Scott,

I too am in the embyronic stages of building a Split Rig.

For state of the art advice and knowledge of the Split Junk Rig, Slieve McGalliard has to be the person best placed to help, as he both concieved the idea and has done a lot of real life development work on what works and doesn't. As he has essentially been working solo and, in my opinion, done a fantastic job, by building our own SJR's, you and I are about to become developers in this project which might quicken the pace of development and knowledge of the rig.

In another thread, he advised me that the slot should be placed forward of the mast.

I've built 3, ¼-scale, jib panels and attached them to a model mast and battens and this got me thinking some more as well about the slot and where to put it and the aerodynamics of it all.

Obviously I'm no expert on aerodynamics or sail interactions but,

As I understand it, the purpose of splitting the rig is to remove the essentially aerodynamically useless area (in windward mode) forward of the mast which increases turbulence on the leeward side on a standard balance lug rig sail and reduces lift/thrust/drive development. (sincerest apologies to all owners of standard balance lug junk sail owners who find their rigs fantastic, which they are).

Instead, again as I understand it, the split in the rig allows;

1. Camber to develop right to the luff of the sail area forward of the mast, eliminating the turbulence and developing a lower pressure zone on the leeward side of the jib panel and thus creating more lift/thrust/drive.

2. Allow the wind/airflow on the windward side of the jib panel somewhere to escape with relative ease in an aerodynamically smooth fashion (reducing drag) and at the same time, hopefully help create a lower pressure on the leeward sail of the main panels, similar to the leeward side of the jib panel, increasing lift/thrust/drive of the main panel.

The sheeting angle in combination with the slot gap, I think, needs to be large enough to allow the airflow on the windward side of the jib panel to escape to leeward of the mast without being constricted too badly by the mast's presence. As the amount of air trying to get through the gap between the jib leech and the mast will increase with increased windspeed, it's probably better to leave a larger sheeting angle and/or a larger slot forward of the mast to allow as unrestricted an airflow as possible. I think that the whatever camber is designed in will have to take account of the sheeting angle to eliminate back-winding when sailing to windward at your preferred sailing angle into the wind.

Drawing a line on the plan view of the jib panel development from the luff of the jib panel to the leeward side of the mast would give, I think, at least the minimum angle sheeting angle to help reduce a restriction in the flow between the leech and the mast. The jib leech (when filled out by airflow) is then to leeward of that again.

Also, having a slot forward of the mast further increases the space for jib windward air to escape to leeward of the mast. If the slot is too small and the sheeting angle is too small, the windward airflow will bunch up and slow down, causing heeling and loss of drive.

I've wondered if also having a 2nd slot aft of the mast might allow more airflow over the leeward side of the main panel or if it would just create an aerodynamic mess by interfering with the airflow coming off the jib leech.

But Slieve has probably tried and discarded that idea.

These are just my thoughts based on limited knowledge and making a couple of model panels. I certainly stand to be corrected and definitely, Slieve knows best.

Regards, Dave.

As I get ready to make the 1/10 scale model, I've finally figured out what is meant by "sheeting angle".  If I have this correctly, it's the fixed angle between the batten and the straight line from the jib luff to leech.  This is controlled by the width of fabric in the 45 degree shelf foot.

What I'm calling the Designed Slot, for lack of a better term, is the distance from the luff of the mains (which should be the centerline of the mast) and the leech of the jibs where they attach to the battens.

What is the state-of-the-art opinion on these items? 

For the sheeting angle, I'd think that there'd be a trade between sheeting angle and camber: the camber would be inversely proportional to the sheeting angle, the two perhaps always summing up to something like 20 degrees.  That truly a wild guess - both the implied relationship and the 20 degrees.  Somebody with experience, please weigh in.

For the designed slot, I'm even less sure of anything.  Arvel Gentry's treatises on the interactions between jib and main indicate that the overlap of the two (say, 120 percent genny) seem to maximize the upwash critical for better pointing.  Obviously that ain't happening on a split rig - the mast would snag up everything.

But would it be best to minimize the designed slot width, putting the jib leeches just a cm forward of the mast?  Or does the slot effect need more than that? Do we even care about the slot effect on a split rig, or is that a minimal player in our world.  Does the bulk of the mast so mess up the slot effect that we can simply ignore its existence and focus optimizing the jibs' leading edge?  If the slot does matter, can it be opened up with higher sheeting angles? 

So many questions for a Monday.  I bet you can tell what I was doing instead of getting my holiday shopping done.