Hi Andrew
I understand from earlier posts that you have now found an acceptable position for your mast partners. I also used that location you are proposing as did Sunbird Marine which fitted a junk rig to a Sabre 27.
I note that you are considering building an aluminium mast to your Sabre 27. In my earlier posts I mentioned that the mast I fitted to my Sabre 27 was built by Hawk Marine Services ( previously Needlespar masts). It might be of assistance to you to know more about what they shared with me about the building process.
My mast is 5.5" diameter at the partners as recommended by Sunbird Marine and tapers to 4.5" at the masthead. It is built of 3 sections of 6082A grade aluminium 1/4" wall thickness. This is in keeping with Needlespars design parameter to minimise the windage of the spar.
Needlespar masts both stayed and unstayed, were very popular in the dinghy racing world some years ago because of their comparatively low windage due to the small section tubing used in their construction, which conferred a significant advantage togther with their designed bending profile under load.
The lower 5 meter section of my mast has an internal sleeve 2 meters long of 5" diameter x 1/4" wall to reinforce the area through the partners. The mid-section (3.5 meters x 5" x 1/4") is inserted into the lower section and pinned through the joint by an aluminium bar , the ends of which are beaten to form a domed surface to retain the bar in position. In effect the sections are held by a substantial aluminium rivet. The same construction is used to secure the upper section of the mast. This is 4.5" diameter with 1/4" wall thickness. The mast weighs about 90 kg.
Hawk Marine Services (who now produce masts based on Needlespar's construction methods) explained that although in principle each section should just fit inside each other that in practice the extruded tubes are not perfectly circular and so do not fit together. To overcome this issue they turn down the section on a large lathe and then press the sections together using thickened epoxy as both a lubricant and an adhesive. I would expect it also filled any gaps between the tubes. The mast sections are also chamfered at the joints.
I don't have details of the degree of overlap of the tube but for the aluminium battens I have used 3x the internal diameter as an overlap for joints in the battens. I had a length of solid bar turned down to use as a joint between lengths of batten to avoid piercing the batten to attach the batten parrells. I have had no issues to date with the battens failing at the joints.
Prior to ordering my mast I calculated the safety factor for the proposed spar using Arnie's suggested method. This was published some years ago in the junk Rig magazine. Following this method gave a safety factor of around 2 which seems acceptable. I think the derived safety factor is probably significantly larger given that the equations used are those relating to a cantilever beam with all the load at the head of the mast. In reality the load would be distributed more evenly along the length of the mast although it would be higher at the masthead given the geometry of the junk rig sail which has significant area in the upper part of the rig in the event of a knock-down under full sail, but is such a thing likely?
I have read of other methods of building freestanding aluminium masts. I understand that Roger Taylor (who fitted out Ming-Mig 2) used a spun aluminium lighting pole of much larger but thinner wall section but reinforced the lower part by cutting a longitudinal slot in a spare piece of the lower part of the pole. This made it possible to compress the tube and insert it to form an internal sleeve. Epoxy resin was used as a lubricant/adhesive. Calculations based on the above method show that the mast is of adequate strength.
Other junk rigged craft have according to their builders, used large diameter aluminium thin walled tubes in mast construction. I recall reading the account of the builder of Easy Go employing masts of this kind and this vessel successfully completed many open ocean voyages. The masts employed appeared to be too weak according to my calculations based on the cantilever beam equation but have proved otherwise. There do seem to be other factors involved in determining whether a mast is of adequate strength.
Best wishes, Frank