While I was drawing the sailplans below, David T produced a posting, suggesting limiting the SA to 64sqm
David has a good point, so I dug out another Johanna 60 master sailplan, this one with AR=2.08. When scaling it to B=6.20m, the area landed on 64.0sqm.

I think these designs with a mast balance of  25%,  are better than my first ones (from 2020):

• ·         The mast is now moved further away from the bow.
• ·         The original hatch can be kept as before.
• ·         The mast can be made lower
• ·         The CE sits closer to the JR mast, and when running before, it will sit well within maximum beam of the boat. Easy steering downwind.
• ·         As positioned here, with a mast balance of 25%, there is room for moving the sail a little forward or aft, to get the steering balance just right.

To make it as offshore capable as possible I would...

• ·         ..tie the topping lifts only 2/3 aft on the boom, and...
• ·         fit a thin, one-meter long extension to the yard’s top, and and...
• ·         ..fit a FUP; a fan-up preventer, and...
• ·         ..use a separate upper and lower sheet, as David suggests
• ·         .. and  -  possibly  -  fit batten downhauls.

YHP, THP and HK-parrels are already standard on my Johanna style sails.

Cheers,
Arne

(full size diagram in Arne's sketches, section 9 )

Looking at https://sailboatdata.com/sailboat/seal-36/ I see that the pointy-top mast is 15.9m above LWL. With an untapered mast section and with the weight of forestay, backstay and cap shrouds added, I’d suggest that its windage and height of CG is always going to be greater than that of any JR mast, whatever the construction or material. My experience with Tystie, not quite as large as the Seal but with shoal draft and limited stability, leads me to think that excessive mast height with any form of JR is not a serious concern. In fact - more is better, as it permits a higher AR with shorter battens and yard to be used, and I came to the view that this was the best way to go, in designing a large rig. 

The basic-triangle sail area of the Seal is quoted as being 58.27 sq m. Add 10% and we get 64 sq m, which I would consider to be quite enough for an offshore cruising boat of this displacement. An SJR of 64 sq m with battens somewhere between 5m and 6m would be practicable and manageable. I’d suggest adding one more batten to the Amiina mk 2 planform, and using upper and lower sheets, ie, three battens for each sheet. Further, I agree with Slieve that spanning the SJR-style downhauls isn't necessary or perhaps practicable in this size of sail.

I do think that more control over the position of the boom than is provided by the usual form of SJR downhaul may well be desirable in a seaway. When scaling up the SJR to the maximum practical size for offshore cruising, there just might be a need to look beyond the simplicity of the small inshore SJR, well proven though that undoubtedly is. 

Yes, the rule of thumb these days seems to be to have the midpoint of the boom and battens of a junk rig matched to the CE of the original bermudan rig. ie to assume that the CE of both rigs will coincide on tha same vertical line. That puts your current proposed mast position about right, to a few inches, for a 33% balance sail, I would say, though you should draw it accurately and get a second opinion.

Here is a little spiel on mast height.

For a normal junk rig with soft batten parrels (that means currently nearly all junk rigs) there are some additional horizontal forces necessary on the sail and battens, to keep the rig from swinging forward. This is why other extra running and standing parrels are often added, to keep the sail draping nicely without severe creases etc. If there is a harmony between the mast-balance and the yard angle, on most junk sails, these forces are kept to a minimum.  As a general rule, we can say that high yard-angle goes best with low mast-balance, and low yard-angle is in harmony with high balance. This is one reason why the Amiina SJR has a relatively low yard angle. The sail is very well designed and harmonised, and horizontal forces on this sail are quite low, and pretty much taken care of by the running parrel downhauls (a feature of that particular design) - no other parrels being necessary (at least on my little boat).

Paul Th has an excellent diagram which demonstrates the relationship between yard angle and mast balance (which assumes a fixed mast height and a fixed halyard sling point close to the centre of the yard).

Arne has analysed the problem in another way, by considering the angle between the halyard and the mast when the sail is fully hoisted. (He refers to it as “halyard angle”). The halyard is normally attached to about the midpoint of the yard. The lower the halyard angle, the more vertically it pulls and the less are the horizontal forces required to counteract the halyard’s forward pull, to make the sail drape nicely – a better behaved rig. A correct relationship between yard angle and mast-balance  amounts to the smallest halyard angle, for a given mast height.

Properly designed, a low balance/high yard-angle sail does not need to have a mast as high as the peak of the sail, as you will see in all of Arne’s drawings,

Another way of reducing halyard angle is just to increase the height of the mast, but that can mean using one fault to overcome another.

However, there is another reason why the mast must have sufficient height. Even though the halyard angle is kept to a minimum, when the sail is hoisted to its highest, the halyard still has some angle away from the mast, which means that it has a horizontal component which does not help, and a vertical component which is less than the pull on the halyard. What this means is, if the halyard angle is too high, the effort involved in hoisting the last metre or so of the sail becomes quite great.

In the case of a SJR sail, that means two constraints. For the Amiina planform, with its low yard angle, the mast height must not be lower than the peak of the yard, in fact a little higher is desirable. The other constraint is that the Amiina sail is designed for and works well with running parrel-downhauls. But these lines are spanned so that each span controls two battens. The lowest batten (be it the boom, or some other batten when the sail is reefed), must be sufficiently clear of the deck if there is to be enough room for the span. If the mast is stepped forward of a cabin top, this presents little trouble. But if the mast is standing on a flush deck or over a deckhouse, then the height of the boom above the turning block for the parrel downhaul (ie the deck) must be at least  just a little more than 50% of the distance between battens. That is a further constraint on the height of the mast, for a given sail area. You need to think about these things when designing a SJR – minimum height of boom above the parrel-downhaul turning blocks, and minimum height of masthead.

You can make some scale drawings and you will find that for a given height of mast, a low balance sail can be made to have a little more sail area than a high balance SJR. Or, conversely, for a given sail area, the SJR will require a slightly taller mast.

Here is another "eyeball" comparison, but you can see where the mast height is, in relation to the original bermudan mast height, and compare in each of these two sails

The SJR is a powerful sail and Slieve will advise (correctly I think) that there is no reason to pursue an excessively large sail area. Also, within reason, a slightly taller mast is not such a disadvantage as may appear (apart from a bit of extra windage when the sail is reefed.) The height of the centre of gravity of the mast, sail and battens is not increased all that much by a bit of extra topmast, and the effect on initial stability is next to zero. When the boat is heeled to a great angle, then the taller mast will start to show some effect on righting moment. Then you will be glad you have a junk rig anyway, because it is so easy to reef.

You just need to do some drawings and do some calculations to see that we are not talking about a serious disadvantage. Unless the boat happens to be very tender under its original sail area, then it is just one of those things we tend to over-think. I hadn’t thought about that when I made the rough sketch, and didn’t really consider sail area, it was just eyeball. When drawn properly to scale, a SJR rig with a comparable sail area usually comes out with a mast about the same height as the original Bermudan mast, and a Johanna type, of moderate aspect ratio, usually comes out with a mast a bit less than the original Bermudan mast. If you are worried about it, you might like to consider a hybrid mast, with the top, say 25 – 30% made from carbon fibre. In practice I have not heard of any SJR which suffers from too high a mast (though I know of one or two which have masts a bit too short).

Obviously, everything within reason. These are just things to think about, and maybe to discuss with people like Slieve or Arne, and there are others, who have a wealth of practical experience.

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...