SibLim 4 metre dinghy

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  • 14 Jul 2021 01:03
    Reply # 10749590 on 10741742

    I didn't think about that either (the bendy battens potentially affecting sail balance).

    My battens bend a little downwards (due to tweaking a  bit too hard on the downhauls sometimes). So there are now two good reasons to give a SJR stout, rigid battens.

    In reply to Dave D I would agree it is important to be very clear exactly what is meant by "balance" on a SJR. I believe for the purpose of calculating balance, the sail and its perimeter should be considered as a single entity, including slot, and I do not like to calculate "area of sail in front of mast" etc. In the case of SJR with vertical luffs and leeches, it is sufficient to consider the balance axis as a vertical line which divides the lower battens in the required balance ratio. Period. I see no reason why a dinghy sail should not balance in the same way as a larger sail (I guess we will find out) but you are certainly right to look closely at sheeting systems with a view to keeping sheet friction to an absolute minimum.

    On the smallish sail of Serendipity sheeting forces are low but I never had a problem letting fly in a gust. I am wondering now if this is an unexpected benefit of the double sheeting system I use, rather than a single long mainsheet going through many blocks. This leads me to contemplate, for Little Dipper, to try tiny dyneema sheetlets, no more than 4 sheeting points, and paired mainsheets made of soft and very moderate diameter rope, which I have found works very well on Serendipity (the only difference being 5 sheeting points).

    Rather than lose some of the advantage of SJR (and also in order to test the limits at small scale) I will certainly go for 33% balance on Little Dipper, and try to reduce sheeting friction - rather than reduce sail balance.

    Last modified: 14 Jul 2021 02:35 | Anonymous member
  • 13 Jul 2021 19:21
    Reply # 10749034 on 10741742

    With that sheet system, you're pretty much using the running spanline and euphroes that Tom Colvin used, aren't you?

    Thanks for the info on geometry, David. I'm right on the limit, as drawn, which is where I want to be for investigative purposes.

    Thanks, too, for the caveat on the battens. I hadn't really taken on board that with a SJR, both ends tend to bend to leeward, putting more incidence on the jiblets, less on the main. This would very likely explain any bad behaviour. I am planning to use 22mm x 20mm x 2000mm carbon fibre battens which should be OK, I think.

  • 13 Jul 2021 16:37
    Reply # 10748653 on 10741742

    For quicker sheeting, you might consider the "Ahs-up" style of sheeting mentioned in one of the magazines a few years ago.


    It used a split system with a single sheet for the helm, attached via a turning block on the transom, to a triple block, through which the conventional Junk style of sheeting was attached to the battens.

    The author used it on his Sabot dinghy and when he reefed, he tied off the excess Junk sheeting line at the batten end.


    I modified it slightly and brought the end of the line forward to the mast, through a turning block and down to a cleat on my tabernacle.


    It worked well enough and meant that, in a gust, the helm sheet only had to run through a single turning block on the transom, and not through multiple turning blocks on the batten ends and triple block.  Giving a much quicker response to release of the sheet.


    Disadvantage is having to make a further adjustment of the Junk line each time you reef, to get the twist out of the sail.


    I would be careful about the amount of SJR sail area you put in front of the mast on an unballasted dinghy.

    Too much and it definitely will not feather.  Certainly no more than about 25% of total sail area maybe less in an unballasted dinghy, with the luff not more than 33% ahead of the mast centre line.

    I misunderstood Slieve's original instructions, put 33% of sail area in front of the mast, plus the split, and in one particularly heavy gust, the sail didn't feather and I powered along, basically out of control,  until the gust passed. Lighter gusts, fine. The heavy one, NO!.

    The fact the Wayfarer has a lot of stability for an unballasted dinghy and I hiked like crazy is what saved me from a bath. 

    Apart from that one incident, it was great. Went upwind and uptide, fast downwind and easy to reef or drop completely. Ideal for a solo sailor.  

