keel stepped mast with no partner??

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  • 17 Jun 2019 22:43
    Reply # 7584464 on 7580599

    What this comes down to is the engineering of a stub mast with loading taken into the boat's structure in such a way as to take care of forces resulting in fatigue..... more easily achieved on a monohull, where dynamic loading is not as high or is considerably dampened - compared to a multi.

    This is where staying is the lightweight and simple solution to get around problems on a multihull.

    Sure, it would be possible to set up a suitable lead between CE and CLR by putting enough area into a rudder/rudders or boards aft, and shrouds could be slackened to allow some squaring of the main when on a run or off wind, making the handling more demanding, although feasible in practice.

    My Jung rig worked like that, so I can see a way to configure a  catamaran rig along similar lines, but having a wing, with headsail, rather than a conventional balanced or split junk sail. The headsail would be on a luffspar rather than a forestay and would  pivot about the fixed mast along with the wishbone and  the hinged batten foil elements/tubes.

    Bracing of the mast partner with diagonal struts will provide at least 10% bury from step to effective partner.

    Last modified: 17 Jun 2019 22:59 | Anonymous member
  • 17 Jun 2019 15:24
    Reply # 7583748 on 7583288
    Anonymous wrote:

    Howard,

    your first posting about a free-standing mast without partners, only bolted to a keel platform via a flange, made me think: "Why spend calories on such an awkward solution?"

    It doesn’t take a Ph.D. in engineering to know that

    Moment = Force x Arm, and thus Force = Moment/Arm

    Therefore, if you have an un-stayed mast with a bury of, say, 125cm, and then decide to only hold that mast upright by a 25cm flange in the bottom end, the forces will be roughly five. times higher (or a bit more, actually). Well and fine if the material in the mast and boat can handle the high, concentrated loads, but otherwise not so wise. That’s why we have the rule of thumb saying that the bury of a free-standing mast should not be less than 10% of the LAP of the mast. Practical rules of thumb make life easier.

    There is no law against having stayed masts in junk rigs. It is just that the low compression loads involved in a JR allow us to do away with the shrouds (if there is enough bury for the mast). The reason is that the JR doesn’t use staysails, which calls for quite some tension to stay well. The reward is that the sail can be squared out properly, which makes sailing downwind simpler and safer.

    Tom Colvin used staysails in his hybrid rigs, and my guess is that the shrouds had to be added as a consequence of this.

    As for getting the lead right with a mast through the cabin of a catamaran, I would have a closer look at the possibility of shifting the CLR aft, either by making new, big and strong swing-up rudders, or by adding a set of aft-set dagger-boards or centre-boards.

    Arne

     



    Arne:

        I agree with you about bury.......... and of course would point out again that the mast itself does not know weather it's buried or not.  The total bending load is at the last (upper) point of support, and is the same on the mast tube weather it is buried an additional 2M or not....... the issue is the load on the structure of the boat itself........ or at least that's the way I see it.   

         I also tend toward moving the CLR aft rather than the mast forward.  It's been determined that on a catamaran, the further aft the CE is the better..... within reason, due to the weight sensitivity / relatively small displacement.  Many cats place the mast forward of the main cabin rather than through the roof of the cabin, and that means struts would be needed.  Few would allow simply dropping the mast into the original location, and adding carbon tow to the saloon roof in strategic locations to support the partner.  


    If the bury is cut in half, the loads on the supporting structure of the partner and step are doubled, if cut to 1/3 they are tripled. 


    Here is a photo and drawings of one of the very few factory built cats that meets most of my criteria. 9.9M LOA, 5.33M beam, fractional rig would allow the mast to sit where it is without concern about balance...with the right rig, but of course bury is very limited........ but I can see where struts to tabernacle could work, depending on the underlying structure of the boat.  The rig itself would be the least of the problems, the structure, significant in several respects.   A biplane rig would result in chopping up the interior unacceptably, making access difficult......there is a reason we don't see more cats converted to junk rig.

    I'm not proposing this, just throwing it out there as an illustration of the problems involved.


                                               H.W.




    2 files
  • 17 Jun 2019 07:35
    Reply # 7583288 on 7580599
    Anonymous member (Administrator)

    Howard,

    your first posting about a free-standing mast without partners, only bolted to a keel platform via a flange, made me think: "Why spend calories on pondering over such an awkward solution?"

    It doesn’t take a Ph.D. in engineering to know that

    Moment = Force x Arm, and thus Force = Moment/Arm

    Therefore, if you have an un-stayed mast with a bury of, say, 125cm, and then decide to only hold that mast upright by a 25cm flange in the bottom end, the forces will be roughly five. times higher (or a bit more, actually). Well and fine if the material in the mast and boat can handle the high, concentrated loads, but otherwise not so wise. That’s why we have the rule of thumb saying that the bury of a free-standing mast should not be less than 10% of the LAP of the mast. Practical rules of thumb make life easier.

