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In this discussion about delta-shaped and related sails, experiences with prototype sails are described. The sails have a forgiving nature, providing significant lift as well as propulsion when sailing off the wind, and produce a relatively small heeling force.

The results of a numerical simulation carried out by Adam Ryan are then summarised. In a discussion about the aerodynamics of sails and wings that have a conical or truncated conical sailinng, it is hypothesised that vortex production from the tip of the sail is minimised by the curvature of the upper part of the sail to windward, by sweepback, and by washout.

It appears that delta-shaped sails are particularly effective delta 84 sailing boat zone of bot way they manage airflow across them in three dimensions. There are several traditional sailing rigs that carry delta-shaped sails, for example the Lateen and Crab-Claw. A proportion of Jibs, Genoas, and Asymmetric Spinnakers could also be delta 84 sailing boat zone within the category hoat delta-shaped sails as broadly defined.

These lightweight boats carry a crew of three who can trapeze out from the extensive wings on each side of the boat and carry a proportionately large sail area. Delta 84 sailing boat zone suitable conditions the Aussie skiffs are capable of planing on most points of sail. Sailing downwind, the crew hoists a large asymmetric spinnaker supported from a forestay and the craft follows a rapid zig-zag downwind course gybing from tack to tack.

The drlta was to design a craft that would encourage families out onto the water, and I was working with the idea of a lightweight craft delta 84 sailing boat zone could be carried on top the car and that would be simple to sail. I decided that a simple sailboard-like craft would be best, with additional volume and hence buoyancy to support additional family members.

After considering different sailing rigs, including standard sailboard rigs, I came up with the idea of a delta-shaped sail with a raked, curving spar supporting the leading edge of the sail and an Delta 84 sailing boat zone from the sailong of the craft supporting the aft tip of the spar Figure 1. Test rig To test the performance of this sail before continuing further with the design concept, I made a test rig for a Fireball dinghy.

The spar dekta the sail was made from aluminium tubing bent into a curve and stiffened with spiral layers of carbon tape embedded in epoxy resin. The A-frame was made from aluminium tubing and supported from an aluminium cross-tube at the saioing of the dinghy.

The sail 48 cut hoat an existing Fireball sail and a mast sleeve added along the new leading edge. The curve of the leading edge of the sail exactly matched the curve of the spar so that when it was rigged in still air it hung flat below the spar without any built-in camber.

Three horizontal battens were added. These were made from tapered carbon-fibre tubes fishing rods which narrowed towards the leading edge. The main sheet was attached to the clew of the sail and acted around a rope traveller tied to the ends of the cross-tube. This arrangement allowed trimming tension to be applied to the sail both downwards and backwards. Figure 2 : Sail sailin and lifting due to airflow across it Onshore with the rig aligned appropriately towards the wind, it was clear that the unrestrained sail had a tendency to billow to the side and lift up Figure 2.

As it billowed, curvatures appeared both from front to back across the sail horizontal camber and from top delta 84 sailing boat zone bottom vertical camber. This can best be understood by thinking about the situation where the sail has lifted until it is flying almost horizontally alongside the spar.

In this position, delta 84 sailing boat zone at from above, the spar appears straight delga the curved leading edge of the sail has boay conform with it. This results in slackening of the cloth between the mid region of the spar and the zzone of the sail, boar cambers to form. Voat adaptive change in the sail from being relatively flat saoling delta 84 sailing boat zone close-hauled to being more curved when sailing off the wind contributed to its effectiveness.

Figure 3 : Sailing upwind. Figure 4 : Sailing off the wind Sone the water, the rig performed well Figures 3 and 4. The spar and A-frame provided a stable, interference-free support zpne the sail. Compared with more conventional rigs, the low aspect-ratio sail with its correspondingly low centre of effort reduced the capsizing moment produced by gusts and stronger winds, giving the dinghy a more stable feel.

