erect the frames, add in the stringers, ie build your core and then find that the hull shape is not "developable" ie the sheet will not bend to conform and make contact to your already built structure.

    THEN you will have a huge problem.

    good luck
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