    There is no law against having junkrigs with stayed masts. It is just that the low compression loads involved in a JR allow us to do away with the shrouds (if there is enough bury for the mast). The reason is that the JR doesn’t use staysails, which calls for quite some tension to stay well. The reward is that the sail can be squared out properly, which makes sailing downwind simpler and safer.

    Tom Colvin used staysails in his hybrid rigs, and my guess is that the shrouds had to be added as a consequence of this.

    As for getting the lead right with a mast through the cabin of a catamaran, I would have a closer look at the possibility of shifting the CLR aft, either by making new, big and strong swing-up rudders, or by adding a pair of aft-set dagger-boards or centre-boards.

    Arne

    PS:
    The two main arguments for not using a stayed mast with a JR, lose much of their power if you go from a monohull to a cat:

    ·         The catamaran is much wider, which lets one have a much wider staying base for the shrouds. This reduces the compression loads on the mast pole and the boat’s structure.

    ·         The aft shrouds, which prevent one from squaring out the sail, has less consequences on a cat, since it often pays to broad-reach downwind.



    Last modified: 17 Jun 2019 15:37 | Anonymous member (Administrator)
  • 17 Jun 2019 03:13
    Reply # 7583087 on 7582984
    Anonymous wrote:
    Anonymous wrote:

    I just received a communication from the fellow who wrote the bit about the mast bolted down by it's base only in his Gazelle.   I'm even more confused now.  It clearly lacks backstays, and in the  quote in red that I provided in the original post he says "completely unsupported", keel stepped, and no partner.  From what he wrote me today, it appears that  it is indeed simply all street light pole bolted down to the step at the keel by it's flange with no support at deck level....no partner whatsoever.  This suggests placing an inordinate amount of faith in that base........


    that leaves me more confused, as all the Gazelle's shown on the  web have standing rigging


                                                            H.W.It

    Taking a guess now, after connecting the dots back to people on the Wharram builders and friends forum - this might well point to anecdotal evidence from 'Budget boater', who is pretty keen on the idea of experimenting with Junk rig on a catamaran, after some experience on, maybe, the Gazelle mentioned here.

    It could be a case where this instance relates to some very limited usage under benign conditions and is probably not acceptable otherwise.

    In general, a Junk mast requires a fixed step and partners with wedges, while stays/shrouds can and do add support if there is need to restrain a very light and whippy mast.

    I have had fairly hard usage of such an above configuration and  called it a Jung rig, in an attempt to cover the difference between it and the HM version.

    Colvin possibly felt he had the credibility to stand his ground and ignore the semantics, instead being content to live within the technical limits of his chosen parameters.

    At this stage of rig development it seems wise to accept the burden of whatever it costs to go unrigged and freestanding.....that is if you wish to avoid controversy, but if on the other hand there are financial limits, you might need to be a little creative.

    As far as back staying goes - I found that the Junk sheet system provided heaps of support in that direction.

     


     In an ideal world, we would all have a pile wrecked freedoms from a recent hurricane with good carbon fiber masts just down the road a piece that could be had for a song  ;-)


    I started this thread because I found the concept a bit outrageous.... why would one NOT have partners when you have a deck to support them?

    Budget Boater is as you said, a JR enthusiast, and the Wharram community is just about the only place one finds junk rigs on multihulls, excepting a few mavericks..... Multihulls is where my interest lies, though not Wharrams.   My ideal would be a catamaran at about 30' with a single free standing mast and a junk rig.   I feel that this is very possible with a bridge deck cabin design, though nothing I've seen places the high portion of the cabin anywhere near the CLR, which means that the CLR would need to move aft, or the cabin forward, the former being preferable in all respects except sheeting angles.  A high balance rig like Paul's Aerojunk would seem pretty ideal in such a case.

                                                            H.W.

    Last modified: 17 Jun 2019 03:23 | Anonymous member
  • 17 Jun 2019 01:33
    Reply # 7582984 on 7582422
    Anonymous wrote:

    I just received a communication from the fellow who wrote the bit about the mast bolted down by it's base only in his Gazelle.   I'm even more confused now.  It clearly lacks backstays, and in the  quote in red that I provided in the original post he says "completely unsupported", keel stepped, and no partner.  From what he wrote me today, it appears that  it is indeed simply all street light pole bolted down to the step at the keel by it's flange with no support at deck level....no partner whatsoever.  This suggests placing an inordinate amount of faith in that base........


    that leaves me more confused, as all the Gazelle's shown on the  web have standing rigging


                                                            H.W.It

    Taking a guess now, after connecting the dots back to people on the Wharram builders and friends forum - this might well point to anecdotal evidence from 'Budget boater', who is pretty keen on the idea of experimenting with Junk rig on a catamaran, after some experience on, maybe, the Gazelle mentioned here.

    It could be a case where this instance relates to some very limited usage under benign conditions and is probably not acceptable otherwise.

    In general, a Junk mast requires a fixed step and partners with wedges, while stays/shrouds can and do add support if there is need to restrain a very light and whippy mast.

