The "Sib-Lim" Challenge

  • 12 Jun 2015 06:14
    Reply # 3384067 on 3144241

    Hi Annie,

    thanks for the comments. I think that your concerns on construction of the radius chines may be a bit of a lions and kittens thing. They should be quite easy as the design is for two  layers of 9 mm strip planks 50 mm wide laid at about 30 degrees to the horizontal on opposite angles for each layer. The strip planking should edge set sufficiently so that at most every sixth or eighth plank will need shaping, if any at all. The radius will be achieved by twisting the planks through 90 degrees with a slight amount of bending as well. This should be relatively easy and quick and will result in a much better looking hull. 

     The boat should sit quite nicely on the box keel if she is grounded on a hard bottom. This will allow a relatively easy scrub down of the bottom with a long handled broom.

     The tabernacle is intended to be a welded steel fabrication with a wide base and tie plate forward in the foredeck and back along the cabin top. Preliminary calculations show more than adequate strength if there is extra strengthening in the forward cabin front and the foredeck. Having a support all the way to the keel would kind of spoil the double bunk!!

     The bilge boards are two inches thick solid hardwood and should be more than adequate in strength. I had a thirty two footer in England with a drop keel of Oak and 1800 pounds of lead on the bottom, drawing eight feet, and had no problems with her. The bottom of the boards is rounded on the leading lower edge and sloped aft so that if it hits a shallowly sloping bottom it should push the board up. I did toy with the idea of forward sloping bilge board, similar to my nesting dinghy design, which would definitely push up the case if she hit an obstruction, but that seemed like overkill!! It could also be a first class kelp catcher!!

     The swing rudder seemed like the best option as you get the area that a junk rig demands while still having shallow water ability. She could have twin rudders but the complications seemed a bit more than on the option shown and they would not have the depth or area that a junk rig needs.

    I hope that this answers some of your comments.

    Just bought an Adams 33 to spend the winter on in Australia. Needs a bit of work but should be a nice boat. Unfortunately not junk rigged !!

    David.

    Last modified: 12 Jun 2015 06:17 | Anonymous member
  • 11 Jun 2015 06:42
    Reply # 3382637 on 3144241

    I note Annie's desire for some feedback from other members.  I am not going to endorse any particular entrant to this challenge, but note with interest the different approaches to meet the design criteria.  I am not qualified to make a mathematical assessment of any particular design, but a lifetime of sailing, and studying successful designs has given me an eye for what's needed.

    Some general points.  Firstly, despite Annie's  understandable wish to restrict the draft to 2 feet, I think all designers need to bear in mind that Annie has also stated a desire to go cruising in this boat, not just potter around in sheltered waters.  By cruising, I'd hazard a guess that Annie will be satisfied with making coastal passages between Auckland and Whangaroa.  She will pick her weather and most likely never face a severe gale with dangerous seas, but one can get caught out and the boat needs to have sufficient stability and lateral resistance to sail itself out of trouble and the ability to recover from a serious knockdown.

    To that point, given shallow draft, the boat needs to have high freeboard amidships, with a good radius to the deck, to make the boat unstable when it has gone beyond its natural righting angle, and sufficient fixed ballast to bring it back onto its feet smartly once it does start to recover.  Whether one can achieve this with 2 feet of draft I'll leave to the mathematicians.  I think it is possible, with sufficient ballast ratio and displacement.

    Rudders on such shallow boats offer their own challenges.  There are only three ways to go.  You either have a hinged rudder, or one that slides up and down inside its own case, as many trailer-sailers do, or you go for twin rudders.  Twin rudders can be a lot smaller, not just because there are two of them but because the leeward one, when sailing with the wind forward of the beam, is deeply immersed and, if canted, can offer a better angle of attack.  Getting the feel right, as I discovered on my previous yacht, is a challenge in itself, requiring delicate foil shape and making them toe in a little.  One thing they may not do, if they are far enough apart to be effective, is allow the boat to lie over on one bilge for easy access to the bottom, although possibly, if they are canted out sufficiently, the boat may lay over somewhat, with the lower rudder in a vertical position.  If not, you are going to sit upright on those rudders and it might make sense to incorporate small skegs to take the strain.  For lying over gently on one side, to avoid slithering around on one's belly scrubbing the bottom, the hinged or lifting rudder might work best.  They can be engineered well.  A deep, single rudder, combined with a deep bilge board or leeboard, will give the boat the sweetest handling characteristics.  I like the idea of twin rudders myself, with stout skegs to let you sit comfortably upright, indeed live aboard dried out comfortable upright, and to hell with slithering around in the mud!  Just slip the boat once a year and hang the expense.  An added advantage of twin rudders is that you can sail with the boards up, if sailing in thin water, with some loss of efficiency.