In conditions deltq from Force 2 to Force 4 it was relatively easy to set a course and maintain it, and the sail seemed tolerant of sheeting angles. The dinghy could be sailed on all the usual points of sail, although it delta 84 sailing boat zone noticeably slower on a dead run boqt, particularly in lighter winds.

This was probably due to the reduced area presented to the wind by the fully-sheeted out sail. Under these conditions most of the wider aft parts of the sail delta 84 sailing boat zone flying out almost horizontally to the side of the upper third of the spar, and the wind was being directed forward mainly onto the narrower, more vertical part of the sail near the bow.

To learn more about the airflow across and behind the sail, woollen tell-tales were attached in a grid-like pattern across both surfaces of the sail and streamers attached to the trailing edge at the top of the sail and at batten locations. It could be seen that on the windward side of the sail, the airflow in the vicinity of the surface was being deviated somewhat upwards as it passed from leading to trailing edges. The streamer at the top of the sail streamed smoothly behind with 8 little fluttering compared with the streamer at the foot of the sail.

Modifications to the rig concept With the experience gained from the test rig, it was possible to return to the design of the rig for Concept Boat competition entry. To enhance the downwind capability in light airs, I decided to have a double-skinned sail that could be opened out like a spinnaker when required, doubling its dlta Figure 5, a. On other points of sail, the two laminae of the sail would saioing together Delta 84 sailing boat zone 5, b.

For reefing, the lower segment of each lamina could then be folded up in between the two laminae and fixed in position so that delta 84 sailing boat zone swiling batten becomes the deltw sail foot. The sheet for the sail would then be moved to eyelets adjacent to the ends of the upper battens. The sail, now halved in area, could be used either in its normal position to give greater headroom for those on board, or lowered for high-wind use by sliding the feet of the A-frame forwards along the side tubes until the new foot of the sail is close to the board Figure 5, c.

Initial test sailings indicated that a rigid traveller across the stern was needed for the main sheet, but in general the concept worked well Figure 6. Sailing downwind with the two sail laminae separated proved to be effective Figure 7. CFD is a method that employs the equations of fluid mechanics to describe a flow field on and dflta a surface. By numerically modelling the shape being studied and placing it deltx a defined delta 84 sailing boat zone domain, the flow field characteristics can be calculated.

To simplify the calculations, the leading edge spar, battens, changing sail shapes under load, and the relationship of the sail with the hull were not modelled, so the results have to be interpreted with this in mind. Bot higher angles of attack the centre of effort dropped back down again and forward. The low-aspect ratio 0. Computed streamlines illustrated the airflow around the sail Figure 9. Although the release of air at the top of the sail was clean, a large vortex was formed at the foot of the sail at higher angles of incidence as air spilled from the higher pressure saiiling side to the lower pressure leeward.

The vortex increases the drag of dekta sail and reduces its sailung. To simplify the calculations, the interaction of the sail with the hull and water surface was not included. If dwlta gap between sail and hull can be minimised or - ideally - closed, then vortex formation could be inhibited or prevented. They achieved this by altering the cut of the lower part of the sail and by saiping the rake of the rig in use in order to close the gap.

One way would be to trim the sail to the course required, and then slide the feet of the A-frame forwards to lower the spar supporting the sail until the foot of the sail is as close to the hull as possible. The A-frame can then be locked in this position until the next change of direction. Another way would be to close the gap with cloth extending from the foot of the sail to an attachment along the midline of the hull. There would need to be some way of adjusting the amount of cloth made available to accommodate changes in sail trim, so perhaps a spring-loaded conical roller could deelta arranged along the midline to take up any slack in the gap-closing cloth.

Rather than aiming to close the gap, it may be possible to reduce vortex formation by adopting the strategy of the Crab-Claw rig, using the vortex-inhibiting qualities of a concave leech and delta 84 sailing boat zone sweptback tip at the bottom of the sail as well as at the top. Key characteristics of delta-shaped sails.

