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The "Sib-Lim" Challenge

  • 30 Jul 2015 02:02
    Quote
    Reply # 3457069 on 3446280
    Anonymous member
    Arne Kverneland wrote:

    Ah, Annie,
    I see now that the displacement spec has always been at 3 tons. I haven't followed this thread for a while, so had forgotten. However, I am still worried that it will be difficult to make a 26' boat boat with 3 tons displacement and shoal draught sail well to windward. A coastal cruiser must sail well to windward unless one is willing to use the motor a good deal.

    Arne 

    Dear Arne

    Your insistence on my lack of sanity here is wearing me down.  I don't believe that I'm asking for the impossible.  Surely, the point is that when the boards are down, the boat isn't shoal draught?  And my design criteria are not producing a lightweight boat.  There is a trailer sailer, called the Noelex 25 that is very popular in New Zealand and they are not only sailed in open water, but regularly raced: and we get a fair bit of wind in this country.  If a trailer sailer, equally limited by design constraints, can be persuaded to go to windward, why can't my Sib-Lim concept?
  • 29 Jul 2015 22:05
    Quote
    Reply # 3456844 on 3455572
    Anonymous member
    Peter Manning wrote:

    Why not make the boards equally open at the top and bottom and with the vertical spars DT has referred to. The top and bottom of the board could be strengthened by adding layers of plywood or hardwood tapered inwards so as to avoid retaining sand and stones whilst leaving enough space for water to enter and drain easily.

    Peter, are you managing to lift your boards OK with a three to one tackle? They are heavier than the hollow boards I propose for Sib-Lim.
  • 29 Jul 2015 22:03
    Quote
    Reply # 3456843 on 3456277
    Anonymous member
    Arne Kverneland wrote:

    Another armchair idea; handsfree boards

    Since the suggested boards are fitted in trunks, their to-in angle (if used) cannot be adjusted (unlike on true leeboards). Why not rather align the trunks and boards parallel with the centreline (or with only 1 – 2° toe-in) and make the boards with symmetric and generous sections (for added stall resistance). Then one could sail with both boards down when close-hauled and thus save a lot of work: Tending the boards at each tacking doesn’t sounds to be more fun than tending genoa sheets.

    Arne

    PS: The drain holes could well be drilled in the side of the boards  -  they need not be in the bottom plate.

     

    The difference is - you don't have to work the boards at each tack if you don't want to! For short tacking, I'm usually lazy, and don't bother.

    symmetrical, non toed in boards would have no performance advantage over bilge keels, just the advantage of less draught, so I wouldn't want to go that way.
  • 29 Jul 2015 12:33
    Quote
    Reply # 3456277 on 3144241
    Anonymous member (Administrator)

    Another armchair idea; handsfree boards

    Since the suggested boards are fitted in trunks, their to-in angle (if used) cannot be adjusted (unlike on true leeboards). Why not rather align the trunks and boards parallel with the centreline (or with only 1 – 2° toe-in) and make the boards with symmetric and generous sections (for added stall resistance). Then one could sail with both boards down when close-hauled and thus save a lot of work: Tending the boards at each tacking doesn’t sounds to be more fun than tending genoa sheets.

    Arne

    PS: The drain holes could well be drilled in the side of the boards  -  they need not be in the bottom plate.

     
  • 28 Jul 2015 22:56
    Quote
    Reply # 3455587 on 3144241
    Anonymous member

    Arnie, David,

     at four inches thick and without holes in the bottom, the bilge boards, when fully immersed will displace about 200 pounds of water, my original two inch thick solid hardwood boards weighed in at about 130 pounds, still very heavy for Annie to lift. They will need to weigh well over 200 pounds if they are to deploy under their own weight. They will also need this much lift to fully raise them and I think that this will be too heavy for Annie to manage, correct me if I am wrong Annie. This is with no allowance for friction of the board in the case. If there are holes in the bottom then the structure of the board will weigh about 60 to 70 pounds and this is all that Annie will have to lift plus friction, but this is still marginal for her strength I would imagine. With a two to one tackle and a small winch I would see it as  quite possible. This would need a sheave embedded in the top of the board and a sheave with becket at deck level,  with the tail led back to the cockpit and a small winch on each side. If the boards are open at top and bottom then ant stones or mud could easily be dislodged with a stick poked down from deck level.  What are your thoughts Arnie, David, Annie.

