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Aluminium pole masthead crane fabrication

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  • 24 May 2016 12:26
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    Reply # 4036389 on 4001698
    Deleted user

    This morning I discovered that aluminium is actually quite conductive so a separate lightning rod and copper wire leading to a ground plate is unnecessary.  I will simply wire the foot of the mainmast to the external ground plate on the hull.

    The separation of the main halyard blocks is an interesting idea - I'll think about that.  It would give me an option to add an additional block and turn it into a 5 part purchase moving the bitter end to the yard end. The only downside would be a longer halyard to store when the sail is hoisted.
  • 18 May 2016 15:30
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    Reply # 4026837 on 4001698
    Anonymous member

    Looks good! I'd add the sixth U bolt, starboard aft. Whatever power of main halyard you're going to use, it's better if the fall comes downwards from a single block on the starboard side, with the bulk of the purchase on the port side. There's less unfair wringing load that way.
  • 18 May 2016 12:38
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    Reply # 4026516 on 4001698
    Deleted user

    Adapting David Tyler's description I've fabricated an aluminium masthead crane using 6 x 10mm buttresses spaced 60 degrees apart. The top plate is 5mm thick and another 5mm thick top plate will be bolted on complete with lights and lightening rod. 5 U-blots will be attached with the buttresses in the centre. 2 for topping lifts forward, starboard for spare block, port for mast lift, port aft for main halyard block.

    The diameter at the top of both masts is 76mm and the diameter of the plates is 160mm. The top plate, U-blots and fittings will be attached just prior to stepping.

    I attach links to pictures of the recently welded crane plate.

    https://alberg37junk.files.wordpress.com/2016/05/aluminium-masthead-crane-top.jpg

    https://alberg37junk.files.wordpress.com/2016/05/aluminium-masthead-crane-bottom.jpg
  • 07 May 2016 13:26
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    Reply # 4007876 on 4001698
    Anonymous member

    Sorry, I can't find a picture, but think in terms of a disc 200mm diameter for a tube 100m diameter. I fitted five U bolts, between five buttresses. Two aft, for two single halyard blocks (3:1 purchase), two at the sides for the lifts, and one forward for an extra halyard. I kept a strong, new and therefore reliable block shackled to the forward U bolt, with a messenger line that I could use to pull through a length of 9mm climbing rope, which worked well with a Grigri and a handled ascender, for mast climbing.
  • 07 May 2016 11:17
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    Reply # 4007754 on 4001698
    Deleted user

    Thanks for the ideas. If I were to use an aluminium crane it would have to be strong enough for extended cruising. 10mm thickness sounds about right.

    Regarding the masthead crane of Tystie I'm guessing the diameter of the top plate extends beyond the diameter of the tube and is supported by 3 or 4 triangular buttresses. The steel ubolts then just hang down vertically, if I'm visualising it correctly.

    Sounds simple and sturdy, which I like. A picture would be great if one exists.
  • 04 May 2016 16:41
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    Reply # 4002154 on 4001698
    Anonymous member

    On Tystie's mainmast (100mm diameter at the top), I had a disc of 10mm aluminium plate welded on top, with triangular buttresses welded between disc and tube, also of 10mm plate. Into this disc, and  between the buttresses, I fitted 10mm diameter stainless steel U bolts for the halyards and lifts. This system gives a little bit more freedom of movement than shackling a block through a hole in a tang. Nearing 90,000 miles and still going strong.
  • 04 May 2016 12:54
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    Reply # 4001831 on 4001698
    Anonymous member (Administrator)

    The wooden mast of the 48sqm sloop Johanna was fitted with a mast cap of aluminium. Recently I had a new mast cap made the same way for my 35sqm rig of my Ingeborg (see below). This last mast cap will be fitted soon while Johanna's mast cap has been in use for over12 years now, with no problems. The aluminium mast caps come out a lot lighter than steel version. I see no reason why such a mast cap cannot be fitted to a metal mast top.

    Arne  

     
  • 04 May 2016 11:22
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    Message # 4001698
    Deleted user

    I am currently awaiting the delivery of two aluminium poles to use as replacement masts for my Alberg 37 junk-rigged schooner.

    So I'm fishing around for ideas on fabricating and fitting the masthead crane.

    My first guess is to have masthead cranes fabricated out of steel tubes with tangs welded on as I used for my wooden masts as pictured:

    https://alberg37junk.files.wordpress.com/2011/07/mast-cranes.jpg

    A steel crane as such would then be fitted with a couple of through bolts to hold it in place.

    But I'm interested in how others have fitted cranes on aluminium poles as there may very well be better solutions I can't think of. 

    All ideas considered. Thanks for reading :)
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