Mast Breaking Strength - Junk Rig Calculation Tools

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  • 20 Apr 2018 04:02
    Reply # 6111951 on 6111096
    robert self wrote: [snip]
    Hi--Do you really need a 6" OD for a Cape Dory 25? The boat specs indicate a 262 ft^2 sail area which gives a SA/D of 16.7 on a 4000 pound displacement. That's a pretty hefty ratio already. Given a 262 ^ft sail area a 5" OD x 0.156"W x 33 ft overall mast (27ft abv deck?) might work for you.

    [snip]

    For 262 ft^2 I estimate a 33,000 in-lb righting moment at 12 knts at which point you'll want to reef anyway. The bending "strength" of the above mast (#ESR30C51) is around 70,000 in-lb. Safety factor around 2. 

    Once you decide on sail area and height of the sail a shorter mast might work. This company does sell a: 27.5 ft ov'all x  5"OD x 0.188W x 146 pounds.


    At this point I really don't know what the spec should be for this scenario. I'm trying to figure that out and open to all input on the subject.

    You are correct about the design total sail area on this boat but I was considering about 300sqft but I'm not settled on that yet either. I might go 270 or so and use a shorter stick.Oscars calc tool tells me I need a minimum 24ft mast LAP for 301sqft

    If 6063 T6 is acceptable (and there is some discussion about that here) where I am in N IL I have two relatively near by suppliers. One about 35 minutes away and another about   two hours away. Both have one piece poles in the dimensions we're talking about.




  • 19 Apr 2018 18:55
    Reply # 6111156 on 6101804

    So far I'm finding this all very interesting albeit the costs of alloy spars do continue to rise in an unhappy making manner.

    Most all of my personal experience has been with laminated wood and wood composite masts which seem (especially the wood composite area) to be a more budget friendly alternative and I'd love to correspond with anyone else with experience with wood composite to throw some ideas around.

    You might say I have something of a cunning plan...

  • 19 Apr 2018 18:41
    Reply # 6111096 on 6108441
    Dave Rieger wrote:

    Actually looking a little closer. The spec on my new option is still 3.98mm wall thickness but the base diameter is 177.8 (7") rather than 152.4 (6") 

    previous pole:

    • Length -     10.05 m
    • Base dia -  152.4 mm
    • Top dia  -   88.9 mm
    • wall thick - 3.98 mm
    • mass -  39.6 kg
    • Breaking  - 1845 kpm

    New pole:

    • Length -     10.05 m
    • Base dia -  177.8 mm
    • Top dia  -    101.6 mm
    • wall thick - 3.98 mm
    • mass -       46 kg
    • Breaking  - 2541 kpm

    Can you have too much 'Breaking strength' ?


    Hi--Do you really need a 6" OD for a Cape Dory 25? The boat specs indicate a 262 ft^2 sail area which gives a SA/D of 16.7 on a 4000 pound displacement. That's a pretty hefty ratio already. Given a 262 ^ft sail area a 5" OD x 0.156"W x 33 ft overall mast (27ft abv deck?) might work for you.

    https://www.ebay.com/itm/30-ft-Flagpole-Aluminum-5-Bottom-External-Halyard-Flag-Pole-Made-In-the-USA/380825909760?hash=item58aafe3a00:g:VGAAAOxyOMdS4qpF

    for $1080 and free shipping. The specs are here:

    http://imgs.inkfrog.com/pix/flagpolesetc/30x5x156.jpg

    and "shipping weight" shown here (click size comparson):

    https://www.united-states-flag.com/architectural-elite-series/shopby/flagpolesize-30ft.html

    is 155 pounds.

    Not clear if it ships in 1 or 2 pcs. I'd elect 2 pieces. The joint is sleeved and looks solid.

    For 262 ft^2 I estimate a 33,000 in-lb righting moment at 12 knts at which point you'll want to reef anyway. The bending "strength" of the above mast (#ESR30C51) is around 70,000 in-lb. Safety factor around 2. 

    Once you decide on sail area and height of the sail a shorter mast might work. This company does sell a: 27.5 ft ov'all x  5"OD x 0.188W x 146 pounds.


    Last modified: 19 Apr 2018 19:10 | Anonymous member
  • 19 Apr 2018 18:40
    Reply # 6111095 on 6101804
    Anonymous member (Administrator)

    Dave,

    the 6063 alloy is OK  -  it is just that the 6061-T6 alloy sems to be stronger, so you can get away with a lighter pole.

    On my last two boats, the lower section has been of 6082-T6 alloy with a yield strength of 250MPa.

    Arne

    Last modified: 19 Apr 2018 21:33 | Anonymous member (Administrator)
  • 19 Apr 2018 18:16
    Reply # 6111071 on 6110681
    Phil Brown wrote:
    [snip]

    In the US 6063 T6 is used in architectural applications and things such as door frames and roofing.

    6061 T6 at a very minimum of240 MPa is the common structural aluminum here. I bought 6061 t6 tube listed at 40 Ksi or 270MPa.

    Straight tube 6061 is so much cheaper than a flagpole. If building a hybrid mast is not in the cards for you, could you have it done locally for around the same total cost as a pole?  It would likely save a fair amount of weight. 24 feet is a standard available length.

    I'm not sure what some of this means. So you're saying that 6063 T6 doesn't work for this application or that 6061 T6 is just better?

    I've never thought about a hybrid pole so I'd have to go back and rethink this of I was going to head in that direction.




