keel stepped mast with no partner??

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  • 15 Jun 2019 14:56
    Reply # 7581323 on 7580762
    Anonymous wrote:

    Essentially, a Junk rig uses a cantilever mast and theoretically should have a min bury of 6x mast diameter (at the partners).So elevating the partners, by means of a Targa frame could do the trick on a multihull, when and if deck stepping is the only possible option.


    Jeremy:

        The description suggests zero bury in the Gazelle, and no frame above the deck.... I'd be very interested in seeing this close up.   I'm not sure what a "Targa Frame" is, but I presume it entails struts or some other high strength structure to spread the load out in the horizontal rather than vertical plain.   A tabernacle structure that  did this would work for a "deck stepped" mast.   Of course if a 1.5 M bury was called for, and the structure bore load both in compression and tension, it could span that 1.5M, and the resultant loading would be the same as at the step and partner of a buried mast, thus 750cm each way would accomplish the same thing, provided there was substantial sub structure such as bulkheads or beams where it terminated.   I considered such a set up when looking at trimarans, where penetrating the hull crowded the interior to the point of making it almost unusable.   Vertical loading should be minimal at the step itself... just the weight of the mast and rig, but the bending loads are going to be substantial at times.   This would suggest that an optimal location for a deck stepped mast tabernacle built in this way would be dead center between two bulkheads with the load carrying structure at 45 deg either way to the longitudinal axis of the boat terminating on the two bulkheads such that mast bending loads would become vertical tension & compression loads on the boat structure.  This would work fairly well structurally, as most bulkheads are penetrated by doorways, so those loads would not be centered over openings in bulkheads.


          This however is not what is described in the quote.........  I'm very curious to see what the mast step sysem in the Colvin Gazelle the author is referring to actually looks like, and how the loads were distributed....... the Gazelle is steel isn't it?   That would make it easier to handle loads than with a lower strength material I suspect...... It still seems wrong to me



                                                     Howard


  • 15 Jun 2019 03:50
    Reply # 7580762 on 7580599

    Essentially, a Junk rig uses a cantilever mast and theoretically should have a min bury of 6x mast diameter (at the partners).So elevating the partners, by means of a Targa frame could do the trick on a multihull, when and if deck stepping is the only possible option.

  • 15 Jun 2019 02:32
    Message # 7580599

    I read incessantly, (in my spare time) and just this evening I ran across a reference on the Wharram Builder's Webring...... I can't imagine this:  http://wharrambuilders.ning.com/forum/topics/narai-with-modern-lug-junk-rig

    "On my Gazelle, the tapered aluminum masts (made with commonly available light poles) were stepped on the keel. They were completely unsupported where they passed through the deck. Even during storms they never touched the deck when flexing. From the base of the mast to where the foot of the sail started, it was about 10' (3.2m). So the force imparted on the keel stepped mast occurred very high. On a catamaran, it is possible to have the foot of the sail start much lower to the base of the mast, thereby exerting much less force on the structure."

          While the mast  itself is going to be indifferent to bury, it seems beyond silly not to have a bearing partner with a keel stepped mast, both from the standpoint of transmitting the loads to the structure of the boat efficiently without extreme loads, and making the mast carry more unsupported height than necessary.    Am I misunderstanding something here?  Most light poles I've seen  have a welded on base that bolts to the ground, and if they are intelligently designed, the weld will NOT be where the pole projects from the base, but rather the pole will project through the base and be welded on the bottom side. 


                                                           Howard

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