Using rectangular aluminium tubing for the framing?

Discussion in 'Boat Design' started by RSD, Jan 20, 2023.

  1. RSD
    Joined: Nov 2022
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    Location: Red Sea, Egypt

    RSD Junior Member

    I came across this photo while looking at the website of a aluminium boat builder that makes about 30 boats per year - these boats are high end power cats running twin outboards in the 300-425hp range, and are used by well funded recreational fishers to go far offshore on day trips chasing marlin etc.

    One of the interesting things that the photo shows is that instead of cutting frames from plate aluminium they instead use rectangular aluminium tube. What is everyone's thoughts on this idea? I've never seen it used amongst other aluminium boat builders. You may need to save the picture and zoom in to see more clearly the tubing etc.
    upload_2023-1-21_4-40-46.jpeg
     
  2. bajansailor
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    bajansailor Marine Surveyor

    I did save a copy of your photo to my computer, and then enlarged it, but I could not see if the rectangular hollow sections are used for the hull framing as well?
    They should work ok in those areas of constant hull width where the shell plating can lie flat against the hollow section, but if you try to use them where you have hull curvature you will then have to weld the shell plating on to the corner of the hollow section.
    We had to do that with the Kayak Kat (described below) on the stations forward of amidships.

    She is the 15 metre ally cat shown in my avatar - we built her 22 years ago using square hollow sections for framing above the upper chine, although the frames in the hulls below the upper chine are simple flat plate.

    We were originally hoping to build a truss framework first for the bridgedeck, using the square hollow sections, and then use this upside down as a building jig for setting up the hull frames on - but there was a delay in receiving the shipment of hollow sections, and the Builders had lots of flat plate in stock already, so we started on the hulls first instead.
    I still had a conventional film camera then - attached below are some scanned photos of the build.
    The first sheet shows the hull construction; the next one shows the two hulls set up right side up the correct distance apart, and the individual truss frames (forming the bridgedeck) set up at each frame station.
    The third sheet shows the finished boat.
    She has been working well now for 22 years, carrying up to 50 passengers on snorkelling tours along the west coast here.

    001.jpg


    002.jpg


    003.jpg

    She was originally designed for carrying up to 40 passengers and 20 kayaks on kayaking tours (as per the kayaks in the photos below) - they did this for a few years, and then realised that the average punter coming off a cruise ship for the day much preferred to just go snorkelling with the turtles and then quaff a rum punch or two.

    Kayak Kat on beach.jpg Kayak Kat underway.jpg
     
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  3. comfisherman
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    comfisherman Senior Member

    With the speed and ease of cnc cut panels, let alone the ability to absolutely nest for maximum utilization of plate.... why?

    Never mind the difficulty sourcing decent marine grade extruded shapes...
     
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  4. RSD
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    RSD Junior Member

    Unfortunately in Egypt things are very different to life in more developed countries, and unfortunately that means not having access to things like CNC machines - most industries still use labour-intensive methods, and wood is still the most common method of building any vessel up to around 50 metres or so. But there is a massive aluminium refinery 150 miles from the coast so getting 5083 extruded shapes is relatively easy.
     
  5. RSD
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    RSD Junior Member

    Looks like it works well! many thanks once again Bajan!
     
  6. RSD
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    RSD Junior Member

    See my reply above for our situation in Egypt, the photo I posted though was from the website of a boatbuilder in Australia - I'm unsure why they would use rectangular extrusions rather than CNC'd plate though as I would expect them to have a CNC cutter of some sort given that they are putting out over $5 million of boats a year.
     
  7. fallguy
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    fallguy Senior Member

    The tube sounds like big trouble and an ideal spot for crevice corrosion.

    Skipping the CnC idea, still best to cut plate methinks. But I am no aluminum expert, so take me with a (puddle) of salt.
     
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  8. bajansailor
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    bajansailor Marine Surveyor

    @fakkfuy, that is a good point re crevice corrosion - I certainly would not want to use square hollow sections for transverse hull framing in the bilge where it could quite easily sit in a puddle of water, and you don't know what is going on underneath the frames.
    But for the Kayak Kat in my photos above, it has worked well over the years (with no obvious signs of corrosion underneath) as transverse deck beams, and for supporting the underside of the bridge deck, and for the hull shell topsides between the gunwhale and the upper chine.
     
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  9. fallguy
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    fallguy Senior Member

    Fakkfuy? Perhaps a bit too much rum punch on a Sat nite?

    Someday, I may visit Barbados.
     
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  10. RSD
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    RSD Junior Member

    It could have been worse:D
     
  11. comfisherman
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    comfisherman Senior Member

    Two things, I've seen bad ideas maintain adoption for well after they were decidedly a bad idea. (In the alloy construction boom of the 80s and 90s all manner of stuff was tried. In My time directly following school I made a tidy sum cutting out bad ideas and rebuilding them. Cut the bottoms out of more than a couple boats and built them anew. Most times related to water trapping in a void somewhere. )
    Two, these days 5 million in boats isn't all that much unless your talking raw materials.

    Mavrik, rozema, kvichak, cold water, bayweld will all use plate cut on cnc. With proper spacing of limber holes and good qc on the proper alloy plate. I'd hand cut over tube. Knew an outfit that hand cut up until about 12 keeping some old shop hands employed. They eventually bought a relatively inexpensive 4x8 plasma table that while not as clean as router was much cheaper and faster in todays us labor market.
     
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  12. bajansailor
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    bajansailor Marine Surveyor

    Oooops, sorry, I should have proof read it before sending.
    In my defence, the 'l' is just to the right of the 'k' on my keyboard, and similarly the f is to the left of the g.

    And I cannot even blame it on the rum - only coffee this evening.
    But hey, if you ever decide that you need a new Forum name, Fakkfuy is rather neat.... :)
     
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  13. Ad Hoc
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    Ad Hoc Naval Architect

    It only shows RHSs being used for the main deck beams. It does not show, or at least cannot see, the side frames nor the bottom frames.

    You are also overthinking the reasons for such.
    No two boats are the same, in terms of their design and build. In other words, what one builder does, does not equate to, aahh ok..i can do this too. ..without understanding the why's.
    It could simple be, for example, that RHS sections are very cheap and easy and quick for them to buy....and so on such a small boat, makes sense for them financially. But this also does not mean it is the best solution technically.
    Thus, without knowing the why's...it is merely speculation.
     
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