Hybrid Aero split junk sail.

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  • 07 Sep 2023 15:13
    Reply # 13251304 on 13245393
    Anonymous wrote:

    Steven, your mast appears to be several sections of telescoping aluminum tube, can you share any details, please?

    Sorry I have just seen your message. It has been rather a crazy month answering lots f questions from all over the world. My Mast was purchased second hand but appears to be double skinned for the first 1.7m, there is then a sleeved pivot and a further 7m the first 5m which appears to be double skinned again too. As it tapers at the top it reduces to a single skin. Looking at it closely, it appears to have been formed in some sort of folding process as it is made up from what appears to be many tiny folds. It is incredibly sturdy, but does have some flex in very high winds.  
  • 05 Sep 2023 23:46
    Reply # 13250509 on 13234064
    Anonymous member (Administrator)

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

    I am afraid I did not note measurements at the time. I have just been up to the boat (which is under a tarpaulin) but measured the top and bottom sections of the mast by using a piece a string and getting the circumference. 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 over-strength for my boat. 

    I would like to replace it with a lighter mast, and keep this one as a possible mizzen for a bigger boat.


    This is starting to digress from the subject of Steven's thread.

    If you don't mind, I am going to see if I can copy this conversation over to a new thread.


    Last modified: 06 Sep 2023 12:58 | Anonymous member (Administrator)
  • 05 Sep 2023 02:44
    Reply # 13250081 on 13234064

    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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  • 04 Sep 2023 22:24
    Reply # 13250009 on 13249976
    Anonymous member (Administrator)
    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 https://youtu.be/6vNtklr4W04?feature=shared] - 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 

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    Last modified: 05 Sep 2023 01:01 | Anonymous member (Administrator)
  • 04 Sep 2023 21:02
    Reply # 13249976 on 13234064

    Greame, thanks! Do you have the dimensions of your mast's sections handy? My rough calculations are suggesting at deck level I'll need a 100mm tube with 2.5 to 3 mm wall thickness for a sail area (18.6m2) comparable to yours, is that fairly close to your mast?

    Starting with a Hobie cat with SA/D of over 50 in the original rig pushes for a perhaps overpowered junk rig. If I do a more sedate design it will be easier to learn on but hopelessly underperforming compared to the Hobie sail, and I would like to compare it to other Hobies at some point. 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...

    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.






  • 01 Sep 2023 23:55
    Reply # 13249264 on 13234064
    Anonymous member (Administrator)

    Doug: Steven, your mast appears to be several sections of telescoping aluminum tube, can you share any details, please?

    I too would be interested in the details of Steven's mast.

    Doug, if you are looking for ideas, I have made a couple of masts from telescoped, differing sections of aluminium tube, and found it not too difficult.

    Ideally, I suppose, one tube should fit nicely into the next one, but sometimes you have to make do with what you have. With my scrap pile of tubing I found in each join, the annular gap between the two tubes was too great, and some packing was needed. Here is a schematic diagram: 

    In one case, where the gap was small, I stuffed the gap with epoxy-saturated short-pile synthetic carpet - it was just because that's what I had on hand - and jammed the two tubes together before the resin cured.

    In another case I made a strip of wooden battens (laid out on a strip of duct tape) wrapped it around the smaller tube and epoxied it in place, filling the gaps between the strips with epoxy filler, sanding it down until the fit was nice.

    (As a matter of fact, you can cast the epoxy with the two tubes assembled, to get a perfect and perfectly concentric fit, but ideally you want to be able to get it apart again so that's a bit risky. I've done it though, using baking paper as a release agent. Probably not necessary to go to those lengths for a small mast.)

    On the other mast I made the annular packers from wood and also incorporated the cone-shaped fairing which smooths the transition between the two diameters (necessary, for two reasons). The wooden packers looked like this (schematic):

    The fairing serves two purposes: (1) to smooth the transition between the two diameters and (2) to prevent the small diameter tube from "telescoping" down into the larger tube due to the downward force of the halyard.

    Metal fastenings (rivets etc) are not necessary, and undesirable. 

