Aluminum Tube/Pipe Question: Extruded vs Drawn -Trimaran beam

Discussion in 'Multihulls' started by Jetboy, Mar 17, 2015.

  1. Jetboy
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    Jetboy Senior Member

    I'm buying some aluminum pipe/tube this week for my trimaran project.
    Both options are 6061 T6.

    I have the option of 3.5" drawn tube .125 wall, or 3" sch 10 extruded pipe - which BTW is 3.5" OD .120 wall.

    The pipe is $4.50 per foot, whereas the tube is $8/ft and a couple weeks wait time. Both will sleeve inside 4" OD .125 tube (which interestingly enough is $6/ft - cheaper by 25% than 3.5"). Pipe and 4" are in stock on the shelf.

    I believe the 3.5 OD will not sleeve inside a 4" OD .250 very well. It's just too tight.

    So... what would you suggest? Is there any reason the drawn tube is preferable? The only difference I can find in specs between the two is that the extruded pipe has greater tolerances in dimensions. Since I'm somewhat limited by the 4" to having an 1/8" tolerance anyway (which is more than I think would be ideal), is there any other reason to use the drawn tube?

    Also this is a 15' beam 18' long trimaran with nacra 5.2 floats. Any reason to think 3.5" OD beams with .125 wall (1/8") are insufficient? They will span approximately 5 feet unsupported - although water stays are always an option later if it's too flexy. And of course I could also jump up to .25 wall or schedule 40 pipe for more rigidity.

    Any thoughts are much appreciated!!

    Thanks

    Jetboy
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    You first need to establish what loads you expect to encounter. And also how frequently you will use the boat and then how you intend to attach the beams/tubes to the boat. Without first identifying these basic questions it is pure guess work and your guess could be way way out.
     
  3. Jetboy
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    Jetboy Senior Member

    The boat weight is roughly 1500lbs max - 700lbs rigged + passengers and gear. Beams will be continuous 15 foot sections run through the sleeve in the main hull. The cantalever from the edge of the main hull to the end of the beam(outer face of the float) is approximately 54".

    Attachment will likely be a pin or set of pins or in the alternative a clamp mechanism.

    My core question is whether there is a strength difference between extruded 6061 pipe and drawn 6061 tube with similar profiles. The data sheets don't seem to indicate any difference.
     

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  4. redreuben
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    redreuben redreuben

  5. Jetboy
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    Jetboy Senior Member

  6. gonzo
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    gonzo Senior Member

    A very important question is whether the pipe is seamless or not.
     
  7. redreuben
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    redreuben redreuben

  8. Jetboy
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    Jetboy Senior Member

    I'm not trying to be argumentative, but the table compares apples and oranges; 6063 T832 Drawn vs 6063 T6 extruded.

    6063 T832 always has a higher strength than 6063 T6 because of the temper. 6061 is stronger than 6063 because the base material is stronger. And 6061 T6 is stronger than 6063-T832 for the same reason - base material has a higher yield strength.

    I cannot find any direct comparison between 6061 T6 drawn and 6061 T6 extruded. (both seamless) None of the suppliers I've talked with could confirm any difference structurally.

    FWIW every aluminum mast with a formed track is likely extruded like pipe. Essentially all aluminum spars are closer to pipe than tube by manufacturing process. Is that bad? I don't know. I don't want to sound like i've made up my mind. The price difference is not that great that I wouldn't buy a superior product if it is in fact superior. I simply have no evidence to suggest there is any difference in structural qualities.
     
  9. Marmoset
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    Marmoset Senior Member

    I guess my question ties in with " is it seamed or not?" Cause I would want to know,which has more accurate profile in order to not only fit, but fit snugly in the receiver.

    Barry
     
  10. Jetboy
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    Jetboy Senior Member

    I believe it is not seamed. In the aluminum pipe world seams are rare because most aluminum pipe is used for pressurized liquid or gas and seams are problematic for obvious reasons. And I think the nature 6061 being brittle means making pipe usually involves seamless extrusion. I could be wrong. I'll find out more when I get down to the supplier on friday.

    Sometimes touching and feeling a material can tell you a lot about it.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Material properties do not change i with the profile. The only change is thus:

    In other words, is the section extruded from one shape...or whether it is "welded" together to make the same.
     
  12. AnthonyW
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    AnthonyW Senior Member

    Jetboy - is your build on a thread of its own? Would like to see how it progresses. These questions are similar to what I will be asking once my hulls are done.
     
  13. Jetboy
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    Jetboy Senior Member

    No build thread yet. I figured I might do a quick writeup when it's done. It's been a very long process. Almost 4 years now. Long story short 40" snow storm plus 70yr old flat roof garage = collapse. So I stopped mid-build and stored it while I built a new garage.


    Now I'm back at it. I'm going to pick up the beams today. I'll report on my thoughts. I suspect 3.5" .120 6061 tube is more than adequate for this use. The likely failure point is at the hull. If I can I'll reinforce it in a few critical inches at the point of likely buckling. Otherwise I'll let it fly and see if I break it. Sometimes destructive testing is one of the best ways of finding the limits in a dynamic system.
     
  14. oldsailor7
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    oldsailor7 Senior Member

    IMHO you can't get away without water stays.
    Water stays put the tubing into compression, which tubular spars can happily endure, and reduce the terrible bending loads on unsupported beams.
    By using 6061T6 tubing on my B24, for instance, I was able to get away with 4 1/2" tubing on the Akas, which was approved by Lockie, and never failed me in some horrific sea conditions. :cool:
     

  15. Jetboy
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    Jetboy Senior Member

    I went to the aluminum supplier today. Turns out they were out of stock of the schedule 10 3" nominal pipe (3.5 OD .120 wall). So we did some playing around with what they had and I ended up with:

    4" nominal schedule 40 pipe (actual 4.5 OD .223 wall thickness - extruded seamless) which has an ID of 4.026 ID. And 4" drawn pipe .125 wall. It makes an excellent telecopic fit with just a slight amount of play. The general rule for steel is usually a minimum of 10 thousandths for a telescoping fit. This is greater, but still a really nice fit. I would recommend it as a very good combination for this use. It's much nicer than 3.5 OD inside of 3.75 ID.

    FWIW, as a cash customer I paid Just under $500 for 40' of 4" tube, 20' of 4" pipe, 12' of 2x1/4 flat bar, and a 2 sf scrap of 1/4" plate. All in all it seems pretty reasonable for all of it. 4" tube was $6.15/foot.
     
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