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Tuesday, January 21, 2014

IOM; A new A-rig

After reading up on the latest widgets and thoughts on IOM rigs, I decided to build a new A-rig for my Tempest IOM Juno. My old rig was getting on, but I'll keep it for those borderline A-B rig days.

One thing I wanted in this rig was an adjustable gooseneck axis, as covered in detail by Lester Gilbert. Essentially, changing the aspect of the gooseneck relative to the mast by shimming the bottom (or top) in and out, you can adjust (slightly) the leech of the mainsail to optimize performance. After some research, I purchased a very nice adjustable, ball-raced, variable geometry  gooseneck from the folks at Radio Sailing Shop, along with a standard rig kit. The installed gooseneck looks as follows:

Carbon fiber, ball-raced, variable geometry gooseneck. 

A screw (seen above on the bottom mast band, facing aft) allows adjustment of the gooseneck, seen here in "neutral". The mast, incidentally, is 11.1 mm, with some forward pre-bend.

I also wanted this new rig to employ a jackline, rather than simply attach the sail with tied off rings as I had done previously. Again, Lester Gilbert has a good explanation of the advantages of this system. Essentially, the leech of the sail stays aft of the mast rather than rotating around if simply tied off in rings. After discussing this with several local skippers, I ran a wire from the masthead, through a pocket in the mainsail leech, to a loop at the bottom, and bowsied it off around the gooseneck. This way the jackline tension can be adjusted, and I am told it is important not to run with the tension to tight, it can be deceptively slack to be effective. A few pictures of this attachment:

Jackline attachment with yellow shrink-wrap, at the masthead wire.

Jackline, bent into a loop and bowsied around the gooseneck.

The new sails for this rig, come from Power Sails, and had the luff pocket already added. I like these sails very much thus far, and they have four small cut-outs to allow the jackline to be fastened to the mast with a loop of line. Also visible in the picture above is the auto-easing Cunningham, once again based on work by Lester.

I find it easier to adjust the jib line and jib luff tension at the top, so simple bowsie attachments for these adjustments here:

Following some advice from Radio Sailing Shop's Building Guide, the shrouds attach at a central point on the front of the mast, and otherwise have simple chainplate attachments:

All in all I am very satisfied with the finished product, though I need to make a final jib sheet attachment, I just tied if off for the test sail. I may lower the jib a bit as well.

A few photos of Juno testing in 10 mph, she went very well, and the gooseneck adjustment definitely helped downwind performance!

Sailing along, sails full and drawing some power.

Tacking onto starboard...

A nice study, the sails full and powered up for light air.

Before an adjustment, leech of the main was bleeding out some pressure.

Please post your comments, observations, and thoughts below! I'm particularly interested on any comments on the sail shapes above.


  1. Now you can adjust the leech tension when the boom is against the shrouds, with the vang, and a separate adjustment for the leech tension when close hauled, with the lower gooseneck adjuster.

    1. Thanks, that's what I'm hoping... It will no doubt take a while to work through all the permutations of the additional control adjustments!