    The overlap between the two tubes needs to be "sufficient" (10% didn't look enough to me so I doubled it, although in theory it should be enough). The main stress on the joint, I believe, is rotational, so some kind of adhesive is necessary. (Junk rig puts rotational forces onto the mast). I used epoxy glue (carefully cleaned and primed with liquid epoxy) and it has proven strong enough, but I now believe this is the wrong material to use and it has been suggested that a polyurethane rubber glue such as Simsons would be more appropriate. I coated the faired joins with an overlay of glass cloth and epoxy - this is now showing signs of movement/stress on the surface, maybe better coated with something a little more flexible, or not coated at all.  Not sure. 

    3-piece alloy tube mast for Serendipity


    These are just ideas that have worked, maybe there are better ways, anyway it is not difficult to make a "tapered" alloy mast from scraps of tube of varying diameter, and timber topmast sections using the same principles have also been successfully made and reported. Hope that gives you some ideas.


    Last modified: 02 Sep 2023 05:05 | Anonymous member (Administrator)
  • 25 Aug 2023 00:11
    Reply # 13245393 on 13234064

    Steven, your mast appears to be several sections of telescoping aluminum tube, can you share any details, please?

  • 13 Aug 2023 08:23
    Reply # 13240212 on 13236100
    Anonymous wrote:

    On Miranda, my AeroJunk Etap 23, I used this halyard system. The halyard went to the mast top to a double block, down to a single block on the top batten (Yard), back up to the double then terminated at the rear end of the yard. 

    Paul McKay

    I dropped my mast yesterday and did as you showed. So much easier! I can easily lift everything with an 8mm halyard now..I love the etap rig.


  • 08 Aug 2023 18:55
    Reply # 13238243 on 13236100
    Anonymous wrote:

    On Miranda, my AeroJunk Etap 23, I used this halyard system. The halyard went to the mast top to a double block, down to a single block on the top batten (Yard), back up to the double then terminated at the rear end of the yard. 

    Paul McKay

    Thanks for that...I may well be having a change of my haylard arrangement. At least wit a pivoted mast I can easily make the changes.


  • 04 Aug 2023 01:15
    Reply # 13236359 on 13234064
    Anonymous member (Administrator)

    As already discussed on another forum, your sail is outside the norm when considering the relationship between yard angle and mast balance, as shown on your sketch diagram, which will result in a larger than normal halyard angle

    Len wrote: I am not sure that is true.

    I think it is still true in principle - probably more correct to say that in the case of the aerojunk rig it is somewhat irrelevant.

    In any case, I stand corrected. There is something in what Len is saying.

    The conventional junk sail (including the SJR) hangs from the halyard and is held to the mast by an array of standing and running parrels which handle the horizontal forces on the sail bundle, which would otherwise tend to swing forward - and to keep these horizontal forces easily manageable there needs to be some harmony in the outline shape of the sail, the relationship between yard angle and mast balance, yard attachment point and height of crane etc. This harmony also serves to ease the effort of hoisting that last metre or so when the angle the halyard tackle makes with the vertical is at its greatest.

    However, as Paul Mc's photo shows, evidently the aerojunk sail is set up quite differently, with a different halyard arrangement and also requiring no parrels - rather more akin to the gaff sail, with its throat and peak halyards, and the role played by its yard throat and boom throat in holding the sail from moving forward on the mast. Evidently the aerojunk sail design thus somewhat more free from the normal constraints on junk sail outline shape. I guess that is what sets the rig apart from the others, the key feature of the aerojunk rig being the double wishbone cross bars - or the D-formers in the case of Steven's rig and Miranda's latest rig - which play that same mechanical role as "throats" in maintaining the correct fore and aft position of the sail.  (I think we see the same mechanism at work in David T's junk soft wingsail).

    I can see now that Steven's sail is within that category, and get a better understanding of Paul Mc's article on his "hybrid junk" on P9 or Magazine #84

    Looking forward to successful trials.


    Last modified: 04 Aug 2023 04:56 | Anonymous member (Administrator)
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