15 m mast parallel then tapered wall thickness

  • 07 Mar 2022 14:42
    Reply # 12645329 on 12644978
    Anonymous wrote:

    Thanks everyone for the calculations and advice

    I have split the mast at the joint it is a two core  12 mm  total so I intend to replace the bottom end in 12 mm

     to ensure I don't build in a  weak point below this I am  then going to make the sleeve in 12 mm  and extend it to the full length of the bottom section 

    This maybe overkill but my mast has been in the sea once 

    In the UK we use 6082 T6 

    Bill






     Bill:

          That looks like a good choice........ I was concerned about the 6063... which as I mentioned I personally consider an inferior product in many ways.     Here is a link showing the properties of 6061 and 6082, both T6, and they seem pretty comparable........ One of the great things about the internet!

                                                           H.W.

    6061T6 / 6082 T6 properties comparison

  • 07 Mar 2022 09:17
    Reply # 12644978 on 12630955
    Anonymous member (Administrator)

    Thanks everyone for the calculations and advice

    I have split the mast at the joint it is a two core  12 mm  total so I intend to replace the bottom end in 12 mm

     to ensure I don't build in a  weak point below this I am  then going to make the sleeve in 12 mm  and extend it to the full length of the bottom section 

    This maybe overkill but my mast has been in the sea once 

    In the UK we use 6082 T6 

    Bill





  • 04 Mar 2022 01:57
    Reply # 12638661 on 12638160
    Anonymous wrote:

    Hi Howard,

    yes you are correct but it is not always available, whereas I have found that 6063 is much easier to find and in a larger number of sizes, at least here in New Zealand. If using 6061 then the thickness could be reduced by about 20%. I would keep the diameter the same to allow for the splice.

    David.



    David:

              Availability is an issue in many places I suspect.   6063 is usually a simple extrusion where 6061 T6 is usually drawn tubing.  That alone makes a big difference in strength.  My biggest objection to 6063 is that it does not tolerate flexing nearly as well.  It will fatigue much faster........As you say it needs to be thicker so that any flex is within a very limited range because of the nature of fatigue.   6063 just does not give me that "warm and fuzzy feeling".  Again, I think some sort of elastomer surrounding the mast so there is not an abrupt sharp pressure point would be something I would look seriously into.... I'm talking about virtually no detectable flex without extreme pressure.  An outer sleeve only slightly larger than the mast with a welded circular flange that bolts to the partner, the intervening space filled with an elastomer.   It would not be hard to build, and would give me at least some additional peace of mind.   Losing a mast is the stuff of nightmares!

    ..........Maybe I'm making it too complicated, but I've been fixing broken stuff all my life.   Wedges made of appropriate material perhaps.....

                                                              H.W.

    Last modified: 04 Mar 2022 02:00 | Anonymous member
  • 03 Mar 2022 20:51
    Reply # 12638160 on 12630955

    Hi Howard,

    yes you are correct but it is not always available, whereas I have found that 6063 is much easier to find and in a larger number of sizes, at least here in New Zealand. If using 6061 then the thickness could be reduced by about 20%. I would keep the diameter the same to allow for the splice.

    David.

    Last modified: 03 Mar 2022 20:55 | Anonymous member
  • 03 Mar 2022 16:28
    Reply # 12637621 on 12636294
    Anonymous wrote:

    Hi Bill,

    I did some rough calcs and a 200mm diameter with a 6mm wall thickness 6063 T6 alloy will give a rough safety factor of 2 at the deck, based on sail loading. I would suggest that the original mast was undersized. If you get a 5 or 6 meter length of pipe of the above size and use a filled epoxy/fiberglass  bond/fill for the splice to the existing mast, which would need to be shortened as required to achieve the correct overall length. Splice should be at least 3 times the diameter of the pipe. Then you should achieve a good result.

    All the best with the repair, David.


     Let me point out as someone with a metallurgy background that 6063 in any temper is a far inferior alloy to 6061T6
  • 02 Mar 2022 22:04
    Reply # 12636294 on 12630955

    Hi Bill,

    I did some rough calcs and a 200mm diameter with a 6mm wall thickness 6063 T6 alloy will give a rough safety factor of 2 at the deck, based on sail loading. I would suggest that the original mast was undersized. If you get a 5 or 6 meter length of pipe of the above size and use a filled epoxy/fiberglass  bond/fill for the splice to the existing mast, which would need to be shortened as required to achieve the correct overall length. Splice should be at least 3 times the diameter of the pipe. Then you should achieve a good result.

    All the best with the repair, David.

    Last modified: 02 Mar 2022 22:06 | Anonymous member
  • 02 Mar 2022 17:26
    Reply # 12635707 on 12630955

    15M  is a pretty tall mast for an unstayed aluminum mast..... or so it seems to me.  The natural failure point is the cabin top, which is where the stresses are concentrated.  I do not know what alloy of aluminum your mast is, but that is very important.   Aluminum is subject to fatigue cracking..... what  metal is not.   Different alloys have different properties.  Fatigue is a function of stress cycles.  The higher the stresses the fewer the cycles......The function is a parabolic curve.   Different alloys have different properties in this respect.   Worse yet stress reversal aggravates this.   One of my favorite alloys is 6061 T6....... very tough and resilient, but they all have their limits.

           I've often thought that a high durometer rubber collar at the partner would alleviate this somewhat by at least reducing the stress concentration at this single point, distributing it out over a distance.  It would take some trial and error experiments to find out how much and what durometer was needed.... You don't want it moving significantly except perhaps in extreme load conditions.    My favorite product in this respect is Devcon's Flexane 80.  An additive is available to allow you to change the durometer as needed.  How thick and how wide the band of rubber is would make a difference here.  An interesting engineering problem, but one that could be tested at scale.  

  • 01 Mar 2022 15:46
    Reply # 12632910 on 12630955

    Look at what Roger Taylor did on Ming Ming 2

    https://youtu.be/ei6_MlMrYX0

    2.5mm wall thickness, thickening to 5mm at the top

    The flagpole I’m considering is .156” wall thickness

    Note also that Roger had about 8” thick partners to distribute the load…that may be your issue??

  • 28 Feb 2022 20:45
    Message # 12630955
    Anonymous member (Administrator)

    I have a 15 M aluminium keel stepped  unstayed mast that is 7 in diameter at the base 

    What wall thickness should I use to replace the bottom section 

     My boat has a 400 sq foot sail on a 33 ft yacht  and the mast snapped 2 m up at the cabin top 



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