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.

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

Sib-Lim boat?

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!

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.

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.