    I have had fairly hard usage of such an above configuration and  called it a Jung rig, in an attempt to cover the difference between it and the HM version.

    Colvin possibly felt he had the credibility to stand his ground and ignore the semantics, instead being content to live within the technical limits of his chosen parameters.

    At this stage of rig development it seems wise to accept the burden of whatever it costs to go unrigged and freestanding.....that is if you wish to avoid controversy, but if on the other hand there are financial limits, you might need to be a little creative.

    As far as back staying goes - I found that the Junk sheet system provided heaps of support in that direction.

     

    Last modified: 17 Jun 2019 01:41 | Anonymous member
  • 16 Jun 2019 22:17
    Reply # 7582777 on 7580599

    Well, if it's a steel Gazelle, the decks will be massively strong and, apart from the risk of denting the masts, partners are possibly unnecessary.  I find it hard to believe that the mast didn't flex to touch the decks: as anyone will tell you who has had the wedges fall out, the mast will move around in its partners.  Assuming a gap around the mast and no shrouds/stays it would bang back and forth in its step.  If there were no gap around the mast then it was effectively supported in partners - even if only 3mm deck plating.

    Gazelles don't have standing back stays any more than gaffers do. 

    This is a one-off comment: I'm not sure I'd take it to reflect the realities of junks in general and Colvins in particular.


  • 16 Jun 2019 17:32
    Reply # 7582422 on 7580599

    I just received a communication from the fellow who wrote the bit about the mast bolted down by it's base only in his Gazelle.   I'm even more confused now.  It clearly lacks backstays, and in the  quote in red that I provided in the original post he says "completely unsupported", keel stepped, and no partner.  From what he wrote me today, it appears that  it is indeed simply all street light pole bolted down to the step at the keel by it's flange with no support at deck level....no partner whatsoever.  This suggests placing an inordinate amount of faith in that base........


    that leaves me more confused, as all the Gazelle's shown on the  web have standing rigging


                                                            H.W.

  • 16 Jun 2019 03:52
    Reply # 7581964 on 7581727
    Anonymous wrote:
    Anonymous wrote:

    If the "Gazelle" was a Colvin design, then AFAIK it uses a stayed mast.

    Sure, as far as I know the Colvin Gazelle does have forestays and shrouds, along with a Junk main, which is something along the lines of a rig that I have experience with. Then the thread ilnk (to the Wharram builders site) brought the multihull issue into the discussion...... and is something that I had also been tied into.

    Making sense of it all, at the same time as keeping within bounds of the collective knowledge here on this forum, I have chimed in with the statement about a cantilever mast being the essential Junk component.

    Rigging can be included (as in the case of a Colvin Gazelle), which can confuse things, unless we come to agreement in that "Junk Rig" means means a sail that can be swung to feather on almost all points of sail, regardless of wind shift.So for this to be possible on a multihull, some sort of above deck support structure/Targa frame is required, and I have seen this done on a bridge deck catamaran, where the cabin was the structure.

    Then there is a proa configuration that eliminates rigging and offers a variety of cantilever mast stepping - such as keel stepping in one hull or support both above and below the hull's connecting structure- in the way of diagonal struts comprising a frame. 


     Thanks....... I could have answered my own question by looking at a Gazelle photo ..... look first, then ask  ;-(  So few junk rigs use stays and shrouds, that I simply never looked... In my mind free standing masts go with junk rigs..... getting all that "garbage" out of the way, and eliminating the maintenance and countless potential failure points that go with it, is a huge plus as far as reliability, and cost.  It kind of defeats a great deal of the benefit to use a stayed mast....


                                         H.W.

  • 15 Jun 2019 22:48
    Reply # 7581727 on 7581608
    Anonymous wrote:

    If the "Gazelle" was a Colvin design, then AFAIK it uses a stayed mast.

    Sure, as far as I know the Colvin Gazelle does have forestays and shrouds, along with a Junk main, which is something along the lines of a rig that I have experience with. Then the thread ilnk (to the Wharram builders site) brought the multihull issue into the discussion...... and is something that I had also been tied into.

    Making sense of it all, at the same time as keeping within bounds of the collective knowledge here on this forum, I have chimed in with the statement about a cantilever mast being the essential Junk component.

    Rigging can be included (as in the case of a Colvin Gazelle), which can confuse things, unless we come to agreement in that "Junk Rig" means means a sail that can be swung to feather on almost all points of sail, regardless of wind shift.So for this to be possible on a multihull, some sort of above deck support structure/Targa frame is required, and I have seen this done on a bridge deck catamaran, where the cabin was the structure.

    Then there is a proa configuration that eliminates rigging and offers a variety of cantilever mast stepping - such as keel stepping in one hull or support both above and below the hull's connecting structure- in the way of diagonal struts comprising a frame. 

    Last modified: 15 Jun 2019 22:53 | Anonymous member
  • 15 Jun 2019 20:46
    Reply # 7581608 on 7580599

    If the "Gazelle" was a Colvin design, then AFAIK it uses a stayed mast.

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