Personally, I have a strong preference for high balance, and I like my 33% balance SJR very much, but there are some theoretical matters which need to be considered. 

Here is a theoretical answer, referring (for simplicity) to your single mast proposed rig.

(I suspect you might know all this already, but for the record, here goes):

The question of split junk rig, or non-split rig. for a single mast rig, depends on where you intend to place the mast. SJR usually only makes sense if your sail is intended to have high balance, ideally 33%.  Anything much less than this and you are probably better off with an unsplit sail. 

If the boat sailed well (satisfactory helm balance) with the original rig, then a good starting point, if possible, would be a scale drawing showing the original sail plan, or at least the position of the geometric centre (CE) of the original sail plan. You can then draw your proposed junk sail with its geometric centre (CE) positioned over the same vertical line. (Usually the mid point of the boom establishes this vertical line through the junk sail CE, with sufficient accuracy.) 

After that, your desired mast balance for the sail will determine the position of the mast.

Or, conversely, your desired mast position will determine the amount of balance you are going to end up with.

The SJR, with its higher balance, will mean a mast placed a little further aft than it would be for a lower balance sail such as the Hasler McLeod sail you have drawn.

[Is the "present CE" you show on your drawing, the CE of the original rig? If so, then the vertical line through the midpoint of the boom you show on your junk sail drawing should align with this point. In other words, I think your your proposed sail plan may be placed too far aft.]

[If the mast position you show on your drawing is where it has to be, because of internal accommodation requirements, then it follows that your sail will need to have high balance, in which case SJR may be your best choice. If you were prepared to have the mast further forward, then your sail would need a lower balance, and a contiguous (unsplit sail) will make more sense.]

Assuming the CE you have drawn is the CE of the original rig, this is what I think a SJR would look like. (This is Slieve's well-proven Amiina Mk2 sail). 

This is an approximate drawing only, I don't claim to be a rig designer, but it looks to me as though a 33% balance SJR would suit your chosen mast position very well. Perhaps you should check with Slieve. (Note, you will need a slightly taller mast for a given sail area, with this type of sail).

My opinion is, if you decide to go for an unsplit sail, such as one of Arne's Johanna sails, or a HM sail, then you are going to have somewhat lower balance, and will need to move the mast further forward than shown on your proposed drawing. Perhaps you should check that with Arne. (Also, the lower balance/higher yard-angle sail would allow a slightly shorter mast for a given sail area, than for a high balance/low yard angle SJR sail).

My final thought is, if you decide on schooner rig such as you have drawn, then the same principle of matching CEs applies - and also, in that case, I see no point in SJR and in any case I would urge you to consult with Arne or others who have experience designing junk rigs.

Eero.

now I had a look at that calculation tool, made by Oscar Fröberg.

When choosing mast material as Norwegian Spruce, its density is given as 0.43.
Then I look at your .xls file where you say the density of spruce is 0.63 (630 kg/m³). That makes a whole lot of difference:
If you multiply your found weight of 298kg with 0.43/0.63, the mast will come out with

W_m = 298kg x 0.43 : 0.63 = 203kg.

The rule of fancy algorithms is: “Put garbage in and you will get garbage out”.

In my world we reckon the density of dry spruce to be around 0.52, give or take  2%.
If the other algorithms are ok and the density is 0.52, then the weight should come out at

W_m = 298 x 0.52 : 0.63 =246kg.

That would only add about 2.5% to the boat’s displacement, so should be good.

Arne

PS: I also note that the Sigma of spruce is set to 72MPa. In my calculations I have used the much more modest 45MPa. Maybe I am too pessimistic...

according to the junk rig calculation tool of oscar froberg a steel mast for 16300kpm could be a 300/6mm tube – a bit heavy…

ueli

Greetings Arne,

In your various suggestions for a large sail sloop rig for Falkor you state “The biggest challenge is to build a tall and strong enough mast at a moderate weight and cost.”

What would be the specs for a suitable mast in, for example, your most recent iteration of a 65.6 square metre sail?

Arne, David, Graeme,

Thanks for your comments and input. Very much appreciated!

Arne, in the scale (0-100) I would say I am 45% sailor. Pure cruising. Focus is on safe, comfortable, and enjoyable sailing. The crew is 60 year-old couple, whose idea is to leave the regular work within say 2-3 years and the boat will be our liveaboard for maybe 6 months per year. Nowadays we sail the ‘normal’ summer holiday sailing i.e. daily trips along coasts of Gulf of Finland and Northern Baltic Sea. But once we get rid of the tight schedules, the trips will get longer but leisured. We can keep the boat abroad and no need to dock the boat for every winter. Perhaps not crossing oceans but exploring canals, rivers etc might be nice. For that purpose, a shorter mast(s) would be nice as they could be carried on deck if the overhang is not too much. For that reason, we have also considered if we should switch the boat to a smaller one with a smaller draught. But on the other hand, this is a nice and cozy liveaboard boat giving very safe and comfort ride at open sea.

The present sail area is main 25.7, furling genoa 37.3 making 63m2 in total (hopefully I managed to measure them correctly). We have a few more sails, but in practice we keep those home. I would guess that a JR of the same size would be much more efficient downwind, as without a spinnaker/gennaker we loose a lot of effective area of the genoa. The present balance is quite good I would say based on the rudder angle. Difficult to say because the hydraulic steering gives almost none sense of the pressure. I am used to tiller, which I keep as my favorite way for steering.

Eero