Another thing to remember is that some members have reported good success with quite high balance contiguous sails ( ie, NOT SJR), so even if the best mast position for your needs is not suitable for an SJR, you may still be able to build a quite high balance one-piece sail to get some of the benefits of better balance, going downwind. 

I think Arne has some lower-yard-angle higher-balance sails in his portfolio, though you do lose some luff length and sail area for a given mast length. 

Just a thought. 

Hi Andrew, I read with interest your intention to fit a Junk rig to a Sabre 27. I fitted out, from a bare hull, a Junk rigged Marcon Sabre 27, twin keel. To my knowledge my vessel was the second Sabre 27 with a junk rig. The first was by Sunbird Marine, which is no longer trading following the death of Robert Blain the founder of the company. Sunbird Marine also fitted out other hulls with junk Rigs. 

Robert shared his experience regarding positioning of the mast and components used ( partners, mast clamp for attaching blocks to lead running rigging aft to the cockpit, etc). Sunbird Marine fitted alloy masts from Hawk Marine Services. This company took over production from Needlespar and built masts to that design. Hawk Marine built my spar.

I opted for a HM flat sail, alloy battens and boom. Purchased a length of dinghy mast from Z spars and associated fittings to make the yard. I had the sail made professionally.

Regarding mast position, I followed Sunbird's example and positioned the mast just forward of the doghouse. I was working with a bare hull and redesigned the interior layout, specifically for the first mate (wife) and myself. Our boat has a large double berth f'd with a large  hatch above, as per the original design of the Bermudan rigged boat and I wished to retain that feature. 

I appreciate that you wish to retain the original mast position and accordingly are considering a split junk rig. The CE of my current sail coincides with that of the BR sloop. My original sail was 35m^2 AR 1.5 but although fine upwind, it generated too much weather helm when off the wind particularly with the wind aft of the beam. I had designed the rig to enable canting of the sail across the mast for off-wind sailing  but this was not enough to remedy the excessive weather helm in winds F4 and above when off-wind. I had 1m cut off the leach and this significantly improved the situation. There is a touch of lee helm when close-hauled in light winds but I find this acceptable and once the boat heels this disappears. My current sail is 30m^2 which is a bit too small for light airs but generally fine for the west coast of Scotland where I've been sailing this season.

I hope you find the above helpful and if you wish for more detail, I'm pleased to assist. 

Best wishes, Frank

I've measured the available space on my Sabre 27. I can move my mast position forwards by approx. 22 inches. Is this likely to be enough? This is the current position.