    Lifting (as opposed to hinged) asymmetric bilge boards seem to be a tidy solution to gaining lateral resistance, though one does risk damage to the case in the event of accidental groundings on hard bottoms.  Leeboards will just swing up, offering a useful built-in depth sounder, but you really need to haul up the windward board to avoid excessive loads on the hinges, which can be a pain when one is short tacking short-handed. Hinged bilge boards, as on Tystie, are perhaps the best solution but will take up an awful lot of space in a small boat.  Asymmetric twin keels can be effective but, to be so, they require more draft than this commission allows.  I don't see a solution here that does not involve some compromise.  In the end it will come down to individual preference. 

    The rig is another area where individual preference will be a dominant factor.  The cambered Van Loan rig will work very well, with minimal loads on it, though my experience with a reasonably high-AR version of Arne's cambered HM sail, with the intermediate panel giving three, flattish fanned panels in the head of the sail, strikes me as the best junk rig development yet seen.  The top three panels make a terrific heavy weather rig and I am of the opinion that they are superior in these conditions to the Van Loan design.

    Well, I guess that's enough cats in the pigeon house for now!

    Last modified: 11 Jun 2015 08:21 | Anonymous member
  • 11 Jun 2015 00:52
    Reply # 3382474 on 3377888
    Deleted user
    Annie Hill wrote:

    Hampus - your design program is quite amazing.  I'd love to see the twin rudder arrangement.  I am bemused as to how you have managed to get such astonishing accommodation into such a small boat, but assume that the beam has a lot to do with it.  Getting almost to catboat proportions!  Is the rig still at the development stage?  I'm not sure how the sheeting is going to work for it and the mast seems very short: there's not much drift at the masthead; nor would I expect the halyard to pull at right angles to the yard.  I assume the leeboards are what allow the mast to be placed so far aft?  How do you propose to support them along the side of the boat?  I gather the strains can be quite considerable and, from the drawing, they swing right up through the after end of the bracket.  It's hard to tell from the drawings, but the tiller looks to be very high?  Are you still thinking of having little bilge keels?  I'm not sure how the boat will sit upright when she's dried out.


    Hello Annie!

    There are a few more pictures of Work In Progress here the interior is coming into place. I think I will halt the interior design a bit to draw the twin rudder arrangement and the lee boards. There will no longer be small bilge keel but each rudder will have a sturdy skeg. The skegs together with the keel will give her three resting points. You won't like this ;) but the rudders will still have to be folding ones as I'm quite convinced that the rudder surface you would get from 0.6 meter deep rudders would be far from enough in adverse conditions, even with two rudders.

    The rig is still very much in the design phase. Depending on how you would ultimately like it, it may have to be moved forward a few decimeters which will put it in the bunk. Unless that would bother you for other reasons I don't think this will be a big problem as there will still be 1 meter between the mast and the inside of the hull. That's with a mast of 30 cm diameter.

    The leeboards will be supported about half way down using small rails. The entire design of the leeboards is shamelessly stolen from the dutch tjalks. Althogh this is not a tjalk, the general idea of the arrangement can be seen here. There is no bracket for the leeboards, they are mounted on the outside of the hull.

    Yes, the software is very nice. Although it isn't specifically made for marine design it has some nice features that makes it suitable being a 3D CAD software rather than just a 3D modelling software. I'm having loads of fun with it :)

    /Hampus

  • 07 Jun 2015 05:12
    Reply # 3377889 on 3144241
    As you know, David Tyler is on passage from Canada towards Hilo, Hawai'i.  He has Iridium on board, which allows him to send text emails, so I sent him Hampus's comments about the righting abilities of the various Sib-Lim offerings.  He asked me to post the following on his behalf:

    Hampus,

    You are quite right. We have designed with reference to Annie's clearly expressed wish for a shoal draught boat that tends towards "a houseboat that sails", rather than "a sailboat suitable for living on". But if a sistership to Sib-Lim is to be taken offshore, I would strongly advise the builder to increase the draught, by adding a deadwood between the slab of ballast and the hull. I would consider a draught of 800 - 850mm to be appropriate. But not more. My experience with Tystie leads me to feel safer and more comfortable with a boat that does not have a deep keel to trip over, but yields and skids sideways in steep seas.

    A boat that is intended to fulfil the requirements of "Category B, Offshore" must have a AVS of 97 or more, and a STIX number of 23 or more. I think these are achievable, but such a size of boat has little hope of meeting the requirements of "Category A, Ocean", with an AVS of 107 or more, and STIX of 32 or more. [Tystie, somewhat larger, has an AVS of 132 and a STIX of 33 - demonstrating that with shoal draught, the first requirement is relatively easy to meet, the second, less so. Tystie recovered quickly from a knockdown that must have approached 132, as all the sheets and halyards, stowed in bags in the cockpit, ended up in the water].

