I have been wondering about these things too. I don’t have any aerodynamic theory about the SJR and have yet to discover whether or not it has any aerodynamic advantage, all I know is: it works well.
I took this photo a couple of weeks ago, close hauled and as near as I could tell in the “sweet spot” – which is fairly narrow on my boat but it makes quite a difference if I can stay within this narrow zone.
The lee tell-tales on the jibs are horizontal and showing correct air flow.
The main luffs are still nicely inflated (just).
At the “sweet spot” the main luffs are close to collapsing, but still just inflated. If I point any higher, the main luffs will begin to tremble and show signs of collapse – if I bear away any, the lee tell-tails on the jibs begin to revolve (showing airflow is losing adhesion?)
I have to concentrate hard to keep in this zone, but beginning to find it makes a difference if I can manage to do it.
(This is actually a screen shot from a video clip – you can watch the clip by clicking on it – then you will see the bottom panel which is setting badly due to a lack of downhaul. The luff of the slack bottom main panel is actually just past the point of collapsing, acting as a convenient indicator).
Since the mains seem closer to the point of stall than the jibs, at this “sweet spot”, I too was asking myself if there would be an advantage if the slot were to be opened up a little. I am not sure that increasing the jib camber would achieve this, but I did wonder if the sheeting angle of the jibs could be increased a little – or would it help if the “shelfs” of the jibs were cut with a greater angle?
Somewhere there is a limit to how much sheeting angle is possible– we don’t know what it is. We also have to consider how this might affect pointing ability.
As for shelf angle, increasing it would open up the slot (vertically) to freer air flow – but only if there is enough wind to support the shelfs. Even with 45 degree shelf, the jibs sag a little and will not fill out nicely in very light air.
Rudolph (Oceaan 22) has reported something quite different - with 10-% camber on both mains and jibs – he reports that his boat will sail to windward, when pinching, with the jibs collapsed and the mains still pulling. (see thread "new slit junk Oceaan 22). Though he also reports that jibs and mains both collapse at the same time.
Slieve has mused in the past about possibly increasing camber and sheeting angle, but recently informed me that he strongly believes 45 degrees is about right for shelf angle.
Without a comparable boat to trial against, I think it will be difficult to resolve the questions of ideal camber/sheeting angle/shelf angle/slot width – each parameter can be considered separately, but it is likely they have an effect on each other as well – which suggests to me that to improve this quite effective sail will involve a lot of trial and error and it may be that the current thinking is about right. There is also an aerodynamic theory approach but I leave that to others, except to observe that the apparently restrictive slot may have the effect of actually accelerating the air flow over the lee side of the main panels, in which case it would be an advantage?
As far as I can tell, those of us who have followed the Amiina-style plan form have all settled on pretty much the same parameters, and I would like very much if the various SJR-owners would complete and correct this spreadsheet. We would then have some actual figures for comparison, between this fleet of small boats.
Some numbers are from memory and some I don’t know, so additions and corrections would be appreciated.
“Tin Plate” camber refers to shelf cut sails and represents the theoretical camber which would exist of the panels were cut from tin plate. Of course, for comparison with other sails, the true soft cloth camber needs to be measured when the sail is inflated – we probably all need to do this.
It would be good if we could correct, add to and build up this database.
(PS to change the subject:
1. the sail catcher, supported by just fore and aft lift pairs, removes the need for lazy jacks - and the lifts interfere very little with the sail.
2. Advantage of wooden battens - hit a marker pole the other day and broke one. Was able to add splints using screws and a drill-driver, and keep sailing. )