aluminum grade for mast

Discussion in 'Metal Boat Building' started by fritzdfk, Jul 17, 2004.

  1. fritzdfk
    Joined: Jul 2004
    Posts: 45
    Likes: 2, Points: 8, Legacy Rep: 32
    Location: Alaska

    fritzdfk Junior Member

    I am building a 40' sailing catamaran that will have a mast made out of 8" aluminum pipe. The mast is 40' high so two 20' lengths will have to be welded together. My question is what type of aluminum should I use?

    fritz koschmann
     
  2. sorenfdk
    Joined: Feb 2002
    Posts: 511
    Likes: 27, Points: 28, Legacy Rep: 394
    Location: Denmark

    sorenfdk Yacht Designer

    I'm not sure about what grade you should use as long as it is seawater resistant.
    I'm also not sure that welding the two pieces together is such a good idea, as the mast will be quite heavily stressed. Riveting the pieces together over an internal sleeve would probably be better, even though this makes a part of the mast somewhat stiffer than the rest.
     
  3. gonzo
    Joined: Aug 2002
    Posts: 15,249
    Likes: 951, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Have you considered buying mast sections?
     
  4. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,005
    Likes: 210, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    Round pipe is not good for a multihull mast, for two reasons: One, round is a poor shape for aerodynamics on a fast boat. A more oblong shape is better, such as a standard mast extrusion shape. Second, pipe is heavier wall, typically, than you need, and if there is anything that multihull sailors agree on, it is getting rid of excess weight.

    In the US, you should be looking at 6061-T6 aluminum as that is what most mast extrusions are made of. It has adequate strength, corrosion resistance, and is easily extrudable. I agree about riveting and not welding. The welded strength of aluminum is always a lot less than the virgin metal strength, unless you re-anneal it. All production masts are riveted.

    Eric
     
  5. fritzdfk
    Joined: Jul 2004
    Posts: 45
    Likes: 2, Points: 8, Legacy Rep: 32
    Location: Alaska

    fritzdfk Junior Member

    more about the boat

    Thank you for your replies. The boat is a Bolger design and some photos of the building and some lines drawings can be seen at www.thegreatsea.com

    The mast has no spreaders and the sail is what Bolger calls a chinese gaff. It has full battens, each having jaws around the mast. There are also three sheets. A drawing is on my webpage. I am not building a multihull for speed but for "real estate" and the ability to carry a load. The boat will be slow compared to other modern cats and it is heavier.

    The idea of riveting sounds interesting but the mast would have to be smooth on the outside.

    fritz
     
  6. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,005
    Likes: 210, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    When done a professional spar maker, they use flush rivets just like on airplanes. Alternatively, they may use stainless steel flathead machine screws which, of course, sit flush. So a flush surface is not a problem. Figuring out how many screws is the engineering problems that has to be attended to.

    Eric
     

  7. Dutch Peter
    Joined: Aug 2004
    Posts: 645
    Likes: 7, Points: 28, Legacy Rep: 66
    Location: The Netherlands

    Dutch Peter Senior Member

    On the grade of aluminium:

    Aluminium 5000 series alloys (5083, 5086)
    Aluminium 6000 series alloys (6061T5/T6, 6082T5/T6)

    These are Marine grades.

    On the welding: welding can weaken the aluminium, but doesn't always. It depends on the ally's condition (heat treated, hardend). But if you know the properties of the material and the stresses at the weld you can asses weather welding is an option or not.

    Remember: using stainless steel in aluminium, take measures to prevent galvanic corrosion.

    Hope to have been of help.


    Peter
     
Loading...
Similar Threads
  1. John Stream
    Replies:
    12
    Views:
    3,582
  2. MrNewman
    Replies:
    17
    Views:
    562
  3. Eciton
    Replies:
    7
    Views:
    599
  4. Northeaster
    Replies:
    6
    Views:
    441
  5. Kemmer
    Replies:
    3
    Views:
    1,583
  6. cthippo
    Replies:
    65
    Views:
    4,175
  7. Ry Scott
    Replies:
    40
    Views:
    2,832
  8. jwells11
    Replies:
    58
    Views:
    7,796
  9. Pen Gwyn
    Replies:
    37
    Views:
    2,875
  10. fpjeepy05
    Replies:
    50
    Views:
    4,599
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.