Shopping For Used Masts, How Close is Close Enough ?

Discussion in 'Multihulls' started by Chotu, Jun 27, 2020.

  1. Chotu
    Joined: Mar 2018
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    Location: Florida

    Chotu Junior Member

    Sorry for the Imperial Units in advance.

    I need a mast section that is 12 inches x 6 inches in size with a 1/4 inch wall thickness and 60 feet length.

    How much can I deviate from this when trying to find a used rig?

    It also is a fractional rig with no back stay.

    I need to rig it on the cheap because $70,000 is something I can’t afford.

    Any thoughts on how to best achieve a rig on a budget?
     
  2. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    That doesn't give enough information about your mast. You should determine the first and second moments of area, "X-X" and "Y-Y" axes, that the mast cross section needs to have, in order to find an equivalent one
     
  3. Chotu
    Joined: Mar 2018
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    Location: Florida

    Chotu Junior Member

    Thank you for the advice.


    Ok, fair enough. I do not understand the moments of area for the mast. I don’t understand how moments would change given the same diameters, wall thicknesses and lengths, which, to my lack of understanding, would be all the dimensions specified to creat two identical masts.

    But you know what you’re talking about. I don’t. So I looked up the values you mentioned.

    The Selden 304 mast was used on a sistership.

    The dimensions for the Selden 304 are as attached.

    X = 272,0 cm^3 Y=194,0 cm^3



    How would I conduct my search given nobody knows even their wall thickness never mind the moments of area on their used mast?
     

    Attached Files:

  4. TANSL
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    TANSL Senior Member

    I mean the values that appear in the columns called "Ix", "Iy", "Wx" and "Wy". Those values must be the same or very close on your current mast and on the new mast you wish to find.
    And, without knowing anything else, and to avoid problems, "very close" means that they should be the same or slightly larger on the new mast.
    If you could draw a cross section drawing of your mast, I could give you the Ix, Iy, Wx, Wy values of your mast or, perhaps, help you select one from the Seldem catalog.
     
  5. Chotu
    Joined: Mar 2018
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    Location: Florida

    Chotu Junior Member


    Ah, I see where the confusion is.

    My boat has never had a mast on it before. It is newly built.

    A sistership, rigged by one of the top professional rigging houses in the USA used the Selden 304 section. 18.3 meters length.

    I want to find used masts in aluminum or carbon fiber.

    When you are looking for masts, the ad is like this:

    “18m sailboat mast for sale- $6000”
    “Amel SuperMaramu mast for sale - $6000”

    How do I know if a mast is ok to use on my boat assuming a Selden 304 is the correct mast?

    These extra quantities are not known on used masts.
     
  6. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    If you cannot know those properties of the masts you are trying to buy, I don't see a way to check if that is the ideal mast for your boat.
    At least try to get them to specify on which boat and with which sail area that mast was used (displacement and sail area should be similar to your ship's).
     
  7. Chotu
    Joined: Mar 2018
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    Location: Florida

    Chotu Junior Member

    Ok. Thanks for the help. I see a lot of people buy a used mast and put it on their boat.
    I’m at a complete loss on how people usually do this after finding the proper way from you.
    Thank you for the help.
     
  8. TANSL
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    TANSL Senior Member

    Don't assume that that's the right way. I am not an expert in buying second hand masts. I am really sorry that my help does not seem to help you at all.
     
  9. Chotu
    Joined: Mar 2018
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    Chotu Junior Member

    Well, it is better to know the real engineering situation than to imagine life is simple. I thank you for showing me.

    Now, I just have to find out how people usually do this. Then I can apply the engineering to a possible mast once I find one that’s close.


     
  10. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    It is not an engineering problem, absolutely no, it is simply about buying something without knowing how is what you buy and, therefore, without knowing if what you buy is what can solve your problem. The same would happen to you if you buy a nut without knowing if it is the size and the pitch of the screw to which it must be screwed.
     
  11. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    How did you calculate the needed spar in the first place? Or do the dimensions come from plans?
    In order to be able to deviate you need to recalculate the new section as TANSL said.
    As for buying used the only option is call and ask. If the seller is not willing to go out with a tape measure and a phone in order to mail you photos of the dimensions, he does not deserve your money.
    You can ask at boatyards and marinas in your area about masts, they often have some laying around.
    Another option is buying the bare extrusion and doing the mast building yourself. Splicing, cranes, spreaders, etc. all cost a lot in labour.
     

  12. Tom.151
    Joined: Jul 2009
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    Tom.151 Senior Member

    If the Selden 304 is correct for the boat, then you know the moments of inertia ((X = 272,0 cm^3 Y=194,0 cm^3)) which enable you to compare used masts to the Seldon 304.

    If the seller cannot provide the mast manufacturer info, they may be able to tell you what make/model of boat the used mast is off of - from that it's often possible to research (even from the boat builder) what the technical specifications are for that mast.

    A detailed comparison may just end up taking a while.
    If the seller want's to sell, then expect them to be helpful, if the mast is from an unknown boat then you need to get precise dimensions, thickness, and materials info (also you should ask for a "pressing" of the mast cross section shape from the bottom or top of the extrusion - pressing heavy paper or cardboard stock against the butt of the mast - to get an clear impression of the shape and thicknesses.)

    Not to be ignored is the way the mast is rigged, which must also be the same as for the sistership that is using the Seldon 304.

    Good luck.
     
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