    Another factor that has a large impact on ultimate safety is the form of the deck. The "rounder" and more barrel-like the midship section, the less likely it is that the boat will remain inverted, and the more quickly she will shed solid water from the deck. My version of Sib-Lim has as convex a deck as I think is consistent with day-to-day comfort and convenience."

  • 07 Jun 2015 05:08
    Reply # 3377888 on 3144241
    I've been cruising around in an area where my access to the Internet has been good enough for emailing and to look at websites, but not good enough to post.  Well, finally I have a decent signal.

    Puffin: it's good to see your updated design, David.  You've done a great job of getting in the accommodation, she looks very roomy.  There are some very clever ideas incorporated, too - I like the window through to the chart table.  Ideally, I would like a U-shaped seating area - it's that quart in a pint pot again.  I am interested to see the debate about the leeboards - certainly something that needs careful consideration.  Are they perhaps a bit thin?  The tabernacle is something that concerns me: I'm not sure how it would be engineered to be sufficiently strong for the mast, without going through to the keel.  I also have my doubts about building the radiused chine - I think it might tax my wood working abilities more than a little.  The hull is very much that of a barge yacht and I wonder how I'd go about keeping it clean.  I'm not sure that shuffling around on the tide would work with such a large area.  However, the rocker aft and the long skeg there would certainly assist in keeping the boat heading in the right direction once the boards were up.  I think the lifting rudder might prove an issue, especially with the mizzen/self-steering on the stern.  Where did you plan for a dinghy to be carried?  She's very different from the first design you showed me, David.  I wonder how she would sail in the F2 to 3 that are my favourite sailing conditions.

    Hampus - your design program is quite amazing.  I'd love to see the twin rudder arrangement.  I am bemused as to how you have managed to get such astonishing accommodation into such a small boat, but assume that the beam has a lot to do with it.  Getting almost to catboat proportions!  Is the rig still at the development stage?  I'm not sure how the sheeting is going to work for it and the mast seems very short: there's not much drift at the masthead; nor would I expect the halyard to pull at right angles to the yard.  I assume the leeboards are what allow the mast to be placed so far aft?  How do you propose to support them along the side of the boat?  I gather the strains can be quite considerable and, from the drawing, they swing right up through the after end of the bracket.  It's hard to tell from the drawings, but the tiller looks to be very high?  Are you still thinking of having little bilge keels?  I'm not sure how the boat will sit upright when she's dried out.

    Both boats are very clever - as is David Tyler's design.  I'm disappointed that there hasn't been more discussion about the designs from other members.

    The next magazine is going to showcase David Tyler's offering.  It looks like there'll be plenty to put in the following issue and the one after that.  Thank you very much for all your input.  There's a lot to think about and it makes me very aware of how little I really know about the theory of boat design.

  • 05 Jun 2015 00:04
    Reply # 3375548 on 3374347
    Deleted user
    David Webb wrote:Hampus,

    in my calcs for stability on Puffin I get a positive stability up to about 130 degrees and very little negative stability at greater angles. The ballast is as low as I could get it and the high freeboard, full width cabin and heavily crowned deck all help to prevent the boat becoming inverted stable. She should come back from a complete roll over quite quickly.

      Arnie, your comments on lateral plane are valid and I may well deepen the bilge boards a little more to allow for this. I was attempting to get the leeboard to remain in the slot when raised, but they may have to project above deck level to get sufficient lateral plane for low speed sailing performance.

    Hey David. 

    At first sight I thought 130 degrees sounded a lot considering the shallow draft and that there would be no way my take on Sib-Lim would get that so I did calculations on my unfinished model and got 135 degrees. I expect  it to go down a bit from there though as the center of gravity will creep up a bit as I keep adding interior, rig etc.  

    Those are nice looking drawings of Puffin. She's really coming along nicely.


    Regards


    /Hampus

  • 04 Jun 2015 00:12
    Reply # 3374347 on 3144241
    Hampus,

    in my calcs for stability on Puffin I get a positive stability up to about 130 degrees and very little negative stability at greater angles. The ballast is as low as I could get it and the high freeboard, full width cabin and heavily crowned deck all help to prevent the boat becoming inverted stable. She should come back from a complete roll over quite quickly.

      Arnie, your comments on lateral plane are valid and I may well deepen the bilge boards a little more to allow for this. I was attempting to get the leeboard to remain in the slot when raised, but they may have to project above deck level to get sufficient lateral plane for low speed sailing performance.

