Using dry ice or liquid nitrogen is a recognised way of shrink-fitting bearings and bushes into housings, but I can't find any references to using the method for such a long length of tube.

In order to fit my proposed new sail I need to extend my mast by around 4 feet (120cm). To this purpose I have bought a 2.5m length of 6082T6 Aluminium tube 6" in diameter with 1/4" wall thickness. The inside diameter of this tube is 139.7mm. The straight-sided bottom section  of my mast is 5.5" (139.7mm). I realised that getting the one inserted into the other might be a non-trivial task.

The bottom of the outer extension tube needed a 3" insert cut from the old mast in order for it to fit closely over the mast step. Fitting this 3" section into the bottom of the extension tube would be a good test of the methodology I planned to use to fit the mast into the other end. Here is the set-up:

A length of 6mm short-link anchor chain runs through the tube. At one end, it is fed through the 3" offcut and through a timber packing piece and secured with a stout metal rod through a link (left hand  image). At the other, it is attached to a length of 12mm threaded rod via a lifting eye nut and shackle. The threaded rod goes through another wood packing piece, and a nut and washer system allow the rod to be gradually drawn out of the tube, placing increasing load on the chain and pulling the offcut into the other end (right-hand image).

I calculate the theoretical mechanical advantage of this system as several hundred to one, producing a maximum pull on the chain of several tons. In fact, much of the mechanical advantage would be lost through friction, but the maximal load on the chain should still be formidable.

The amount of pressure required to pull this short 3" section into the extension tube was scary, and the difficulty of the process made it clear to me that using the same method to pull the mast in by 48" would not be successful.

The only alternative I can think of is to cut a 48" long slit in the extension tube, pull the mast in using the above method, then clamping and welding the slot shut. This obviously has implications for the final strength of the extension tube, which will protrude 60cm above the partners. The following information, from the Total Materia database of Swiss company Metals AG, may be relevant:

"When welding weakens the weldment, the weak zone is about an inch either side of the center of the butt or fillet welds. Longitudinal welds in structural members are harmless if their weak zones make up less than 15 percent of the part. Girth welds in piping or tubing may reduce resistance to buckling, but longitudinal welds reduce it hardly at all, because their weak zones are small compared to the total area of the weldment."

The weak zone on the extension tube will be 50/478.8mm = 10.5% of circumference. Observations, and any alternative suggestions for soultions to this problem would be welcome.

Chris

Well, if China Girl turns out to be 'overpowered' with her new rig I will be delighted. There is no doubt that she has been under-canvassed with her current (and probably all previous) rigs, including the original 315 sq ft bermudan. I rarely reef before F6 - sailing her hard-pressed at the top of a F5 with full canvas is the closest she comes to 'exhiliarating'.

To tell the truth, I don't think Ken Evans, her designer, envisaged exhiliarating.

Chris

Chris wrote, “If so, I have 450sq ft of lightweight cruising chute which I set flying from the extensible bowsprit and mast head - it takes pretty good care of light-air performance on most points aft of a close reach.”

I will be very surprised if you will need the chute with the split rig. I never felt any desire to increase area. If the wind was too light to drive the rig it would not have lifted and inflated a chute. I wouldn't waste much thought on flying extra sails.

Annie wrote, “One of the unusual aspects of junk rig, that I've discovered over the years is that you tend to shake the reefs out to sail close-hauled and reef when going downwind: the opposite to pointy and gaff rig. Because of this, I would always make sure to have maximum sail area for windward work, myself.”

That was the problem with inefficient low L/D junk rigs that did not generate lift. With the split rig in a blow it is best to reef up wind and piled it on downwind to go faster than fast, and under full control. 9 knots with a displacement hull of 25 foot waterline is FUN. The split junk is a totally different animal to the old flat junk rigs. Remember we are talking about matching Bermudan performance to windward and approaching spinnaker performance downwind, and all with just one sheet.

Thanks for all your comments, ladies and gents, they are much appreciated. Don't worry too much about shattering my illusions - they're pretty robust.

In the three years (on and off) that I've been planning this rig, I haven't sought much advice from those more experienced and knowledgeable than I. This is mainly for two reasons:

1. I've read enough of what they've written (and generously shared - I couldn't have got this far without it) to be pretty sure of what the advice would be. Slieve's observations about the need for a greater sheeting angle I was ignorant of, and have gratefully implemented.
   
2. As far as learning experiences go, there's nothing like making your own mistakes.

I am a great believer in keeping things simple when it comes to the way a rig is to be used.  Adding a little complexity to the process of building the thing just adds a little to the interest and fun. I'll loft all  the panels direct from the computer model output to cloth, and cut them out, in around a day (I know this because I've already done it on some cheap polytarp). Given how long I expect to use the sail, this is negligible.

Apart from the top batten, which I intend to extend aft, the leech is less severely tapered than that of my current Sunbird sail. I have very few problems with sheets catching batten ends with that. This may be partly due to the fact that I still have a cut-down mizzen mast aft of the steering post from when she was a yawl (Three Sisters), and the bitter end of the upper sheet is anchored some way up that. This pulls the upper sheet and sheetlets away from the leech a little.

Arne's point about having more sail high up in light airs may be important for me - I shall find out and report back. If so, I have 450sq ft of lightweight cruising chute which I set flying from the extensible bowsprit and mast head - it takes pretty good care of light-air performance on most points aft of a close reach.

Chris