Eventually managed to finish the drawing, and the 

CE of the original sails and the new setup, using your 

suggestions on sail sizes, seems very close. 

Thank you 

Thanks David.

OK Mark, looking at your drawing (your first iteration) the geometric centre of your proposed rig seems to be about 1/4 m aft of the green line.

(I have done a rough calculation of where your proposed centre lies, using red. Assuming that the sail areas you have given are correct (330 and 520 sq ft) then your proposed mainsail area is about .61 of the total sail area and the proposed fore sail area is about .39 of the total sail area. This puts the resultant centre roughly as shown – looks like about ¼ m too far aft).

I believe that with these mast placements, In order to match the desired vertical green line, the areas of the two sails need to be roughly in the ratio 50:67  (fore sail: main sail). That is to say, if your total sail area is to be (let us say) 850 sq ft - then the fore sail should be about 850x50/117 =  363 sq ft and the main sail should be about 850x67/117 = 487  sq ft. 

Try making another drawing, (your second iteration) on these mast positions, but with mainsail scaled down to 487 sq ft and fore sail scaled up to 363 sq ft, and see if this gets you closer to centering on that green line.

(By the way Mark, your method for finding the geometric centre of a junk sail is incorrect. I think I can see how you have done it. When the masts are vertical (as they are supposed to be here) just strike a vertical line which goes through the midpoint of the boom. That is close enough. You lucked out on the mainsail but got it wrong on the fore sail you can see I have corrected it. Take care also, and note your drawing is not oriented exactly vertical, though close enough.  I would feel more confident doing this on a big screen and with CAD software).

It's a big boat and I am not qualified to advise. However, I can help you with some of the maths.

OK, let's take one step at a time. Here is an attempt (without accurate software) to determine the CoE of the original sail plan.

I am ignoring the inner staysail and just looking at the main and genoa. Calculating the geometric centre of each.  Calculating that their areas are in the ratio 47:73   (main:genoa) and therefore the CoE is placed at a position of 47/120   or   73/120  on the line marked in green.

I don't have any scale at this stage, but I can see that a vertical line through the centre I have calculated, runs through the clew of the inner staysail on this drawing, or just a little aft of the number 2 port light.

If the drawing is correct (and if my methodology is correct) the CoE of the original sail plan is on a vertical line through the clew of the inner staysail.

Can we agree with that, as a first step?

The next step will be to design a junk sail plan whose geometric centre lies on this vertical green line.

The driving power of this masthead rig (above) seems very much to be in the fore-triangle. David has suggested that a ketch rig would suit this hull better than a schooner. I must say, I can't think of a single advantage of the schooner rig, unless mast placement dictates it, but keeping an open mind, the next step is to look at the accommodation plan, placement of bulkheads, hatches and other structure, and decide what constraints there are on where the masts should be placed.

We could begin by considering the mast placements you have chosen already, and see what sort of two-masted sail plan arises from that.

The chosen rig should be centred around that vertical green line. 

Are we in agreement with the first step (above)?

Are we on the right track David? Arne?

(Further investigation of your proposed schooner rig (very, very roughly) I think your geometric centre is about 1/2 m aft of the green line. Try reducing the main and increasing the fore sail just a little).

She's a cutter rig and I don't know if I should include the stay sail in the CE, but the genoa is 495 sq ft.

I am not competent to answer that question, and I don't have any software other than a simple graphics app so I can't check your latest result. I think with a two masted rig, you always have a little wriggle room when it comes to helm balance, but having said that, I am hoping that someone with CAD software and more confidence than me, will vet what you have done and make sure you have the numbers right before becoming fully comitted.

Thank you Graham. 

You're right, I didn't know this .I will do it again and I should have included the sail areas.