  • 03 Jun 2015 00:27
    Reply # 3369351 on 3144241
    Deleted user

    Hello.


    There are some new work in progress files here: Hotlink

    Arne: You are right about the performance issues. With the shallow draft, leeboards and relatively heavy displacement with a lot of whetted surface none of the Sib-Lims will be great performers. 

    Stability is actually one of my biggest concerns. None of the Sib-Lims will, in my opinion, be suitable for longer passages as none of them will have the ability to recover from a knockdown. There is nothing strange about this, it's just physics. All of the designs rely on stability achieved by having a lot of beam on the waterline in combination with relatively flat bottoms. This will make them stiff initially, like a plank floating in water with the flat side down. Due to the shallow draft, ultimate stability will be low. I'm guessing, without having done any calculations, no more than around 100 degrees. This means that there will be around 80 degrees of negative stability where the boat will be perfectly stable upside down. The chance of recovering from a knockdown in a boat with 80 degrees negative stability is slim to none.

    I would advise against undertaking any passage that would stretch beyond a reliable weather forecast in an area where the conditions can get bad enough to put you in risk of getting knocked down.   


    /Hampus

  • 27 May 2015 23:10
    Reply # 3359544 on 3358752
    Deleted user
    Arne Kverneland wrote:

    Short boats need relatively big “wings in the water”

     I soon gave up trying to design a Sib-Lim. The main design criteria for max length (26’) and draught (2’), and for a loaded displacement of 3000kg was too hard for me to combine into a decent sailing performance to windward, in particular since I wanted the boat to be dead easy to build. I did try, but my attempts either resulted in box-shaped barges, or in real tubs.

     The problem when wanting so high displacement and such a shoal draught is that the beam goes up a lot. Then it is inevitable that the craft will struggle when sailing close-hauled against a chop. This is made worse if the extremities  -  centreboards, daggerboards or leeboards, plus rudders, are on the small side: As soon as the boat stops, the board(s) will stall and it is difficult to get started again. The progress to windward (VMG) will easily become slow or non-existing.

     To remedy this problem, I suggest you do your best to scale up the boards as much as you can. Remember, short, slow boats need relatively much bigger keel areas than bigger boats, to avoid making leeway. A fine example of this scale effect can be seen on sailing model boats: Their keels are huge compared to the size of the boats.

     Arne

     

    To support Arne in his assertion about keel size, if you look at the Atalanta 26 foot, then the keels add another 5 feet to the depth.  The cruisers sailed wonderfully in most conditions - but at an angle because of the barge-like central section, which having the need for good accommodation at this size tends to require along with initial form stability.. 

    The bluff stem made sailing the Solent short chops a challenge unless out early in the morning before all the washes of the motor boats started sploshing from the mainland to the Isle of Wight and back. But to get a sharp stem to cut through on a 26-er will need giving up some forward buoyancy and accommodation. 

    What is salutary is that when Fairey altered the build slightly in the Titania (still 26 foot) and the 31 foot version, neither sailed as well as the original. The 31-er in particular could be outpaced by the 26-er.  

    I don't mean to discourage anyone and write this only to urge contributors on to greater things.

    Apologies that I only have the expertise to contribute occasional experiences and not a design.

    David


  • 27 May 2015 14:13
    Reply # 3358752 on 3144241
    Anonymous member (Administrator)

    Short boats need relatively big “wings in the water”

     I soon gave up trying to design a Sib-Lim. The main design criteria for max length (26’) and draught (2’), and for a loaded displacement of 3000kg was too hard for me to combine into a decent sailing performance to windward, in particular since I wanted the boat to be dead easy to build. I did try, but my attempts either resulted in box-shaped barges, or in real tubs.

     The problem when wanting so high displacement and such a shoal draught is that the beam goes up a lot. Then it is inevitable that the craft will struggle when sailing close-hauled against a chop. This is made worse if the extremities  -  centreboards, daggerboards or leeboards, plus rudders, are on the small side: As soon as the boat stops, the board(s) will stall and it is difficult to get started again. The progress to windward (VMG) will easily become slow or non-existing.

     To remedy this problem, I suggest you do your best to scale up the boards as much as you can. Remember, short, slow boats need relatively much bigger keel areas than bigger boats, to avoid making leeway. A fine example of this scale effect can be seen on sailing model boats: Their keels are huge compared to the size of the boats.

     Arne

     

    PS, 28.5.2015:

    Just to clarify a bit. I have no doubt that the Sib-Lim contenders, as those shown to us, will have enough stability, both for carrying sail and for safely recovering after knock-downs. My concern is only about (possible lack of) leeway resistance provided by the boards.
    A.

     

     

    Last modified: 28 May 2015 09:20 | Anonymous member (Administrator)
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