    Having battens which were too small (19mm) and possibly bending in that bigger gust, simulating sheeting in, as the luff of the jiblet got pushed to leeward, but the leech stayed where it was, may also have contributed to the problem in the large gust. Feathered no problem in smaller gusts, despite the wrong dimensions.


    Unfortunately, I still haven't got to sail it since 2017 due to lack of time, family circumstances and now Covid, but have a new set of jiblets with larger dia battens ready to go, whenever circumstances change.


    Anyway, have a look at "Ah-sup's" sheeting arrangement for a potentially quicker sail response in a gust.

    Dave




    Last modified: 13 Jul 2021 16:46 | Anonymous member
  • 13 Jul 2021 16:37
    Reply # 10748651 on 10741742
    Anonymous member (Administrator)

    I like David’s 4-panel Halibut-SibLim sail.
    My original 3-panel ‘Halibut Special’ rig was a half-hearted go on making an aux. rig for Halibut. Requirement No. 1 was quick reefing, and thereafter stowing along the mast within 60 seconds.
    By adding a fourth panel and generally increasing its seize, David has made a much more useful sail, which still can be stowed along a mast  -   and which btw. is not longer than the boat.

    I think little balance is better on a JR which is smaller than 10-15sqm. Apart from the discussed ability to spill the wind in a gust, there is the problem of sheeting out the sail in very light winds. If the sheet has to run over three sheaves, a high-balance sail will struggle. The 10sqm sail of my Broremann had only 10% balance, but was still easy to sheet, and the sail swung out just fine in any wind when I let the sheet go  -  even though the mast had a little aft rake.

    Arne


    Last modified: 13 Jul 2021 16:38 | Anonymous member (Administrator)
  • 13 Jul 2021 12:00
    Reply # 10748097 on 10741742

    Yes, Graeme, I think you're right. A maximum of four sheeted points, preferably three.


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    Last modified: 13 Jul 2021 12:38 | Anonymous member
  • 13 Jul 2021 11:54
    Reply # 10748078 on 10741742

    Slieve writes “…it is not the wind strength that causes the problems in a dinghy. It is the gusts that catch a dinghy out…." This is particularly true in sheltered waters, where the dinghy is in the shelter of hills and valleys. I would not expect to be sailing a 4m dinghy in really strong wind conditions, and would not expect to be far from shelter at any time, if it cuts up rough.

    I still think it is good to be able to reduce sail in a hurry, though,  if one gets caught out in a sudden weather change, especially as this allows one to be a little generous with sail area for the fine, light weather days. That being said, I do not see the need for multiple reefing options as most junk rigs provide. One or two reefing positions at the most ought to be enough for a cruising dinghy, oughtn't it?

    This ties in with the sheeting issue which, again, Slieve has sensibly raised. It is possible to speculate that the Halibut rig as David has drawn it, with its low balance, only three lower panels, and relatively simple sheeting arrangement, ought to make it possible to ease sheets freely and quickly. I am not so sure about the 33% balance,  5-panel SJR though. The sheeting forces will be pretty light…

    For a couple of reasons, the above sheeting issue being one of them, I think I am going to divide the lower part of my SJR into no more than three – and possibly just two - lower panels, hoping that, among other things, there will be a simplification in the running rigging and less friction in the mainsheet.

    (The other reason is: I don’t want to be the first to try jibs with an aspect ratio of less than 1, but that’s another discussion.)

    If it still turns out that these little junk sails won’t “let fly” in a hurry, there is another solution, isn’t there? I seem to recall reading in one of the magazines of a dinghy (I can’t remember which it was, now) which had been given a sort of “two tier” sheeting system. As I recall, and as I think I understood it, the conventional junk sheeting system came back to a jamb-cleat-block, whereby small adjustments to the sheeting could be made. But the jamb-cleat-block was not given a fixed mounting – it was attached to a second-tier sheet – probably 1-1 and just a single block – which the helmsman kept in hand and could release in a hurry if necessary. Would that do it?


