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Aluminum light pole, engineering and conversion to a mast.

  • 03 Dec 2014 23:01
    Quote
    Reply # 3163639 on 3138712
    Anonymous member

    Freedom 40 cat ketch masts are nominally 56ft, 10" base with 1/4" wall for the first 25 ft or so, then tapers down to 4" at the top.  Burry is 4ft.  Two masts, about 480 sq. ft each in sail area (whishbone, triangle sails).  Boat displaces 22,000 lbs dry.  

    My sense is your choice would work.


    Erik
  • 27 Nov 2014 11:05
    Quote
    Reply # 3158935 on 3158446
    Anonymous member (Administrator)
    Peter Hosford wrote:

     Has. anyone successfully used large tapered aluminium light poles for masts. I am considering 50 ft poles , 1/4 inch wall thickness,  10 in. Dia. At base ,tapering to 6 in at top. Alloy is. 6063 heat treated to T6.     Bury would be about 6ft. Any info would be welcome.  Peter.


    If I got it right, the yield strength of that pole should be over 6200kpm. Whether that is good or not, depends on the size of the vessel and if you are to use only one mast or prefer to spread the load on two sticks.

    Cheers, Arne
  • 26 Nov 2014 22:25
    Quote
    Reply # 3158736 on 3158446
    Anonymous member
    Peter Hosford wrote:

     Has. anyone successfully used large tapered aluminium light poles for masts. I am considering 50 ft poles , 1/4 inch wall thickness,  10 in. Dia. At base ,tapering to 6 in at top. Alloy is. 6063 heat treated to T6.     Bury would be about 6ft. Any info would be welcome.  Peter.


    I think this section would be excellent for a junk mast.  My mast is 10.5m, 200mm at the partners and 110mm at the truck, with a 5mm wall thickness.  I think it is the same grade alloy as the one suggested, and has stood up to a tremendous hammering over the last 3 years and 5000 miles of ocean sailing.  My sail is 35.6 sq metres.  As an example, I recently sailed 65 miles downwind in a sustained 27 knot wind with 3 metre seas.  I started with two panels reefed but eventually put in a third reef as my 24 foot, 5 tonne boat was beginning to surf continuously at 7-8 knots and felt very hard-pressed.  We then carried on at 5-6 knots, feeling much more at ease.  From the cockpit the mast appeared to be as straight as always, though in the past I have gone to the mast and sighted up it, where I noted a slight flexing in the gusts.  It is more than stiff enough, however, and this lightpole you are considering would be strong enough for a 500sq ft sail in my opinion.  PS:  I am an empirical sailor, not an engineer.  All I can say is that my mast has not yet fallen down and shows no sign of ever doing so!
  • 26 Nov 2014 22:07
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    Reply # 3158720 on 3138712
    Anonymous member

    Peter, You might also have a look at "junk information" - "useful links" - "sails, masts and gear". Scroll down and you'll find a database showing what other members are using for masts and links to suppliers of flagpoles and lighting columns in Australia, N America and Europe.

    I'm UK based, also researching this area at the moment and wondering whether lamp posts need additional reinforcement at the partners to make them suitable as masts. I'll be grateful myself for any advice on this.   

    Looking at the size and weight of your boat, I think I'd go for a steel mast rather than aluminium. It may be worth talking to Paul Fay, who has steel masts in his schooner Ti Gitu, supplied by a lighting column manufacturer. He reckons the weight is about the same as solid wood masts of equivalent strength.

     
  • 26 Nov 2014 20:46
    Quote
    Reply # 3158686 on 3138712
    Anonymous member
    Until someone with more knowledge comes back on this, I'll just point you this way: http://www.junkrigassociation.org/junkfaqs_masts.  There's some information in here about alloy for masts that might help you, although this is referring to tubes rather than lampposts.



  • 26 Nov 2014 14:34
    Quote
    Reply # 3158446 on 3138712
    Deleted user

     Has. anyone successfully used large tapered aluminium light poles for masts. I am considering 50 ft poles , 1/4 inch wall thickness,  10 in. Dia. At base ,tapering to 6 in at top. Alloy is. 6063 heat treated to T6.     Bury would be about 6ft. Any info would be welcome.  Peter.
  • 04 Nov 2014 01:30
    Quote
    Reply # 3139967 on 3138712
    Deleted user

    HI Ben,

       The pin wouldn't be under much or any load in the normal course of events, the mast will sit on its rim within a cup - so that will be the main load, wedges to take out any  slack and will add fiction to the walls, so the pin is just insurance really in case the mast gets ideas of jumping or rotating. In my opinion there is  little or no need to reinforce in way of the pin, provided that the mast diameter and wall section is man enough in the first place.

    Dont forget there is nothing like the down force present in a stayed mast, when building my steel roberts 434 a surveyor told me that when beating to windward the down loads can equal the total  displacement.

    No stays, mast that can flex and spill energy like a tree, little down force = Happy Junk Technology

    Ash
  • 03 Nov 2014 23:28
    Quote
    Reply # 3139921 on 3138712
    Deleted user
    It doesn't sound like anyone has had problems with the pin only being through the wall of the mast with no reinforcement in that area. I would have thought the cyclic loading on such a small load bearing area would be a problem.
  • 03 Nov 2014 21:05
    Quote
    Reply # 3139820 on 3138923
    Anonymous member
    David Tyler wrote:

    Alloy extrusions are a little stronger in compression than in tension, circumferentially, so it makes sense to put the mast heel within a cup rather than to drop it onto a spigot.


    One further advantage is the ability to jury rig a solid wooden mast where aluminum tubes are scarce.
  • 02 Nov 2014 05:41
    Quote
    Reply # 3138923 on 3138712
    Anonymous member

    Ben,

    The greatest bending load on an unstayed mast is at deck level. It's here that an internal or external sleeve, if in close contact with the mast, can do the most good. Down at the heel, the loads have decreased a bit, and so it's OK to provide a hole for cable exit, and another for a pin to hold the mast down and stop it rotating. I have a 1" stainless steel tube passing through the 10" heel (used to be 8 5/8") of my mast, with wood wedges around it. Alloy extrusions are a little stronger in compression than in tension, circumferentially, so it makes sense to put the mast heel within a cup rather than to drop it onto a spigot. It's also easier to wedge it in.

    Last modified: 02 Nov 2014 05:46 | Anonymous member
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