Mark Thomasson wrote:

Brett,  <snip>

 One possible problem is the booms hitting the opposite mast when let to swing free.  With the split rig you should avoid this.  

Perhaps the simplest 'balanced' rig is a simple wishbone batten rig, it is more work in making the wishbones but the sail is just a flat sheet.  With a biplane rig the mast can always be on the windward side can be on the preferred side of its mast.

Brett & I are still considering the AR (Aspect Ratio) vs. SA (Sail Area) compromise. Our initial figuring and modeling has kept the low AR/max SA design option with a chord length less than the 16' C-C hull centerlines, so the interference problem you mention is wisely considered.

Masts in hand allow 33' of hoist giving generous boom height and halyard drift. Our models keep this dimension constant while varying the chord.

Simplicity drives centering the masts on the hull CLs; offsetting them outboard to increase separation is still possible, but is not as elegant: the centered position offers nice "walk-around" space and straightforward lowered mast stowing, among other things.

Design max SA is still under discussion as various experienced sources have widely differing opinions. 

The SA compromise is light-air performance vs. the sudden squall potentially carrying the mast(s) away. Nothing is fool-proof. The necessity of timely reefing weighs quite heavily in the compromise. Cruising-weight catamarans do not have the heeling release-valve that monohulls provide. Incorporating wise human decision making into the design decisions is iffy, at best.

My (admittedly biased) assumption is that, in light air, a higher AR will point higher with less SA (though moving through the water slower) than a lower AR with a larger SA. As winds pick up the larger SA needs reefing sooner, so AR becomes the only factor.  I tend to favor "less-is-more" but do not have anybodies' junk experience on this point to judge against. My "pointy-rig" experiences show here. [This paragraph invites discussion!]

Slieve's split-rig design is our primary focus right now. His KISS! design philosophy (and back-up publishing) overrides almost all else looked at and considered. Besides, his affable discussions in emails off-post and general good humor always evident encourage us to build another test-bed for his apparently successful concept. Go figure!

Wishbones are way more complicated than straight Ø2" aluminum tubes (possibly available from the junkyard; shouldn't all Junk boats use junkyards as their chandleries?)

With 33% balance the high AR/min SA of 400 ft² option considered has a chord short enough to allow both sails to be squared-away with both of their leeches inboard without interference. The low AR/max SA design, allowing free passage past with sails on the same tack, is about 550 ft².

The idea of fitting a rigid boom vang, somewhat similar to those Garhauer offers [https://garhauermarine.com/catalog_process.cfm?cid=40], that will allow slack lazy jacks when reefed is perculating. Still to be worked out is the necessary boom section needed for vertical (bending) loading and mast attachment for the horizontal (aft) thrust vector. Fortunately our current (short-term, boatyard cleaning up space) junk dealer has a large selection of aluminium extrusions, so cost factors are minimum if we can keep a step ahead. Carpe diem!

Pax tecum,

Michael

I'm not sure what the best way to attach images onto forum posts.  I just signed up for the "Box", but I'm not sure how that would work for using images in the forums, other than just linking to a file, for those of you who have been around a bit long, feel free to tell me what the best practice is.  For now I'll just link to the fill in my google drive.  

Click here to see a link to a scan of Michael Gian's sail plan from our first round of designing.  We've since dropped some sail area and moved toward Slieve's more recent iteration of his Split Junk Rig, which we'll post next once Michael gets a 400sq.ft. sketch done. 

Best,

Brett

PS  Annie, thanks for the tip, I'm messaging Pete now.  

Michael Gian wrote:

<snip>

42' [12.8 m] mast with boom crossing up 6'-0" [1.83 m] and yard center tangent to Radius 5'-0" [1.52 m] (as per PJR fig. 6-28). 

<snip>

The 5'-0" is 30% of batten length (0.3B)

A little Googling this morning while tending to a feverish baby while mom got some rest led me to this comment Bertrand made on the Wharram Builder's ning site a while back.  

At 5:01pm on January 20, 2012, Bertrand FERCOT said…

Dear Tom

Concerning my own experience with our previous Tiki30 PHA  I was satisfied with her ability to close the wind as a cruising monohull with an angle of 90° on the ground between two tacks.

I can't tell you what is the difference between my wishbone wing sail  and a camber wing sail, because during the rallies  of the Junk Rig Association I was the only catamaran. When, aboard my Tiki46 Grand PHA, I'll have the opportunity to sail with the Tiki46 APATIKI ( Building and Sailing APATIKI ) it will possible to compare these sails on the same type of catamaran.

I think it will be too much to double the sail area. The standard sail area of the Tiki46 is 91m². The first version of my Swing Wing sail was 2 x 46m² and after my ocean crossing with PHA I request to add one panel more, increasing the sail area at 2x55m² (about 20% more). So the masts are 1.6m higher and I can't imagine the supplement of length and weight if I double the sail area!  When you need more sail area, it is at downwind points of sail and happily at these points of sail the biplane is very efficient, so I think 20% more is reasonable.

My biplane Tiki30 PHA was well balanced and her wind-vane Navik worked perfectly even at down wind. I broke its blade when crossing the Atlantic, not due to bad balance of the rig but due to too high speed for this old wind-vane designed initially for mono-hulls with a too much sensible trimmer.

Despite during the winter its more difficult to work easily on my building, I estimate reasonable to plan her launch at the beginning of next spring in April.

After a first sailing test to the Isle of Wight to finalise the electric engine system we'll improvise to join the North America according the weather forecast and after we'll begin a long sailing around  the Earth.

Be careful, a such boat requests a lot of time. In my case, I work mainly alone with occasionally the help of my wife and children or friends when necessary for big operations.

So don't dream too long if you want to sail with your children before they become adults. Pass to the action in building a big cat as ours with a lot of efficient helpers or build a more little one or buy a good second hand you'll customize according your own needs.

Amitiés

Bertrand

So, it looks like Bertrand ended up pleased with about 125% of Wharram's standard sail area.  If we were to follow that lead, we'd be right at 2 x 400sq.ft.  Notably, Grand PHA uses an advanced, presumably more powerful, sail design.  I actually quite like the idea of the split junk rig and imagine that it is within our scope to produce quality sails at home.   

Micheal, what would our batten lengths be at 400sq.ft. per sail based on our previous sail design specs?

Annie, thanks for your contribution.  Do you perhaps remember how much canvas China Moon flew/flies?  

Best,

Brett