Any holes in a tube, or anything other than an entire round tube,  is less than optimal for an unstayed mast - fatigue cracks always start and propagate here. I would only use the length above the access door. But at 145mm dia, there's a good chance that it will fit into a 6" x 10 swg (152.4 x 3.2mm) tube, using the top 4 metres as a topmast, instead of timber. 

When David Tyler speaks, I listen, in particular about mast scantlings for offshore sailing, where I have no experience to speak of.

The term Jester Challenge was also thrown in by Jes, and I realise that I should be a little more conservative when suggesting a mast section for this sort of sailing.

In my Chapter 6b of TCPJR articles, I recommend keeping the wall thickness between 2.5 and 5% of the mast’s diameter. Now I tend to drift towards the lower end if possible. The problem in my world is that I cannot just dream up a mast section, and then have it. I must pick tubes from what is available, so have to compromise.

The fact is that older British junks from the eighties and nineties had very thin masts, probably with quite thick walls. Some of these masts appeared to hold for over 20 years, and then suddenly snapped due to fatigue. Photos of these early rigs in use clearly show quite some mast bend, even in moderate conditions (photo below). In contrast, the mast of my 2.15 ton IF Ingeborg (Ø150 x 5mm 6082T6) doesn’t appear to bend at all, even when dipping her toerail.

From a windage perspective, I am not so afraid of increasing the mast diameter with thinner walls. I have had quite stout wooden masts (Ø250mm on my Johanna), and it worked well enough. However, I still try to avoid building taller masts than strictly necessary. Just imagine what that top metre of the mast does to increase whipping as the boat is dancing about in an offshore gale.

Arne

( see last page of JRA-NL 20 or my Member's Album, Photo Section 8-17)

May I come in on the mast size, Jes?

My preference for offshore sailing was to go for a larger, thinner wall section, so as to get more stiffness for a given strength. Particularly in tidal overfalls, a mast that whips around is most unpleasant. My last boat before I quit sailing, a 22ft 6in Hunter Duette, Weaverbird, has a 177m dia x 3mm tapered aluminium mast, which didn't look too large, and I was glad of its stiffness in rough going. These tapered sections are a little softer than the 6061T6 of a hybrid mast's tube, so for sufficient strength and stiffness for offshore sailing, you can certainly come down to 152mm dia x 3mm, possibly to 127mm dia x 3mm, but as you mentioned The Jester Challenge, I really wouldn't want 100mm dia of any wall thickness. Dirty weather in mid-ocean causes one to have enough to worry about without the mast behaving like a fishing rod!

I know that Mingming II, a larger boat, has a 200mm mast, but can't find the size of the original Mingming's mast (Eric A will know).

Jes,

since my sails generally have a lower section with parallel battens and with the luff and leech parallel to the mast, I define the mast balance of my sails to be the number of percent (of batten length B) that the lower battens protrude forward of the mast.

Arne

PS: The lower mast of my Frøken Sørensen had a section of Ø100 x 4mm. I bet that is enough for your boat as well.

(EDIT: Now I'm not so sure: Jester Challenge  -  see later posting...)

This summer JRA member Paul Schnabel has successfully cruised his Maxi 77 with his junkrig set with up to 27% mast balance.
I have therefore made use of his findings and drawn a hi-balance sail for the Corribee, shown below.

The mast was put through the hatch, with its centre about 200mm aft of its forward edge.

I don’t think the sail will ever need to be moved forward of the shown position. If anything, it rather needs to be shifted aft a little to trim away any lee helm. There will be plenty of room for that.

The Corribee has a smallish rudder. This hi-balance JR brings its CE quite close to the JR mast, which again means that the CE will sit close to the boat’s centreline when sailing downwind. This should help the rudder to cope.

Just an idea.

Arne

PS:
It looks to me that the superstructure of the Mk 2 and 3 are very similar, so I used a Mk 3 version here.

(Hi-res diagram in Arne's sketches, section 8, photo11)