    Last modified: 28 Jul 2015 23:03 | Anonymous member
  • 28 Jul 2015 22:26
    Quote
    Reply # 3455572 on 3144241
    Deleted user

    Why not make the boards equally open at the top and bottom and with the vertical spars DT has referred to. The top and bottom of the board could be strengthened by adding layers of plywood or hardwood tapered inwards so as to avoid retaining sand and stones whilst leaving enough space for water to enter and drain easily.
  • 28 Jul 2015 21:51
    Quote
    Reply # 3455548 on 3448713
    Anonymous member
    Arne Kverneland wrote:

    What about selfdraining boards?

    Would it be doable to make the hollow boards with quite big drain holes at the bottom end, and with ventilating holes at the top? The holes must be big enough to let the boards drain or fill without delay. This would let one produce as thick foils as one needs for high stall resistance and still avoid having to struggle with weight or buoyancy. Careful sealing of the inside surfaces would of course be needed to avoid water ingress and  rot...

    Just a thought.
    Arne

     

    I thought of this. The problem would appear to be that in order to lift a board fast enough, during a tack, there would have to be very large holes in the bottom. Then, bearing in mind that the boards are also to act as legs, those holes will allow the board to get filled up with sand, mud, stones, etc...
  • 25 Jul 2015 09:50
    Quote
    Reply # 3448718 on 3144241
    Anonymous member

    Hi Arnie,thanks for that, good idea and quite practical and doable.

    I can probably increase the board thickness to 4 inches. This would increase the ratio to a bit over 11% and should answer your comment David. There would be very little effect on the interior layout as the boards are behind the seat back on the port side and behind the galley on the starboard side.

    I will need to wait till late October to amend the drawings as we are in Australia until then, and will not get back till then.
  • 25 Jul 2015 09:13
    Quote
    Reply # 3448713 on 3144241
    Anonymous member (Administrator)

    What about selfdraining boards?

    Would it be doable to make the hollow boards with quite big drain holes at the bottom end, and with ventilating holes at the top? The holes must be big enough to let the boards drain or fill without delay. This would let one produce as thick foils as one needs for high stall resistance and still avoid having to struggle with weight or buoyancy. Careful sealing of the inside surfaces would of course be needed to avoid water ingress and  rot...

    Just a thought.
    Arne

     

    Last modified: 25 Jul 2015 23:51 | Anonymous member (Administrator)
  • 24 Jul 2015 23:32
    Quote
    Reply # 3448535 on 3444093
    Anonymous member
    David Webb wrote:

    Arnie you are correct, it would free up the accommodation no end and cost usually goes by the weight rather than the length so if the displacement is fixed then the cost varies very little if the length is changed, as long as you stay within moderate parameters.

    David Tyler. With regard to your comment about the bilge board thickness on my Puffin design. Does the thickness you suggest of 12% apply to fully asymmetrical boards or is it for symmetrical foils? I am not an expert on this subject but my experience has been that an asymmetrical foil can be quite a lot thinner without stalling, is this correct? I could increase the thickness to three inches, about 8% chord thickness, with little change to the accommodation but the weight of the board would increase and may be beyond Annie's capacity to manage if constructed of hardwood as I suggest. It could be built of lighter material but then the strength  and ability to absorb knocks could suffer. Also the displaced volume would increase and require a greater weight to make the board drop. As with all things in boat design it is a matter of compromise.

     


    David,
    My experience has been that with fully asymmetric boards, 8% is not quite enough, and the boards stall when recovering speed after a tack. Once I have 3.5 knots, they are OK, but below that, some extra thickness is desirable. I would propose that the boards are made from plywood skins with hardwood vertical spars to provide the strength and define the shape; the spaces between the spars to be filled with PU foam. I agree that solid wood boards would be too heavy for Annie to manage. But whatever the material, one gets into the way of making the floatation, or weight, work with one instead of against one. That is, for example, raising the board when it is still in the water, during a tack, instead of waiting until it is on the new weather side and out of the water.
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