Weight of 20m sailboat mast

Discussion in 'Sailboats' started by floating, Mar 30, 2015.

  1. floating
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    floating Junior Member

    I am looking for the "typical" weight that a 20m mast could have. I just need a rough ballpark since this is for a non-sailboat application (putting a very high antenna on a buoy) and I am concerned that any mast (antenna) of this length would be too heavy for the buoy to support.
     
  2. gonzo
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    gonzo Senior Member

    It depends on many factors. For example, does the mast have guy wires or is it self-standing? Also, what is the maximum load and where is it located?
     
  3. PAR
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    PAR Yacht Designer/Builder

    A 66' "mast" has a lot more to contend with than just it's weight on the buoy. Windage alone might just topple the whole shooting match.

    You'll need to provide a lot more data, for any reasonable answers or suggestions.
     
  4. floating
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    floating Junior Member

    Some background,
    * Guy wires are a possibility but spread would be limited (say 1m)
    * 20kg weight would be placed at top of mast
    * System would remain deployed during a storm, say 30 m/s wind speed. Lower mast could be hit by waves in storm.
     
  5. gggGuest
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    gggGuest ...

    I think that's spectacularly a "how long is a piece of string" question. Here's a brochure for a non sailboat mast company
    http://www.totalmastsolutions.com/downloads/brochures/premier-mast-200mm-with-clamps.pdf
    and they run to about 100kg for a 20m spar rated for 80kg at the top if I read their brochure correctly.

    Without resorting to any maths at all, my utterly uneducated and uninformed guess is that by the time you have enough stability in your buoy system to keep a 20m spar reasonably upright in a breeze then buoyancy to support the weight of the spar itself is unlikely to be a serious limiting factor.
     
  6. AndySGray
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    AndySGray Senior Member


    Exactly.

    For maximum strength and 'off the shelf' consider the triangular cross braced design used for lighting gantries and aeriel towers (and often for home made wind turbines).

    http://www.solidsignal.com/pview.as...Tower-Assembly-base-stubs-included-(HDBX32BS)


    a bouy the size of a 50 gallon oil drum displaces only about 200 kg of sea water - add the weight of the keel, and of course the chain to anchor it in place, it would go all submarine...
    So let's go to a drum bouy 1.22m diameter (4') by 1m which would give a bouyancy of over 1000 kg, though the bouy is going to be 200 kg+



    If the mast was 100kg, the payload 20, the payload would 'act' at 20m but the mast only at 10m (its center) - so like 140kg at 10m.

    If you put a 5m keel with 280kg it would only balance - 400 to 500 would be required to keep it upright, which is already over 3/4 of the available bouyancy before adding the mooring chain! You'd probably also want to put some large vertical plates on the keel too to act as dampers/baffles to counter side to side...

    A deeper keel would help and the bouy can be made taller, but I think the above gives an idea of the engineering involved.

    :cool:
     
  7. The Q
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    The Q Senior Member

    I've a feeling the mast would have to be much heavier than the above values, unless a freestanding exotic materials mast.
    I know of no Aluminum mast that is 66 ft tall with only a spread of 1M. Even if I'm misreading it and the total spread is 6 foot, it would require a heavier than normal mast with horrendous tensions.
     
  8. daiquiri
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    daiquiri Engineering and Design

    I agree. I've had some fun with few quick calcs for a hollow circular section, to see what results come out.
    They show that 850 kg (roughly, a 300x18 mm tube) is a more likely design weight for a freestanding aluminium mast which heels up to an assumed angle of 60° under a 30 m/s wind. The weight would become 700 kg (a 270x15 mm tube) for 30° heel.
    In the non-attainable case of 0° heel, the weight of the mast would be 400 kg (a 200x15 mm tube), which is the bare minimum. But I didn't make a check for an eventual Euler instability, will do it later.

    Mast.gif

    These data were obtained by assuming an admissible strength of 75 MPa. It is a relatively low value when compared to approx. 190-200 MPa yield stress of an average aluminium alloy, because it considers the fatigue. Being subject to continuous alternating bending stress due to waves, the mast design will have to take into account the metal fatigue.

    Hence, the buoy stability will largely determine the overall weight, because, as the heel angle increases, the transverse component of the weight of the mast become the predominant part of the root bending moment.

    Cheers
     
  9. floating
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    floating Junior Member

    We would consider carbon fiber due to the weight of aluminum (assuming carbon fiber was an option for strength). As you can tell, still in "does this make sense" territory so really appreciate all the suggestions.
     
  10. gonzo
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    gonzo Senior Member

    You can have less weight and windage by building a tower instead of a mast.
     
  11. AndySGray
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    AndySGray Senior Member

    (And as per my previous post link.;))

    Consider the tower cranes used on building sites - far greater in height and with a capacity of 20 Tons not 20 Kilos! Their footprint is not much more than a metre square. Even windfarms are now using this tech;-

    [​IMG]
    [​IMG]

    about 5 times the height your talking at 97m and the turbine aint no 20 kilos :D
     
  12. AndySGray
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    AndySGray Senior Member

    Floating - you're not helping anyone with the 'need to know' BS :!:




    'Where' and 'What for' would get you some much more sensible answers.




    Whats the depth - the cost of a caison or a piling type deployment might work better in some instances.

    You mention 30m/s winds (violent storm force with waves upto 16m) - pretty damn nasty!

    Is this some sort of 'cell tower' ? requiring 24x7x365 availability - ggg already linked to a telescoping design - which would allow it to hunker down to weather a storm better?

    Alternatively a 'Barge' rather than a bouy gives massive potential for stability and guys at very beneficial angles and will head to wind to handle seas better.

    How about what the brits would call a LANBY or USCG calls a LNB - is this close to what you want to survive a beaufort 11, without re-inventing the wheel!

    That mast is 40'.

    [​IMG]

    :?:
     

  13. floating
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    floating Junior Member

    * Potential site is too deep for piling/caisson - we are stuck with a compliant mooring.
    * Availability requirements not defined, so telescoping option may make sense.
    * Likely stuck with modifying an existing buoy structure ("does it make sense to stick a mast/tower on top") but barge/LANBY options are useful if the answer turns out to be "no".

    Thanks to all for the suggestions you have offered - they have been very helpful and I appreciate your time.
     
    Last edited: Apr 8, 2015
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