Graeme,

As you can see, the toilet pan is already offset a little bit so offsetting the mast to starboard would be the obvious choice. Is there any reason not to carry the sail on the port side?

There's a nice, inviting space between the existing mast and the hatch, but whether the width of the doorway below would cause problems I'm not sure. I need to visit again to have a look.

Graham, I concur with all that. Luckily I have a woodworking workshop set up, (I had a cabinet making business for 15 years), so making new knees and partners holds no fear. Whether I can find a decent hardwood supplier locally is doubtful. We moved to Cornwall 6 months ago and I haven't found my way around yet.

The day after I bought the boat I had those terrible doubts one gets but now I'm really getting excited about my new retirement hobby! There's even a big working boatyard in Polruan so I may be able to source the mast from them, they have a pretty impressive pile of metal!

I was thinking i could use maybe a third of the hatch for the mast and make a new hatch cover for the remaining 2 thirds. We'll see.

Graham C wrote: It would be easy enough to make new partners between existing deck beams in that area, and thus minimise structural alterations. 

I agree with Graham about all of that, and would point out, in addition, if the mast seems to block the companionway there, you can always offset the mast a little. It seems a little strange at first, the idea of an offset mast, but it doesn't seem to be all that noticeable especially of you offset (say, to port, where that little locker is) and carry the sail on the starboard side of the mast, which seems to be the usual side (for not very important reasons). That way you would still have easy access to that rather spacious "heads" area up forward (which you also lose if you use the hatch as a mast position). However, with a carvel planked hull you would need to pay a little bit of attention to providing an adequate mast step, in the case of an offset mast.

Your choice of rig will decide where the mast has to go - or, your choice of mast position will decide what sort of rig (balance) you will have. As Graham says, none of this will make much difference to the potential for reconversion at some later date, if you so decide. You can leave all the fittings in place even if you don't use them. (Your chain plates remain a handy attachment point for "legs" if you ever decide to rig them.)

I would still lay modest odds on the feasibility of using the current mast position, in your case, pending a drawing of that original bermudan sloop sail plan. But I would defer to Arne's judgement on that question. 

Good luck.

Andrew wrote: It is very encouraging, especially if I can use the original mast position. At the moment is has a sort of rubber boot to seal it to the deck.

Being able to place a junk rig at the position of the original mast would be an unusual situation, but theoretically possible as a special case. With your rig (which seems to have a large main and relatively very small fore triangle)  - and the SJR (which has extreme high balance) I think it might be possible. I was guessing the proportions of the rig so you would need to get measurements and draw up the rig as it was originally designed, before coming to any firm conclusions. It is also a good idea of yours to sail the boat for a season because this way you can confirm if the designer had things right in the first place, before just following the designed centre of area of the sail plan. (That remark of Graham C suggests perhaps  the centre of area may be a little too far aft, weather helm etc, but you would be well-informed if you were to sail a season with the original rig, and determine that for yourself).

Arne followed a different procedure and found the Centre of Area of the underwater hull (called CLR, see below) and then used his experience with junk rigs over many years to estimate where the sail should be placed relative to the CLR. Arne reached a different tentative conclusion from my tentative conclusion, placing the centre of area of the sail somewhat further forward (which, also, is consistent with the remark of Graham C). It will be interesting to see how it works out. The mast position chosen by Arne will not suit the Amiina rig but he feels it will suit one of his Johanna rigs - and also the Johanna rigs give a bit more flexibility in this respect than the Amiina rig. If you want the mast to go through the hatch (which would not be my choice, but that doesn't matter) then fairly certainly that will be too far forward for an extreme high balance rig such as the Amiina, and you will be looking at one of the beautiful high-peaked Johanna rigs, I would expect.

Finally, I should say that Arne has designed many successful rigs over many years, being one of the pioneers of the modern cambered junk rig, whereas I have only theoretical knowledge when it comes to junk rig, plus the one rig I have built and tested for myself, so I do not belong in the category of "expert". However, I still recommend that you make an accurate drawing first, of the original rig as designed, and if you have a season of sailing with the original rig as well, you will then be in an excellent position to discuss things with Arne and make the right choice.

The centre of gravity, by the way, does not really have much to do with mast placement. From that point of view it is good to have the mast a little bit aft rather than too far forward as it will have some effect on the pitching motion of the boat in a seaway, but in practice that does not seem to be much of an issue for most people. The more important issue is to get the sail itself in the right position so that you have good helm balance in as wide a range of conditions as possible. 

(CLR means "Centre of Lateral Resistance" which is a little bit of a misnomer, because it is calculated as the geometric centre of the profile outline of the under water part of the hull. Nothing really to do with the centre of gravity of the boat itself, but if you were to make a cardboard cutout of the underwater profile and balance it on a knife-edge, it is then possible to estimate where that geometric centre (the CLR) is located. There is some discussion as to whether the rudder should or should not be included in that profile of the under-water part of the hull.)

("Balance" in this discussion means the percentage of sail area which is forward of the mast centre line. The Amiina rig has 33% - 35%  of the sail area ahead of the mast, which is about as much as you can get on a lug sail. More than that causes problems. It is the split in the sail which allows this extra large balance. Ordinary unsplit rigs vary in balance from about 10% to 20% or even 25% - recently the balance has been pushed higher and it seems OK. So, getting the mast in the right place means getting the sail, mast and balance, all three, in harmony.

This balance is not to be confused with "helm balance" which is just a description of how the boat sails, with weather helm (a little bit is good) or neutral helm - or lee helm which is not so good. If the sail is in the wrong place, the helm balance might be all wrong. Helm balance is not entirely a fixed property - many boats develop weather helm as the wind gets stronger and the boat starts to heel. And junk rig sloops are a bit inclined to develop weather helm when reaching and running, though your long keel, cut-away forefoot and good large rudder are all positives here.

"Publications to help learn the basics": my suggestion would be Arne's files which can be found on this website, go to Members' Area/Documents/Technical Articles/Arnes Files and read chapters 1 - 4 an excellent introduction.

Graham, thanks for the pictures of Tarmin. Unfortunately when I click on them to see the larger version I just get an error message. They give me great confidence that Zircon will be a very sturdy and safe boat. I bought her on a whim, knowing very little about wooden boats but it's rather nice that she's only slightly older than me!

I'm going to try a season with the existing rig, especially as I have to try and get 2 non-sailing dogs used to the idea! Cornwall will be very different to the West of Scotland, where I used to sail.

Thanks for your ideas on the positioning of the mast. Basically I need to either position it where it is at present or move into the forward hatch, I don't want to do any chopping on such a classic boat.