    Last modified: 13 Jul 2021 12:02 | Anonymous member
  • 13 Jul 2021 11:44
    Reply # 10748072 on 10741742

    It's interesting to put three possible rigs side by side:


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  • 13 Jul 2021 11:37
    Reply # 10748068 on 10741742

    David W,

    Bearing in mind that this boat is to be used in a shallow estuary where quick reactions with the sheeting will be needed if I touch bottom, I don't think two rigs will work for me. The smaller the sail, the harder I find it to sew up. I'd rather just make one rig of each type at 5.9 sqm. Since the river channels are narrow, I have to be able to skim over the sandbanks, so the board needs to be broad and shallow. I can't immediately visualise a push-up dagger board in that kind of shape, can you?

  • 13 Jul 2021 11:17
    Reply # 10748017 on 10741742

    David,

    may I suggest that you look at possibly using a two masted rig on your dinghy, similar to what I used on my Webb 14. I can see several advantages.

    The rig would be lower so easier to go under the bridges.

    Being lower the rig has less heeling moment and is less demanding to sail.

    The two sails make it possible to balance the boat and sail for extended periods without touching the helm (as I often did with the prototype Webb 14).

    You could use different sails on each mast and so have a direct comparison of say a Split Junk rig and a Halibut rig. With sails of about 45 sq ft each, or use the same shape for both if preferred, say something like the four panel sail on my dinghy designs entered in the dinghy design competition.

    Everything would stow easily within the boat and would be easy for you to rig as all of the parts are smaller and lighter than for a single sail.

    Regarding the centerboard, have you thought of a forward sloped dagger board like the one on my competition dinghy designs. This is simple to construct, easy to work from the center of the dinghy and automatically retracts if you touch bottom. It also helps to push the boat to windward.

    Anyway just some food for thought.

    David.

  • 13 Jul 2021 09:39
    Reply # 10747870 on 10741742

    Thanks for your remarks, Slieve. I too raced dinghies in my youth: Enterprise (I still have a couple of pewter tankard trophies from 1962!), OK dinghy, Fireflies at college. Then I taught children in Enterprises (with the smaller rig) and Wayfarers. On our days off, if it was windy, we instructors would scorch around the lake single handed in the Wayfarers, so I know something about keeping a dinghy upright. Then after I married, we raced a National 12 at Hamble River SC. At age 25, I was comfortable with leaping around a racing dinghy to keep it on its feet; at 75, I'm not. Something nearer to a ballasted dayboat is more my style for sailing these days, but anyway, the SibLim 4m is designed just as much a rowing skiff for exercise and an outboard skiff for family picnics upriver, as a sailing boat.. 

    You might remember my Oughtred dinghy Dorothy, 11ft 6in, that was successfully rigged with several junk rigs. I could handle breezy days in the Solent with no trouble. Last year, I took her out with a western balanced lug on a windy day, with a reef in, and found it unmanageable. I quit and rowed home, before I went for an unintentional swim. JR is much, much easier to handle by a geriatric sailor than a western balanced lug.

    You mention the need for a free-running sheet. That's true. As you are well aware, this is what's always worried me about using a SJR on a small, lightly ballasted boat that can quite easily be blown down in a gust, if the sheet has some friction in it and the sail is so near the point of balance that there is little or no tension in the sheet. So, I've drawn a SJR with 33% balance, with the feeling that this might be too much; but equally, I'm well aware that I need to prove and demonstrate the point, even if it means going for a swim.

    On the other hand, I would expect the extended Halibut rig to have a lot of sheet tension, so if I'm alert, I should be able to let it run easily in a gust. But on a run, the Halibut rig will twist a lot, I suspect (indeed, its camber would come partly from permitting it to twist), so getting into a death roll and capsizing to windward is a strong possibility. 

    These two rigs are at opposite ends of the JR spectrum. I don't know at this stage whether I'd build either, or both, but the boat should be designed such that I could. I think that if I simply wanted the easiest, safest possible rig for this boat, and didn't want to experiment and compare some different rigs, I'd just make a Weaverbird mk 1 kind of rig, with a 45˚ yard, balance somewhere in the middle of the possible range and two fewer panels.


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