A 7 metre variant of SibLim

  • 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.

    2 files
  • 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)
  • 16 Dec 2018 13:34
    Reply # 6961700 on 6010674

    Arne,

    I'm using Freeship rather than Delftship, but that doesn't matter, both will give answers on stability.

    The answer to the question on the stability of both Annie's Siblim, and this 7m Siblim variant, is that both have good initial stability, because of the chine just above the waterline; they should both have good form stiffness under sail in light to moderate conditions. But ultimate stiffness is entirely in the hands of the builder/owner, and how much draught is decided upon. Put the ballast lower, in the form of a deeper fin keel, and you could get more ultimate stability for offshore sailing, obviously. Decide to go for minimum draught, for trailer-sailing, or frequent gunkholing, or drying out at the top of the creek, and you'll get less ultimate stability, again obviously. Decisions, decisions... but decisions that have to be made by the builder/owner, not by me.

    Don't forget, Arne, that both these boats were planned for specific owners with specific requirements. Annie: being a comfortable home for the next twenty years rates as high as, or higher than good sailing performance. Gary: sailing along the NSW coast and into the many shallow inlets there, and trailing down to Tasmania, put different constraints on the design. If someone were to approach me for a variant that was specifically for Norwegian fjord sailing, undoubtedly it wouldn't be the same thing at all, though it might superficially look the same.

    Last modified: 16 Dec 2018 13:54 | Anonymous member
  • 16 Dec 2018 10:32
    Reply # 6961617 on 6010674
    Anonymous member (Administrator)

    Scaling a boat design down.

    I know, in these days of CAD, it is very tempting to scale any design up or down from the original. It can be done with just a few clicks and keystrokes. However, with boats, one has to do this with care.
    The problem with linear scaling a boat is that the righting moment varies with the 4
    th power of the length while the healing moment from the rig only varies with the 3rd power (..sorry, don't know the English  mathematical term for this…)

    If a 7.9m boat is crimped in all direction to 7.0 m, the righting moment will drop to only 61% of the original,

    David, how do you compensate for this, or do you reckon the original SibLim to have such ample stability that the 7m version will be all right, just like that? Or can the DeltShip program calculate stability?

    In the sixties and seventies, a whole range of the Swedish Maxi-boats was developed. None of them was just a crimped or expanded version of a former design. The Maxi 68 (6.8m long) came after the Maxi 77, and it was proportionally beamier than the 77, probably to both preserve space and stability.

    Arne


    Last modified: 16 Dec 2018 10:34 | Anonymous member (Administrator)
  • 15 Dec 2018 13:56
    Reply # 6960833 on 6010674

    The second layer of the bottom and bow transom glued and clamped in place.

    Yesterday, I went into town and brought back some 21 x 9mm PSE fir for the chine logs. At full size, that would be 89 x 38mm, which is generous, but not too far OTT.
  • 14 Dec 2018 08:17
    Reply # 6959460 on 6010674

    Now hang on a minute, Annie, you've entirely misunderstood what's going on here.

    In fact, Gary and I went through the same process as I went through with you: I asked him for a design brief and his input on the methods and processes that he wanted to use and was familiar with. And then we worked up the design together, with me submitting drawings and Gary coming back with what he liked and what he didn't, and making an initial model using his local CNC firm so that we jointly understood how we could meet the design brief.

    Which was for a shoal draught bilgeboarder, junk rigged, of a certain size, to be built in a certain way, because Gary likes doing things that way and wanted to take on this project.

    And such a boat is not available on the market, either used or as a kit, or as a design for amateur build. As you yourself found.

    Yes, if anyone just wants a boat, and any boat will do, I would strongly advise them to go to the current buyer's market and find something that's near enough. But if anyone wants *this* boat, and no other will do, then here it is. It may suit others, and they're welcome to build it, if that's the case. Those whom it doesn't suit will need to look elsewhere.

