Peter, now I had a go.
First of all; I hope that I got the dimensions and mast position right. Better check it.

This sail was taken from my stack of Johanna 60 master sails.
I chose the one with AR = 2.13. By removing the lowest panel and then scaling the master sail down from B = 5077mm to 2250mm, I ended up with this sail with SA = 7,49sqm and Aspect Ratio, AR=1.87.
As with all my Johanna-style sails, the boom, battens and yard are of the same length (B) and all batten panels have about the same sail area.

The drawn mast position makes it difficult to keep the CE where the sprit rig had it, even with the sail rigged with 25% mast balance, as here. I would not worry about that. The sail can be shifted a good deal forward at the boom, or one could build a bigger and better-shaped rudder, like the Chinese would have done.

As for the construction of the sail, with cambered panels and all, I suggest you look up this page on this website, and read Chapter 5 of TCPJR, found up to the left.
Quite a few have been able to make their own sails, using this method.
The Junk Rig Association - Arne Kverneland

Arne

(..full size diagram under Arne's sketches, section 9-16...)

Hello Arne & the JRA,

Many thanks for the sensible questions. It’s taken a little while to get back to you – but I’ve pulled together the plans to give you some substance.

For sail area, the original dinghy was drawn with a sprit sail at 50sq.ft, 4.6m^2 (see image). I’ve found that’s far too small for light winds, and my appeal to the junk is a big rig (far too big) which can be reefed in an instant for a good breeze. Hence, I’m starting with a 4.6m mast, and thinking to test the biggest sail possible, and reduce from there if this is impractical. Say 7.5m^2 perhaps, to give a top-end SA/D ratio. And to make it easily reefed therefore, six panels probably.

The mast position is my key question currently. I want to keep the mast relatively far aft, as the dinghy has a small waterplane area further forward so stability becomes limited if you’re stepping the mast right in the bow (she is a yacht’s tender, so always rigged afloat). In the plan attached, I’ve indicated the approx. centre of effort of the designed sprit rig, and the daggerboard position, along with a starting-point for a mast step and rake. To develop this, I’d be really grateful to some pointers towards appropriate sail plans and CoEs. Keeping the mast fairly far aft, I’m anticipating that the junk battens extend a reasonable distance forward of the mast to maintain the balance. And that the rig drops the CoE vertically as it is reefed, so as to maintain a reasonable weather helm over a range of conditions.

Filling in a little on my overall plans here… I’m building this rig now to retrofit to the existing dinghy and planning to test this over the summer. But, I am also building a replacement dinghy to the same design, with a number of improvements. So, the mast position is variable, and I’m looking to decide exactly where to position the mast step in the new dinghy. My hope is that this junk rig (and the current dinghy) can be chopped about to create a reasonable prototype, and then finalised over the winter ahead.

In the images you’ll see the original sail plan, a photo of the existing dinghy with the mast step in progress at a convenient starting point, and views of my current 2D drawing and 3D model for the replacement.

Looking forward to hearing of any ideas for a rig that might work for this.

Best regards, Peter

Hello JRA, Please could anyone point me towards useful plans to use to construct a junk rig for a small sailing dinghy? The dinghy is 10'4" LOA, a Chameleon design from Danny Greene, which I built in 1996. I am replacing the existing rig and believe that a simple junk rig would be a very effective solution. The mast will be a free-standing reinforced windsurfer spar, which I will keep tall (4.3m potentially) and reef early. I am hoping that a set of plans might exist for a rig which I could scale to suit my needs. Initially I need a sail plan to define the key dimensions and confirm a centre of effort, and then sufficient details to cut and sew the sail, to make the battens and to rig it. I am new to junk rigs, but expecting to learn fast, and hoping to knock up a first-pass to test this summer. So, I'm looking for a rig that is fairly simple and quick to build, and has enough information available to make a good job of it. Many thanks and regards, Peter