Split Junk Rig sail plan

With the mast nearing completion it is time to finalise the sail plan. I’ve been using a scaled-down copy of the Poppy plan for the preliminary design. I’ve now followed through to make sure everything works ok.

As recommended I built a string and stick model of the sail. This is the first attempt – a copy of the Poppy rig. With old garden bamboo it looks pleasingly authentic!

Stick-and-string model – first iteration

I wanted to explore alternatives to the downhauls used on Poppy. I have a couple of issues with them:

  • Too many strings to feed past my (already constructed) turning block;
  • Too many strings to adjust when raising sail or reefing.

I’ve been very happy with the Throat Hauling Parrel on Owl so I gave that a try, as you can see on the photo above. The main issue I hit was the instability of the top section of the sail – while it was possible to get everything aligned a slight change would cause the sail to move around too much.

By playing with the model I eventually concluded that the fan-top to the sail in the Johanna/PJR rig helps to triangulate the top of the sail. While the front of the yard and top two battens can move fore-and-aft, they cannot move far as the front ends are so close together.

As well as making the sail hang correctly it is necessary to get the balance of the boat correct. I did this with QCad (I learned to use Autocad in 1988 and QCad works in a similar way). I played with different yard angles and sail positions using the Johanna sail plan (all battens equal length, 10º boom angle, top batten forms fan). I concluded that a 45º yard angle suited the boat so tried this out on the model.

Model of Johanna sail plan with 40º yard angle

In the picture the yard angle is produced by the Throad Hauling Parrel (THP) – the Yard Hauling Parrel (YHP) isn’t actually doing anything. Note that the THP is lead to an eye close to the mast on the top batten – this is so that the line stays close to the mast and doesn’t interfere with the camber in the Jiblets.

Close-up of the THP

Note that the battens don’t align in this picture. However, since the distance between the front of the battens is small they cannot get far out of alignment.

QCad drawing of the sail plan

Next stage is to check all dimensions, sketch the details of how the sail will fit together, and order the bits.

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