The "Sib-Lim" Challenge

  • 25 Jun 2015 00:26
    Reply # 3403142 on 3144241
    Deleted user

    Hey Annie, and everyone.

    I'm working on the twin rudder arrangement. It has required some thinking but I now have a plan. It will be some time before it is drawn up though. In principle there will be twin rudders canting outwards so that seen from behind they will be more or less perpendicular to the edge between the transom and the bottom of the hull. Each rudder will be attached to a skeg that extends form the bottom of the hull and half way up the transom. The rudder stocks will be attached to the skegs and come up through the transom but will be hidden under the aft bench in the cockpit, the same space that will hide the outboard well. There will be one tiller and a tie rod - ball joint linkage between the tiller and the rudder stocks. This picture (but with skegs) will give you the general idea. 

    Although the canting rudders will make for a better bite while heeled I would still like to make them swinging to provide enough area. Each rudder will have to have around 0.4 square meters of area to match the sail area. Making them 0.4 meters without going deep will make them very long, around .65 meters. That will make for a lot of rudder behind the boat that you can bang up against things. It will also make for heavy steering. Look at a lot of the older cat boat designs, like this one that will be quite heavy on the helm and difficult to control when heeling. 

    Now... The version of Sib-Lim with longer non swinging rudders would still behave better than the cat boats of old thanks to the canting twin rudders as compared to the single non canting rudders often seen on those boats, so this is still a viable option if you wish to avoid the complications of swinging rudders.

  • 24 Jun 2015 22:09
    Reply # 3403065 on 3396804
    Deleted user
    Iain Grigor wrote:

    What about Swallow Boats' Bay Cruiser 26 as the basis of a custom junk-rigged

    Sib-Lim boat?

    I asked Swallow Boats about putting a junk rig on their Bay Cruiser 26 at the London Boat Show.  They were very reluctant to consider it, but then the owner was present in the conversation, so maybe they were being tactful. I still think it would look good and be a nice little boat (with a few modifications to the racing back end and internally). 
  • 20 Jun 2015 15:53
    Reply # 3396804 on 3144241

    What about Swallow Boats' Bay Cruiser 26 as the basis of a custom junk-rigged

    Sib-Lim boat?

  • 19 Jun 2015 00:51
    Reply # 3392440 on 3384067
    David Webb wrote:

    Hi Annie,

     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. 

    David.

    David,

    Strength is not the issue. Tystie's boards have a t/c ratio 0f 8%, and this is really a little too thin, as they stall after a tack and need some speed to get them lifting again. Normally, keels have a t/c ratio of about 12%, so that they don't stall too readily, and that's what I've used on my Sib-Lim's boards, basing them loosely on the well-known Clark Y section.  

  • 19 Jun 2015 00:46
    Reply # 3392439 on 3384275
    Richard Brooksby wrote:

    One question: why do you want her to be so heavy?

    My current thinking is that my next boat will have a single curved bottom made of copper nickel, chine runners, and no keels!

    Here's a quick sketch made on a scrap of paper on a train photographed with my iPhone. Remember you saw it here first!


    It's a question of load carrying capacity, for a boat that is to be a home. My rule of thumb is normally 4 tonnes per person, for reasonable comfort and surrounding oneself with all one's favourite possessions. But Annie, being a minimalist, can get away with 3 tonnes. You'd need to be an ultra-minimalist to be happy with less, on a full time basis.
  • 16 Jun 2015 00:06
    Reply # 3388417 on 3388205
    Iain Grigor wrote:

    What about the David Thomas Red Fox Vision as a SibLim boat?


    This design does meet some of the desired Sib Lim criteria, though it might need to be customised to suit a one-off plywood build.  I would also like to see enough fixed ballast, along with a higher raised deck for reserve bouyancy when knocked down.  Annie does not require a boat that can be easily trailed, which is one of the Fed Fox criteria.  It is said to be capable of offshore sailing but not perhaps with me aboard!  It could form the basis of a custom design though, if one wanted to pay the designer to develop it. 
  • 15 Jun 2015 23:51
    Reply # 3388415 on 3385318
    Robert Biegler wrote:
    Graham Cox wrote:

    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. 

    Chris White, in his book "The Cruising Multihull" describes a daggerboard with a case slanted about 45 degrees, such that the board slides down and forward or up and back.  It has low aspect ratio, with the slot being about as long as the board protrudes from the hull.  Then hitting something pushes the board up as well as back.  The cases will take up more space than a hinged board with the same area in the water, but you avoid having a moving part in an inaccessible place.

    I remember Chris White's slanting daggerboard (Chris White's Juniper Tri, which I have been aboard, is my favourite multihull).  I think it is a great idea, especially for fast-moving multihulls.  It might take up too much space in Sib Lim and also the boat will be moving a lot more slowly, but if it could be incorporated I think it would be a good thing.  The smaller the boat the bigger the compromises!
  • 15 Jun 2015 20:08
    Reply # 3388205 on 3144241

    What about the David Thomas Red Fox Vision as a SibLim boat?

  • 13 Jun 2015 17:40
    Reply # 3385318 on 3382637
    Graham Cox wrote:

    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. 

    Chris White, in his book "The Cruising Multihull" describes a daggerboard with a case slanted about 45 degrees, such that the board slides down and forward or up and back.  It has low aspect ratio, with the slot being about as long as the board protrudes from the hull.  Then hitting something pushes the board up as well as back.  The cases will take up more space than a hinged board with the same area in the water, but you avoid having a moving part in an inaccessible place.
  • 12 Jun 2015 13:12
    Reply # 3384275 on 3144241

    Annie, your requirements are very similar to mine so this thread is of great interest, not that I'm planning to move on from Tammy Norie so early in her development.

    One question: why do you want her to be so heavy?

    My current thinking is that my next boat will have a single curved bottom made of copper nickel, chine runners, and no keels!

    Here's a quick sketch made on a scrap of paper on a train photographed with my iPhone. Remember you saw it here first!


    Last modified: 12 Jun 2015 13:31 | Anonymous member
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