I'd be interested in people's ideas of how much camber is desirable, both in the lower panels and the upper ones.  Would it be, to take one of Arne's sails as a yardstick, about 8-10% camber in the lower panels, tapering to 6% in the intermediate panel, and maybe 4% in the upper two panels?  Would anyone suggest more camber in the upper, or storm panels?

Storm canvas.

i recall from windsurfing days that some camber is beneficial in strong winds.

a smaller cambered sail performed better than a larger flatter sail

Hi Colin,

Thank you for directing me to your video. It does show that the battens are very bendy and generally are not bending in the best direction. I do believe that you're getting poor air flow over some of these panels and are losing out on performance. This means that the performance you are achieving is not necessarily a guide to good sail shape. Even so your experiments are raising some interesting points.

When I was building the Poppy split junk rig I was hoping to achieve a windward performance quite close to that of an equivalent Bermudan cruising boat, so when I first sailed the boat I was amazed to find that the performance was better than some Bermudan cruising boats and on average very similar. If you stand in the bow of a Bermudan rigged boat and look up the luff of the headsail you will see a nice smooth cambered leading edge. With my cambered panels I could achieve the required camper in the middle of each panel but at each battens the camber would be flat, therefore I did not expect to get as good performance as was possible with the Bermudan headsail. So the question arose as to why the achieved performance was comparable to the Bermudan Reg when close hauled.

The other main difference between my SJR and the Bermudan rig was the shape of the head of the sail. This had to be where the extra performance was coming from. Reading Toni Marchaj’s books I could see the inefficiency of the pointy top sail and it seemed clear to me that the shape of the head of the sail was important. My earlier experience of wing-tip shape in model aeroplanes seemed to indicate that the shape of the head of the sail had a large effect on the tip vortex and the overall induced drag of the rig.

The flow off the air across a sail is not parallel on both sides. On the high pressure windward side the air will have an upward direction towards the head from above the midpoint and a downward direction towards the foot at the bottom. On the low pressure leeward side the directions will be in towards the middle and this twisting produces vortices from the end of the boom and the peak of the yard. Concentrating these vortices can seriously reduce the overall induced drag and have a significant effect on the lift drag ratio, which is key to windward performance. Fan topped sails do not seem to encourage the tip vortex upward in the same way.

The big question is what would be the best angle for the yard for best overall performance. No doubt the ideal yard angle will vary with wind strength therefore the best result will be a compromise. In my case I have chosen 30 degrees as it looks right and mathematically it's easy to calculate as sine 30 = ½, but I have not experimented to find the best result.

I have received quite a few letters from people who have built rigs with splits in them, mostly complementary. When I have chased up the ones who have not been satisfied with the performance I have generally found them all to have disregarded the shape of the head of the sail. This encourages me to believe that it is not just the split and the jib that are the strength of the rig but that the overall shape is also very important. The low angled yard also has structure advantages making a low stress rig.

For me I guess the answer is that the jibs of the split rig may approach the lift of the Bermudan jibs but the lower drag of the shape gives a better overall lift drag ratio. It certainly results is a very easy to live with rig. But then, I’m biased.

Cheers, Slieve.

I also have the feeling that Colin is usingsome flexible battens on his Laser. Could you confirm, please.

On the views there are various sheeting, could you explain your choices ?

Eric

Hi guys,

Please excuse the unnatural writing style but this is my first experiment with ‘speech to text’ and it seems to be disrupting my line of thought.

It's nice to see this thread with a bit of cutting and trying in it.

There are a number of interesting points that I believe need closer examination and I believe they come under a number of headings.

First the problem with camber and the various panels.

Secondly a question of twist in the sail.

Thirdly the shape of the head of the sail being square or whatever.

And the laser dinghy called Lores.

Colin’s Collins observation that the position of the cambered panels affects the performance of the sale is particularly interesting. I believe we need more information before making a judgement or decision on these facts.

I noticed that Colin only sheets the lowest batten or boom and that he uses a kicking strap to control the twist in the sail. I also notice or fail to notice any sign of tell tails on the various panels on the sale so that is difficult to determine whether the various panels are stalled or not. Until it is clear that all the panels are in an un-stalled mode it is difficult to assess the efficiency of the setup.

In some of his writings Jan’s fellow countryman Tony Marchaj gave an indication of how the wind speed decreased at lower levels due to the effect of wind-shear , and I remember some years ago calculating that for the Westerly Longbow Poppy where the bottom of the sail was some 10 feet above the water and the head of the sail some 30 feet higher that the twist in the relative wind when sailing close hauled was approximately five degrees over the height of the sail. For a Laser dinghy where the bottom of the rig is some four feet from the water and not such a tall mast the relative wind change over the height if the rig would possibly be something similar to the five degrees (this is only be a rough estimate). However, this can have a big effect on the entry angle of the relative wind to the sharp edge off the sail, and to the performance of the flat cut panels in particular.

There is also the question of what is a flat panel and what is a cambered panel. If we think that 8% camber is fairly normal but remember that that is the camber in the mid height of a panel and that the camber is zero along the battens then the average camber over the height of the rig must be something less like say 5% overall. Most people believe that you need flat sales in stronger winds are therefore flatten the top panels as they are considered to be the storm canvas.**  Colin found that with flat panels at the top and cambered panels at the bottom of the rig when sheeted for best efficiency the performance was not as good as with the cambered panels at the top of the rig and the flat panels at the bottom.

I believe this can be explained by considering the entry of the wind to the leading edge of each panel by the twist of the relative wind. In the untwisted sail the twisted wind when producing drive in the lower cambered panels will be striking the upper flat panels are too steep an angle and there will be knife edge separation and these panels will be stalled, therefore these panels will produce negligible lift but significant drag and degrade the lift/drag performance and windward ability of the total rig.

With the cambered panels at the top of the rig and the sheeting set for best performance the lower flat panels will be receiving the twisted wind at a lesser angle and will be less inclined to be stalled and probably producing some lift and lesser drag and therefore a better total lift/drag ratio of the rig and overall better performance than with the panels arranged in the earlier order. To verify this it would be necessary to attach tail tails to each panel and see what is going on.

I believe best performance would be achieved by having camber in all the panels and with tell tales to assist trimming to make sure that all the panels have attached flow and are not stalled.

I’ll post this and continue later. (I’m not sure I like the ‘speech to text’ as there’s still a lot of editing to do)

Cheers, Slieve.

** I have had a particular experience that makes me believe that storm canvas should not be flat, and I will try to remember to explain at a later date.