Hybrid Aero split junk sail.

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  • 27 Sep 2023 18:17
    Reply # 13259882 on 13234064


    The battens are all the same length. This seems to help with the sheet, it looks like all battens have a sheet attachment, but the forward part seems to have no reason for being long except perhaps ease of manufacture? or conformity of spare parts?

    Maybe, it is because this is a trial? This looks like a great rig.


  • 27 Sep 2023 11:25
    Reply # 13259676 on 13234064

    Let me also add my congratulations to Steve Dawe. I was the one who worried about his forward placement of the sail with its 22% ‘lead’. (Boat of the Month) I thought it might give Leehelm but I’m delighted to say he has proved me wrong. 

    I had luff downhauls on Miranda as shown in the attached photo. Sheetlets spanned the first 3 battens leading to a block at the foot of the mast then back to the cockpit. This added the necessary tension and gave smooth sails. 

    Paul McKay 

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  • 26 Sep 2023 01:36
    Reply # 13259017 on 13234064
    Anonymous member (Administrator)

    In reply to Mark: I have a slightly different view from Steve, as I do not believe the split per se adds anything much to the aerodynamic efficiency of the sail. I don't think we are getting a "slot effect". I would say the same even in regard to my own, higher balance, McGalliard SJR.

    The split in the sail does allow some other advantages in the way the sail works – at least, I think there are advantages. I could list some.  Steve’s unique sail does seem to work very well, as shown in the various video clips, and I think the key advantage of the aerojunk rig is the unique way in which the battens carry all of the forward-seeking forces on the sail, making parrels unnecessary, and requiring only the halyard and sheet to control it. At least, for a sail the size of this one. This, together with its perfect symmetry on both tacks, has a natural attractiveness to those of us who are used to Bermudan and gaff rigs – a kind of psychological advantage, I think. No wonder Steve’s colourful sail (which owes a lot to Paul McKay, see JRA magazine #84 October 2020) has attracted a great deal of interest on social media.

    I agree with Mark, I love the simplicity, and I too thought a tweak on a downhaul on the lower panels would be worth it (straighten out the luffs a little).

    Another factor which makes this little cambered sail work so well is the use of light weight cloth.

    Well done Steve, I say.

    Last modified: 26 Sep 2023 10:26 | Anonymous member (Administrator)
  • 25 Sep 2023 11:08
    Reply # 13258645 on 13234064

    Love the simplicity of Steve’s rig. (And the colours) . Just two main control lines, sheet and halyard, though I guess downhauls would also help. 

    A question for those with split rigs.

    Does it work like a traditional main and jib, or is it more like a single sail with a slot for the mast.

    i have a suspicion that the latter may be better, that is designing the camber behind and in front of the mast, so (as far as practical) the air flow across the jiblet makes a smooth continuous (if broken) curve onto the rear part. 

  • 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)
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