Mast and deck-mounted tabernacle for Hobie catamaran

  • 07 Sep 2023 14:24
    Reply # 13251265 on 13250525

    Graeme, thanks for creating the new thread, I had threadjacked the other too far.

    The range of 6061-T6 tubes available in my area suggests 102mm OD 3.2mm wall, available up to 7.3m long. That's a 20 kg stick even before adding any interface to the boat, but the fwd beam on a Hobie is actually a pretty substantial beast about 104 x 70 mm with a good reinforcement at the original mast step where it takes a substantial downforce into the 14mm dolphin striker. I intend to have an X array of stays/shrouds leading diagonally to the hulls, so both roll and pitch moments can be well reacted. The interfaces to the composite hulls need good load spreading, probably four holes through the lip with whiffletrees to concentrate it down to the stays. At the mast I'll need a reinforcing collar to distribute the shroud loads and prevent local buckling, this will need some design effort and fabrication.

    Yes, I know the capsize moment is not the only large load on the mast, on Hobie forums there are discussions of the joys of burying the lee bow and getting the trebuchet treatment from the trapeze wire, so the pitchpoling moments can be spectacular! Having some elasticity in the shrouds should protect the hulls from shock loads, and happily all my sailing will be in the very sheltered waters of the Intracoastal Waterway. Any speed runs will be very temporary things in slight chop, the average fetch for waves around here is only a few km to the nearest upwind shore.

    As for selling my not-so-gently-used Hobie sails in NZ, I may live near Melbourne- but that's the Melbourne on the other side of the planet from you, alas.

    As you can see below the existing Hobie mast step and dolphin striker have pretty good stiffness. I'll make a foot for the mast with a removable hinge pin (and a bracket on the beam) to make raising and lowering the mast easier. Because I keep the boat in my yard about 1 km from the nearest boat ramp I have to put the existing mast up and down every time, and raising the original sail expands my vocabulary every @#$%& time I struggle with it. This gives me more incentive to make the junk rig. 19m2 on a boat with ~240kg total mass with crew will be fun...

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    Last modified: 07 Sep 2023 14:28 | Anonymous member
  • 06 Sep 2023 00:21
    Message # 13250525
    Anonymous member (Administrator)

    Carried over from the Hybrid Aero split junk sail (General Forum).

    Doug asks: Do you have the dimensions of your mast's sections handy?

    I am afraid I did not note measurements at the time. However I have just been up to the boat (which is under a tarpaulin) and measured the top and bottom sections of the mast by using a piece a string and getting the circumferences. The bottom section is about 4.25" (about 107mm) diameter and the top section is about 3.5" (about 90mm) diameter.  The centre section was obviously somewhere in between. I guess the wall thicknesses are about 2mm.  (The centre section was long enough to extend completely to the foot, so the bottom section is, in effect, double skin.) The result is a mast which I consider a little too heavy and probably way over-strength for my boat. 

    It was just made out of what I had. I would like to keep this mast now, replace it with a lighter mast, and keep this one as a possible mizzen for a bigger boat. (Shoulda kept the old pointy rig in order to re-bundle it as original).

    Doug Posted and Kevin replied:

    Since I don't have the meat to put a tabernacle into, I'm looking at doing some low-angle shrouds from the hulls to the mast only about 600 mm above the foot of the mast stepped on the forward crossbeam- all of the sail would be above the stays. It will be rather strange in appearance, I need to make up some sketches.

    I saw this arrangement in the Decathlon sports shop on an inflatable dinghy - it sounds like the kind of thing you’re thinking of.  But there’s a YouTube video of a very interesting mast step arrangement that might provide some inspiration also [edit - this is it] - also see image below.  I have no info on either of these arrangements but they each act in the way you are talking about, supporting a mast close to the bottom end 

    The two  ideas referred to by Kevin are:


    Doug posted:

    Thanks, Kevin, that's pretty much what I'm designing. From the righting moment calculations, I get a capsize limit of about 172 lb of sail force at 12.5 feet above the deck, giving -866 lb transverse at the stays at 2.5 feet, and 694 lb at the foot reacting the torque. (This ignores the angles of the stays, but those will be shallow just as in the photos you shared.)

    These forces are modest for Dyneema lines, but they may be too stretchy and allow the mast to twang about, I'll do some static testing with a winch before trying it out on the water.

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    Doug, here is another idea for a deck-mounted tabernacle (From Dave Zeiger's earlier barge "Slacktide" . Supporting the fore-mast).

    I can't say I like the engineering of any of the above, especially when mounted on a Hobie Cat tubular beam. My main concern would be fore-and-aft forces on the tabernacle. You have done the "capsize" calculations, but everyone seems to forget the forces on a tabernacle when running downwind at speed and burying a hull or being stopped by a wave - the pitchpole force - probably a considerably greater shock than anything which could be sustained beam-on. So a deck-mounted tabernacle needs to be very strongly supported not only athwartships but also fore and aft. This would seem to require a rigid platform (rather than a tubular beam) and if low-angle rigging is used, very strong connection points at the four corners of the platform.

    Could you replace the tubular beam with a small platform, weld up a couple of circular flanges and mount them back-to-back, top and bottom, something like this, to create a tube stub tabernacle into which the mast could be lifted in and out?

    I hope someone else can take the discussion from here, because the project is an interesting one.

    Doug writes: "Maybe I'm taking on too much challenge for my first effort, but I think it could be the first junk rigged boat to fly a hull..."

    You know, people often seem to think that junk rigged boats don't perform well, but it is becoming increasingly evident and increasingly observed that it is not the rig which is slow, but the hull (most junk rigged boats are very much cruising hulls, or even barges). One always wonders, what would happen if a junk rig was put onto a hull which is capable of exceeding its hull speed. I think Rael ("keep shunting") might have something to say here. For this reason I very much hope Doug will solve the problem of mounting a free-standing mast on a hobie cat - not so much to see it flying one hull, but to see it flying both!

    Here is some inspiration. This is a primitive trimaran with a split junk rig. There are a number of video clips for this  vessel at speed, I am just giving one link here. It shows the boat greatly exceeding its hull speed, on its very first outing, during Florida 120 messabout from about 8 years ago.

    (The owner stated that his rig was "....popularised by a guy .., I can’t remember who his name is …somewhere in Europe I think…” !!)

    I presume he is referring to SlieveMcGalliard.

    The aerodynamics of this sail shown here are worth a separate discussion.

    Have a look at this video  

    "Little Tri"

    [PS Doug here's a guy in New Zealand wanting to buy second-hand Hobie sails. If your boat is in Melbourne it might be economic to do a deal turning back then!]

    Last modified: 06 Sep 2023 12:30 | Anonymous member (Administrator)
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