A 7 metre variant of SibLim

  • 27 Dec 2018 23:15
    Reply # 6974050 on 6973990
    Anonymous member (Administrator)
    Anonymous wrote:
    David T wrote:
    What was I saying about going to sea in a barrel??

    Oooohh, let's hope that the poor fellow has good resistance against getting seasick!...

    Arne

  • 27 Dec 2018 22:07
    Reply # 6973990 on 6962762
    David T wrote:
    David W wrote:

    David,

    with the buoyant and relatively high topsides and the well rounded deck I would imagine that neither Annie's version or the 7 meter version of SibLim will be inverted stable. Is this correct? If so then I would not question their ability to undertake moderate ocean crossings.

    David.


    I can't give a numerical answer to this, only to say that we're about as good as we can get with the deck shape, and the range of inverted stability should be small, and much less than boats with a wide, flat deck. Yes, the closer one can approach to "a barrel with a keel on it", the better for extreme sailing, but in reality, we have to be able to walk along the deck. Moderation in all things.
    What was I saying about going to sea in a barrel??
  • 20 Dec 2018 18:38
    Reply # 6967916 on 6967254
    Anonymous wrote:

    I've used 20˚ from the horizontal on the side decks, the same as on the foredeck of Tystie, and I've observed that some people are nervous and uncomfortable with that much, if they're not roofers or hillwalkers/mountaineers. It's not sensible to put anything down without securing it, and loading stores and water containers onto the deck from a pontoon or dinghy is difficult. I proposed to Annie that the area outboard of the bilgeboard cases should be horizontal, for those reasons. She didn't go for it, but I'm going to do it on my model so that Gary can see it and make his own decision.

    As you say, David, flat deck area is handy to have, and in the case mentioned, there were flat deck areas forward and aft of this(arched) deck part, and admittedly, this did require a a hop or two getting over. But did provide a stiff structure with low mass and CG, to make a contribution to the stability system.

    The hull had  low/no deadrise and not much depth of keel, along with external ballast of only about 15%.


    Last modified: 20 Dec 2018 18:54 | Anonymous member
  • 20 Dec 2018 09:32
    Reply # 6967254 on 6010674

    I've used 20˚ from the horizontal on the side decks, the same as on the foredeck of Tystie, and I've observed that some people are nervous and uncomfortable with that much, if they're not roofers or hillwalkers/mountaineers. It's not sensible to put anything down without securing it, and loading stores and water containers onto the deck from a pontoon or dinghy is difficult. I proposed to Annie that the area outboard of the bilgeboard cases should be horizontal, for those reasons. She didn't go for it, but I'm going to do it on my model so that Gary can see it and make his own decision.

    Last modified: 20 Dec 2018 13:30 | Anonymous member
  • 19 Dec 2018 20:56
    Reply # 6966830 on 6010674

    More deck camber than I see in this design, is ergonomically manageable, and combined wwith the buoyancy of the mast and battens, should provide enough floatation to aid righting, from a bit more than 90deg....depends, of course, on ballast and CG at the other, or bottom side of the hull.

    Coniderably less than 40% has worked for me. 

    Last modified: 19 Dec 2018 20:59 | Anonymous member
  • 17 Dec 2018 15:24
    Reply # 6962762 on 6961900
    David W wrote:

    David,

    with the buoyant and relatively high topsides and the well rounded deck I would imagine that neither Annie's version or the 7 meter version of SibLim will be inverted stable. Is this correct? If so then I would not question their ability to undertake moderate ocean crossings.

    David.


    I can't give a numerical answer to this, only to say that we're about as good as we can get with the deck shape, and the range of inverted stability should be small, and much less than boats with a wide, flat deck. Yes, the closer one can approach to "a barrel with a keel on it", the better for extreme sailing, but in reality, we have to be able to walk along the deck. Moderation in all things.
  • 17 Dec 2018 15:17
    Reply # 6962699 on 6010674

    It took a while and some experimentation to establish a fair line for the chine log, which needs to twist a bit along its length to avoid too much edgewise bend. Now I've taken off the angles where it crosses the bulkheads, I can transfer them to the drawings and add the notches.

    At full size, it might be a bit of a wrestle to get 90 x 38mm bent around, and I'd consider using two 45 x 38mm side by side, or two 90 x 19mm one on top of the other. We did something similar during Annie's setup.

  • 16 Dec 2018 23:20
    Reply # 6961981 on 6010674
    Anonymous member (Administrator)

    David W,

    actually, when I wrote the posting #6010674, I neither had the Fastnet disaster, nor the Roaring Forties and ultimate stability in mind. I was focusing more on the ‘working stability’ at max 30° heel. This stability decides how long you can carry enough sail for windward work. This is something that coastal sailors really need (unless they have a good engine installation). I have had a few boats which in varying degrees could sail against a rising wind:

    ·        My 7.1m/1400kg Albin Viggen, Malena, was a surprisingly good seaboat and very easy to steer downwind. However, her blunt bow, light displacement and moderate keel weight and area made her tricky to sail against rising winds and waves. The junkrig helped me to give her just the right sail area, but on the other hand, I had fitted a way too heavy wooden mast, which did not help at all.

    ·        My 8.83m/3000kg  Alo 28, Johanna had a round bottom section, but also plenty of ballast, so could be sailed more on the ear than Malena. She could work to windward  in quite strong winds, but it was no use to press on downwind or she would just squat and start rolling.

    ·        My 6.5m/740kg Jollenkreuzer Frøken Sørensen was a very different animal. She had no ballast at all, and according to the datasheet, her point of no return was at 83° heel. On the other hand, she was 2.4m wide, with a flat bottom, and her bow was very fine. Max righting arm of 0.43m was at 25° heel. This made her sail like a witch to windward, even against a steep short chop and with only three panels set.

    ·        My present 7.87m/2150kg IF, Ingeborg is at the other end of the scale. Her narrow beam makes her initially tender, but her 1250kg(!) ballast and generous keel area gives her plenty of muscles at 25-30° heel. She can tack against a lot of weather for her size  -  but she is also quite wet, so the sprayhood is a must.

    My posting below is just a mild warning when scaling down a design. I forgot to mention that a scaled-down boat should be given a relatively bigger keel area than the original, since the shorter boat is slower, and the lift of a keel varies with the square of the speed. If this detail is dropped, the smaller boat will be more prone to moving crab-wise under sail.

    Arne

     


    Last modified: 16 Dec 2018 23:26 | Anonymous member (Administrator)
  • 16 Dec 2018 20:08
    Reply # 6961900 on 6010674

    David,

    with the buoyant and relatively high topsides and the well rounded deck I would imagine that neither Annie's version or the 7 meter version of SibLim will be inverted stable. Is this correct? If so then I would not question their ability to undertake moderate ocean crossings. The Fastnet disaster was caused by a rating rule that lead to designs that were inverted stable and inherently unsuitable for ocean crossings of any kind, even though they mostly had fairly deep keels.  The important thing is the righting moment of the keel and the righting arm from the center of gravity of the boat to the center of buoyancy of the hull, this will be much greater in a hull with high and buoyant topsides when in the inverted position than in the right way up configuration.

    The model you are building looks great and is obviously sorting out many minor building and design issues as you go.

    David.

  • 16 Dec 2018 14:44
    Reply # 6961723 on 6010674
    Anonymous member (Administrator)

    Thanks, David, I buy your reasoning, as long as the builders understand this.


    Last modified: 16 Dec 2018 14:46 | Anonymous member (Administrator)
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