Not wishing to chime in over the top of Slieve, Arne and David, whose advice and comments I would always respect - but there is a sketch provided by Thomas, which is now "on the record", which I think needs to be questioned.
Referring to the proposed conversion of Bristol Channel pilot cutter, which was posted on 4 December.
The advice Thomas has received is 100% correct I believe, but no-one commented on the above proposed drawing, which I think, for the record, needs to be questioned.
There is a reason for that long bowsprit.
I think Thomas has placed the proposed junk sail too far aft.
Slieve has expressed it perfectly: if you have an existing sail plan it might be better to forget about lead calculations (which is an art rather than a science anyway) and just follow the CoE on the sail plan which was given for the vessel. The only adjustment necessary should be if the vessel is known to have bad helm characteristics which suggest moving the CoE forward or aft a little from the given position. If the rig, as designed, is known to give good helm characteristics then it would make sense (if using a cambered junk rig) to match approximately the CoE from the original rig.
In that case, of the three choices of CoE which appear on the above diagram, the forward-most one follows most closely the original rig, and if a junk sail is superimposed on that CoE, it will be further forward than on the above proposed rig.
[CoE = CoA = geometric centre of sail plan area. CoE is actually a misnomer, but it is a commonly used expression]
The blue line represents the approximate CoE of the original rig. The Johanna 60 degree yard-angle sail with its high (25%) mast balance seems to be a reasonable and convenient fit. It's about 500 sq ft. Scaling it up or down might move the CoE by a cm or two, but that is roughly how it should look, I would have thought.
I just want to question the record, for future readers, and I am sure that if I have made a mistake someone will point it out.
Thomas might like to consider a similar large-foretriangle cutter rig conversion, done by Pol B some years ago (see Feb 2021 Boat of the Month)
Pol wrote: “With Arne's encouragement we made the 550 sq ft (52 sq m) cambered sail. The gaff rig with tiny topsail was about 600 square feet, but the “smaller” JR seems to give her more power. 5.9m battens and yard are in aluminium tube with Douglas fir plugs in the ends to make them look and feel nice.
My trial sail was a perfect summers day in Crinan Loch. Having eased the sheet, I hoisted sail and she was off, close reaching on her own like an arrow. I just could not believe it! The mast was in the right place after all, and the rig appeared to be perfectly balanced. Heading back in later on, I gradually reduced sail. She only started slowing down when 3 panels remained. Power with ease.
I had worried about the JR's ability to heave to. Experimenting, with 2 panels set in a F5, luffing and falling off, she made about 1 ½ knots. OK, she’s not stopped as she would be with the gaff rig, but - in open water - if she did tack herself she would do no harm, unless there was a big production underway in the galley at the time!
This boat is transformed!! Space on deck, no clutter. The great spread of her single lug sail is a delight, even with the odd home-made wrinkle. I'm a life-long gaffer, but the nervousness about handling her (quite modest) rig short-handed in bad weather has vanished with the JR. Changing up a gear or down three gears was a chore, or worse. A jib that refused to roll up, the clatter of its sheet blocks on deck. All hardships of the past! We love sailing more than ever. Give us a F6 right aft, with tide and wind in opposition. Hateful conditions with more or less any other rig. But now, with no booming out headsails, no preventer, she guzzles the miles and we just grin at the ease of it all.