Interesting replies again!

Graeme, yes it would be interesting to test the practicality of an end plate on the yard, but equally it would be difficult to assess any gain (or loss). If fitted it could be that a different yard angle would work well or be detrimental, so I’m not sure how to learn from the exercise.

The Spitfire semi-eliptical wing plan forms performance depended on a number of factors. It was faster and had a higher climb rate than the similarly powered Hurricane, but apparently could not manoeuvre as tightly. The Hurricane was more of an early biplane with the top wing removed, canvas covered fuselage and a thicker Clark YH wing section, and hence more drag. The Spitfire wing had the thinner wing section which varied over the wing span. There was also a critical degree of washout over the wing. It is interesting that later marks had the wing tips clipped. Whatever, it was a very pleasing shape to the eye.

Edward’s Amiina sailed 3 Round the Island races with the mark 1 rig. First one with the full too big area and horrible handicap, The second with the bottom panel permanently reef out and the third with the top panel removed. I crewed the first two and found it a most interesting experience. The boat was not heavy enough to punch through the Solent chop which was greatly exaggerated by the hundreds of boats sailing in the race, so had to be carefully steered through the nasty sea state, but even so in both races the rig gave great drive and the actual finishing positions in the races were very respectable. If I remember correctly the third race with the top panel removed was a bit of a disaster, not because of the rig but because the winds were so light that none of the smaller boats could complete the course. Edward did anchor up and finish the course the next day and got a mention in the national press. The big point was that when everyone was retiring Amiina was well ahead of and beating the majority of the smaller boats. Considering that it was one of the smallest and lightest boats in a race against some 800 boats the two reduced area rigs matched the boat well. The performance was always in the top quarter of the fleet. How critical was the yard angle, well that is the thousand dollar question.

Regarding water sails, I have experienced them on a 77 foot 1895 sail training built ketch, crewed by teenagers. I came on watch after the opposite watch had rigged them and numerous others sails and I almost had a heart attack. If we had had a man overboard we could not have turned the boat against the rig even under full engine in less than 15 minutes. If long distant sailing over a route with light airs I would seriously consider a light weight panel with large windows hung under the bottom batten and closing the gap, but attached with some running lines so that it could be dropped on deck in an emergency if quick manoeuvring required. Some ideas are good, but they must be safe and practical.

Paul, I appreciate your comments on the vertices produced by the heavies. When an aeroplane is close to the ground the rotating air will expand in diameter behind the machine and spread out on the ground to the sides. With a cross wind blowing the vertices would blow down wind, and the up-wind one would sometimes end up just sitting on the runway to catch any smaller aircraft following. We had to be aware for such situations which could be quite exciting.

I admired Bunny Smith for his pains taking experiments, but couldn’t accept the bumble bee theory. Sure, the bee forms high energy vortices, but that is powered flight, and a sailing boat is more of a glider so we should be looking at soaring flight. Sea gulls, Condors and the such like are where we should be looking.

I think the fast electric aeroplane wing tips will push the vortices out and aft, and reduce the induced drag. I believe this is what we ideally want with a rig, but it is easy for the aeroplane to design it to achieve maximum efficiency at a particularly narrow speed range, whereas a sailing boat sees a much wider range of wind speed, which may require a range of tip shapes, or a very big compromise.

Mauro, your thoughts on bendy ends to the yard are interesting, but experiments with bendy battens seemed to suggest that the range of bending with the range of wind speeds would be difficult to match up. Still, it is certainly worth some thought.

All very interesting.

Cheers, Slieve.

Paul: "...reminder to read those older magazines/articles. I take that as a well-meant kick from behind to get reading!"

No, not all. I just thought that if you haven’t read some of this stuff you will be interested in the level of debate and discussion that was going on within the JRA in the last decade of the 20^th C, on this subject. It seemed to be an especially fertile period. It seems the newsletter was the forum in those days, so a lot of that early thinking is well-recorded.

"However, I feel urged to clear up on the bee flight. It is nothing more than a legend that a bee defies aerodynamics..."

Sure, I wasn’t trying to revive the Insect Flight Theory – what interested me so much about Smith was his insistence that if aerodynamic theory doesn’t fit the observed facts, then the theory needs to be examined. (Actually, his observed facts needed to be re-examined too, because I think he was changing too many variables at once and perhaps not sure what he was really observing).

But the need to modify theory (as opposed to the observed phenomena) is still as valid as ever. According to theory the misbehaving luff on your Ilvy should be upsetting the windward performance. But Ilvy appears to go to windward particularly well. So, instead of trying to change the luff, maybe you should be re-thinking the theory – that was a “tongue in cheek” remark, but it did make me think of Bunny Smith.

He was interested in the idea of a bubble of detached (then re-attached) airflow too, and I did seriously wonder if your “miss-shapen” luff might actually be contributing something – sort of “Bunny Smith” thinking – its all because I was reading that stuff at the time, I guess.

Finally, your interest in the possible fencing effect of battens – Smith attached battens to both sides of his sail, and he thought so too. If he was mistaken in getting his inspiration from the ridges on a bee’s wing, it is still possible he was onto something in those early days. He called them “keeper battens”. It caught my eye and I thought you would be quite interested in that.

I have great admiration for all of those early JRA pioneers, and the history of the ideas which are being discussed today.