    Let's go back to the early stages of your project. I distinctly said to you that I was going to provide the bare bones of the concept of the accommodation, expecting you to flesh it out and lay things out to suit you, and build to a standard and aesthetic that satisfied you. When we built the model together, we stopped at the stage where we had established that there was a blank canvas onto which you could create whatever it was was that you wanted to create, once you'd worked out what that was. Knowing that I couldn't possibly design an interior that would satisfy you because at that stage I couldn't possibly have read that in my crystal ball, seeing as how it's all developing organically as you've gone along. You haven't got the spatial awareness that develops in someone who has worked in a design field, so you couldn't possibly have done it in any other way than working it out as you went along. And so, it's turning out very well, and it's going to be the beautiful home for the next decades that you need it to be - but it couldn't possibly be anything other than an entirely idiosyncratic expression of your needs. Yes, you started out with a low level of woodworking skill, and now you have a much higher level of woodworking skill - that's the case with many people who fit out a  bare hull. It doesn't alter the fact that fitting out a bare hull takes an inordinate amount of time, whatever the level of skill - I know, I've done it. Just to take the example of Tystie, which is freshest in my mind, six weeks from first cutting wood to bare hull, with main bulkheads in, turned over - and then over a year to fit out and complete the boat. And that was mostly a professional build.

    The case is different for builders with a different set of skills and life experiences to draw upon. Different people want and need different things from their boats. But again, in the present case, I'm not dotting all the Is and crossing all the Ts. I'm leaving the galley detailing and the filling in of other areas to be worked out by the builder, and I'm saying nothing about the standard of finish and general aesthetic that is to be achieved. I'm dealing with the structural aspects only, and expecting - nay demanding - that a builder should take up the baton and run with it, making the project their own as you have done with yours.

    Size matters. In a boat of this size, 7m, there's very little scope for laying out the accommodation in a different way. Just make that small step up to the size of your boat, and more things become possible. So if I had a "client" wanting a CNC cut version of your boat, or larger, I'd go through the whole process again of establishing exactly what was wanted, and what constraints there would be.

    Just because you build quickly, from precut components, doesn't mean you're not getting a bespoke boat, just the way you'd dreamed it. You are.


    Last modified: 15 Dec 2018 10:11 | Anonymous member
  • 14 Dec 2018 06:22
    Reply # 6959380 on 6959023
    Annie wrote:

    To me, this 7 metre variant is very clever - a designer's dream.  But the builder is going to end up with the interior the designer has decided s/he wants.  In fact, there is absolutely no flexibility at all.  So why would you build it?  Why not go out and find an existing boat that is almost what you want and remodel it?  It would take little less time, be cheaper and, in the meantime, you could be sailing.

    And what is all this business about saving time?  You don't build a boat to save time, you build a boat because you want to and because the journey is a large part of the pleasure.  The concept is brilliant, no doubt about that, but it is a concept much more suited to a small factory producing several boats than one person building alone.

    Ya, that was my first thought, "how do I visualize the interior?" However, do please note that this can be done one off. That is, the cost of cutting is $x per minute no matter if it is one off or twenty. So it is still possible to customize the interior. The hull shape is pretty much set but the bulkhead position does not have to be every 100 cm (or whatever the designer specs). It is possible to shift a bulkhead for or aft by quite a bit to fit the interior rather than having to build the interior later around preset bulkheads. So rather than getting what the designer wants, you can actually get something closer to what you want with this method. Also a lot of the gluing can be done with more elbow space before the hull is formed.

    The problem I see is still having to decide before the first cut is made. I think maybe something in between where some of the major stuff can be prebuilt in. Then the interior can be finished after. I would give examples here but I don't have the experience of anything more than day and overnight where it is easy to make do. I am more interested in seeing the full size finished out and the time it takes.

    I think if I were to use this method of building I would have to set up some sheets of plywood and add chairs, benches, boxes and tables to help me figure out a layout before cutting. Because the cutting can be per sheet, it may be worth while to cut in smaller batches. Cut a few sheets of bulkheads that I know, set them up (right side up), put one over top if needed and try things out for space. Move things around. Even those bulkheads already cut could probably be moved a bit with minimum cutting or adding at that stage. Then the rest of the bulkheads and longitudinal bits could be cut and assembled... if you have space right side up (or maybe dry fit right way up) and the walk through continue and more interior things could be done at this stage.

    Perhaps the thing to do is start with a boat of similar size and "I want this size but set up this way". Really it just means knowing what _you_ want before you start building and I think that is the hard part.

    As for speed of build, building is fun, but being on the water is still what it is all about.

    Anyway, I am looking at a glass boat/hull for free (24 foot-ish by the picture) to change the rig on... so taking your advice ;)

       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

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