  • 19 Apr 2018 18:07
    Reply # 6111056 on 6109787
    Arne Kverneland wrote:

    Dave,

    The strength of aluminium alloys:

    From a data sheet I find that for the 6063-T6 alloy, yield strength, SigmaY = 200MPa and breaking strength, SigmaB = 245MPa

     

    The strength of a round, hollow pole:

    The maximum breaking (or yield) moment for a hollow round pole is given by the formula:

    [Snip]


    Conclusion: I now think it is better to go for the  thickest, ‘new’ pole with MY=1883kpm (if my calculations are right...)

    Cheers,

    Arne

    Thanks, and...

    Wow, THAT is going to take me some time to digest - I'm a retired IT Tech Manager, not an engineer. But I'll give it a go.

    I don't mean to confuse the issue but the "new pole" (177.8mm at the base ) can also be had with a wall thickness of 4.775mm all other dimensions the same - resulting in 3006 kpm breaking strength (Ocsar's calc). Weight goes up of course.

    Is MY=1883kpm  enough, or do I need to go up another 'notch' (4.775) on a stick that will support roughly 27-28 sqm of sail? Diminishing returns at some point, I',m sure.


  • 19 Apr 2018 15:38
    Reply # 6110681 on 6109787
    Arne Kverneland wrote:

    Dave,

    The strength of aluminium alloys:

    From a data sheet I find that for the 6063-T6 alloy, yield strength, SigmaY = 200MPa and breaking strength, SigmaB = 245MP

    In the US 6063 T6 is used in architectural applications and things such as door frames and roofing.

    6061 T6 at a very minimum of240 MPa is the common structural aluminum here. I bought 6061 t6 tube listed at 40 Ksi or 270MPa.

    Straight tube 6061 is so much cheaper than a flagpole. If building a hybrid mast is not in the cards for you, could you have it done locally for around the same total cost as a pole?  It would likely save a fair amount of weight. 24 feet is a standard available length.

  • 18 Apr 2018 22:54
    Reply # 6109787 on 6101804
    Anonymous member (Administrator)

    Dave,

    The strength of aluminium alloys:

    From a data sheet I find that for the 6063-T6 alloy, yield strength, SigmaY = 200MPa and breaking strength, SigmaB = 245MPa

     

    The strength of a round, hollow pole:

    The maximum breaking (or yield) moment for a hollow round pole is given by the formula:


     (Sorry, the formula will not show here, so look at page 1 of this write-up:)

    ..where ...

    • Sigma=[MPa =Newton/mm2]is maximum stress of the material, yield- or breaking-stress
    • D [mm] is outer diameter of the pole .
    • d  [mm] is inner diameter of the pole, i.e. D minus two times the wall thickness.
    • 1kpm = 1kilopond-meter =9.81Nm (Newton-meter)
    • (1kp=1 kilopond= 1kilgram force = 9.81N)

    The solid pole is just a special case of the hollow pole where the inner diameter, d, is reduced to 0.

    Example 1  -  your ‘old’ tube:

    A tube of outer diameter D=152.4mm and a wall of 3.98mm will have an inner diam, d=144.44mm...

    Putting these numbers into the formula above, should give a max bending moment of
    MY=1368kpm before taking a permanent bend, and
    MB=1676kpm before breaking

    Example 2  -  your ‘new pole’,
    The outer diameter, D=177.8mm and wall of 3.98mm gives inner diam. d=169.84mm

    Putting these numbers into the same formula, should give a max bending moment of
    MY=1883kpm before taking a permanent bend, and
    MB=2307kpm before breaking.

    Now, it is almost midnight here, so I suggest you double-check the numbers.

    The reason why my results are more conservative than yours, is probably because another value of material strength, Sigma, has been used.

    Conclusion: I now think it is better to go for the  thickest, ‘new’ pole with MY=1883kpm (if my calculations are right...)

    Cheers,

    Arne

    Last modified: 18 Apr 2018 23:00 | Anonymous member (Administrator)
  • 18 Apr 2018 19:26
    Reply # 6109434 on 6108621
    David Tyler wrote:

    Dave, the "new pole" spec is the one that I would choose for a Cape Dory 25. Either spec would be strong enough, I think, but I always value the extra stiffness that comes with a greater diameter.


    I agree, and I thank Arne for point out the weight issue. I really didn't know how much was too heavy.

    Now my issue is to figure out why the difference in weight between the two, given the wall thickness is supposed to be the same in both, and the 'new' pole is slightly larger in the other dimensions yet weighs 66 lbs less. This bothers me. Arne's thought that it might be shipping weight vs standing weight is a good one - I"ll check that out


    Last modified: 18 Apr 2018 19:28 | Anonymous member
  • 18 Apr 2018 19:22
    Reply # 6109430 on 6108676
    Arne Kverneland wrote:

    Like David, I prefer the mast to have enough stiffness in it. However, the 150 x 5mm lower mast tube for my own 2150kg Ingeborg, appears to be plenty stiff enough.

    The mast cannot be too strong, but I always try to keep the weight and windage of it as low as possible without spending zillions on carbon.

    If the boat were mine, I would have opted for the slimmest alternative  -  that is, if those 1845kpm is yield strength and not breaking strength. I would not be so happy to return to harbour with a 10° bend in the mast...

    Arne

    PS: What is the specs for the aluminium material; 6063-T6, 6082-T6...?

     



    So far all of the poles I've looked at, Flag and Light, seem to be 6063 T6 (No idea what that really means other than a grade of Titanium, I think.)

    "1845 kpm" and "2541 kpm" came from Oscars JR Calc Tools. How does one determine yield strength compared to Breaking strength? Is there some info on that here?

    The look at carbon fiber was just for laughs. I knew it would be out of line. Just didn't think, by THAT much.


    Last modified: 18 Apr 2018 19:29 | Anonymous member
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