And thanks for your scepticism  of the “end plate on the yard” idea – with the clear reasons given.

I guess that's why Slieve never thought much of the idea either. Slieve takes the “planform” approach to vortex shedding (pushing the vortex up and aft) and has used that swept back aircraft wingtip (in contrast to the elliptical Spitfire wing) as an example of efficient planform, assuming that a lot of design theory went into it, with a view to getting better lift/drag ratio than the Spitfire would have had.

As I am sure you know, the aircraft was designed to be an electric-motor-driven racer – to break the world speed record for an electric driven aircraft. Which it did - by more than 50% !  I would have thought the emphasis would have been all-out max efficiency (ie absolute max L/D) rather than any compromising for the sake of benign stalling characteristics. (It was said to be an awkward thing to fly “you had to keep your wits about you…” so it may well have been prone to stalling a wing).  But I’m well out of my depth, you and Slieve can debate that one.

That aircraft – some nice  glimpses of it at speed, here and a great documentary on the record-breaking flight here.

If only the JRA had the R&D budget of Rolls Royce – who knows, we might be able to achieve an extra 5% performance gain!

Hi Slieve,

as a medical doctor I don’t have any knowledge about aerodynamic at all… nature has also any nevertheless developed on birds a system to cope with tip vortices which has been honed by millions years of evolutionary ignorance (assuming that birds are the descendants of dinosaurs, this process took 66 million years!!!). In the upward stroke, birds fold their wing tips at the elbow to build a kind of winglet; in the downstroke the wings are fully spread and the primary feathers bend upwards acting as winglets.
Would it be thinkable to extend the end of the yard (…and upper batten?) with a bending tube (carbon?) and a very light cloth appendage (spinnaker cloth?) in order to replicate the primary feathers? These could bend on the right side of the sail on every tack…just nonsense? 

Cheers 

Mauro

Thanks for the quick responses.

I will admit that I may have misled you by talking about keels first, and on that subject I reckon most junkies must simply accept the keel on the hull they end up with. My main interest is in the top of the sail, and a set up to have the lowest induced drag/ vortex drag and hence the best L/D ratio.

Winglets seem to be the biggest improver for transport aircraft, but somehow I can’t see that they can be practically adapted for sailing rigs. David wrote, “So my hunch (no more than that) is that we have to decide which way to jump.” I guess that sums up what I am thinking, and in many ways that ties in with the question of sail balance.

Look at the photo below of the elect aeroplane which claims to be the fastest of its type in the world. I’m sure much research will have gone into the wing tip design, and I’ve also seen this shape on many slow speed but high performance model glider designs which also require best L/D ratio.

I’m not surprised that this is considered to be an efficient tip design as back in the 50’s I was building my own design model aeroplanes and found one design with flared wing tips to perform better and faster than expected. Again, about 1970 there was some problem with the wings of the Trident 2 flown by BEA, and some aeroplanes had modified wing tips fitted. The original tips were flared out to the trailing edge, but the modified ones curved in, only a few inches but enough to require an a recalculation of the increased fuel load.

The one thing about these tip shapes is that they are suitable for sailing rigs as they can work on both port and starboard tacks, and do not necessarily increase weight aloft.

Are we to assume that this tip shape pushes the vortex out and back and reduces the drag?

Graeme’s mention of end plates on the yard would increase the weight aloft, and there’s no guarantee that the yard angle might be already concentration the vortex up to the tip. As he says, it would be difficult to appreciate any gain or loss.

I’ve been looking at the videos of the Americas Cup racing and it is noticeable that they have removed the gap below the sails, even putting small skirts to completely seal the gap, and have squared of the mainsail head to horizontal. (This latter is probably some rule about no sail above the black band (Black band with blach sails and black mast!) Anything to raise the L/D ratio.

Just more thoughts!

Cheers, Slieve.

As I understand it, an aircraft's winglet is toed in a few degrees, and cambered on its inner face (a bit like bilgeboards and leeboards), and interacts with the tip vortex in such a way as to add a small lift vector in the forwards direction (at the expense of more friction drag). Well and good, we might be able to put one on the lee side of a yard, the low pressure side equivalent to the upper side of a wing. But hang on a minute, we need our sails to work on both tacks, and having such a winglet on the weather side would cancel out the positive effect on the lee side (a bit like leaving both bilgeboards or leeboards down). So I don't think the type of winglet as generally used on today's aircraft is a workable notion when applied to sails. We might go for the fence type of winglet though, more like a delta shape attached vertically to an aircraft's wing, or horizontally to the bottom of a keel or rudder. We're familiar with those, on shoal draught boats. It might work in modifying the tip vortex in the way we want. Trouble is, the yard would have to be very close to horizontal. So my hunch (no more than that) is that we have to decide which way to jump. Do we go for a near-vertical yard, and lengthen our leading edge and increase our aspect ratio? Or do we go for a near-horizontal yard, and hope to decrease the negative effect of the tip vortex? Or do we go for a yard at 45˚, and fall between two stools aerodynamically but get ourselves a rig which is easiest to manage and sail with? Decisions, decisions...  

I remember David Thomas, the designer of Tystie and no slouch when it came to racing, looking at my cambered panel sail approvingly, and saying "the wind likes to be told where to go". Channeling the wind along the panels with protruding battens might just be a Good Thing.