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!

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."

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.

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

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

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.