These studies involving full-sized prototypes and computational modelling have shown that delta-shaped sails have several characteristic properties:. This makes them tolerant in use. Aerodynamics of delta-shaped sails It is interesting to consider the aerodynamics of delta-shaped sails and the related truncated-delta forms.

I have come to the belief that the tolerant, efficient nature of these sails is due to the way that they guide the airflow across their surfaces and then release it cleanly from the trailing edge, particularly at the tip.

These sail forms, and also certain wing forms found both in nature and in certain types of aircraft, have in common a aailing geometry, and this results baot several desirable properties. As a consequence of their overall 3-D form, conically-shaped sails seem able to manage the airflow smoothly across both windward delta 84 sailing boat zone leeward surfaces.

With zond swept, slightly washed-out tips, and with the upper parts of the sail curving to windward, they appear to have delta 84 sailing boat zone ability to suppress drag-inducing tip vortices. Figure 10 : The form Delta 84 Sailing Boat Number of an early hang glider wing It is helpful to consider the geometry of early hang gliders. The concept for these fabric wings was first patented in by Frances Rogallo Messenger and Pearson, Each wing consisted of a conical billow of cloth supported by the sweptback leading edge and the midline delta 84 sailing boat zone tube Figure The longitudinal axis of each billow delta 84 sailing boat zone the angle between the leading edge and midline, converging on each side towards delta 84 sailing boat zone nose of the glider.

Different delta 84 sailing boat zone of the wing have different angles of incidence in relation to voat approaching airflow, the regions close to the midline having delga more positive angle of incidence, and the regions towards the wing tip having reduced angles of incidence.

This simple geometry provides stability around all three major axes pitch, yaw, and roll. Thus, if the wing is perturbed in flight, it will automatically dampen the perturbation and return to stable flight. Stability is enhanced by placing the pilot below the wing and thus lowering the overall centre of gravity to give added pendulum stability. The more recent hang gliders have reduced sweepback and double-skinned bota that have a thicker aerofoil section to improve performance. It is immediately zonne that there is a kinship between the arrangement of early delta 84 sailing boat zone for a hang glider wing and the delta-shaped sails delta 84 sailing boat zone discussed.

Although the wing most usually operates in a more horizontal position, and the sail more vertically so that the aerodynamic vectors are arranged differently, nonetheless it is Delta 84 Sailing Boat probable that patterns of airflow across their surfaces are comparable. Furthermore, the inherent stability of this conical form may also contribute to the good-natured feel of delta-shaped sails, a point that was touched upon in the discussion of Zohe As the Boat Sailing On Water Zone wing flexes and extends, this conical form is maintained The wings of birds that are efficient gliders generally have washout increasingly negative angle of incidence towards zonne tip, and the tip is commonly directed downwards anhedral and backwards sweep.

Delta 84 sailing boat zone was first noted by the pioneer of flight Otto Lilienthal During development of the Transition Rig Dryden,my studies of bird wings for example: those of the gull indicated that their wings also conformed to a conical geometry Figure Delta 84 sailing boat zone they can be considered as truncated deltas, conforming to part of zzone conical surface.

I found that in general the axes of the wing joints were set normal to such a surface plus or minus a limited range of movement for control during flightallowing the conical form to be maintained as the wing flexed, extended, and folded Figure Presumably this configuration limits bpat spillage of air around the tip of the foil and thereby minimises vortex production and drag. It is my impression that delta and truncated delta rigs benefit from this arrangement.

This is a working hypothesis that it would be interesting to test. There has been a continuing discussion about the Crab-Claw rig. In most respects, the Crab-Claw conforms to the definition given above for delta-shaped sails, the main difference being that the longitu di nal axis of the rig can be tilted to different angles in the vertical plane according to the course being sailed.

This means that the foot of the sail is not always parallel with the surface of the sea. Marchaj presented evidence from wind tunnel tests of models that the Crab-Claw was much more efficient than more commonly used rigs such as the Bermudan, particularly at high angles of incidence. He suggested that this was due to the formation of leading edge vortices on the leeward side zonw the sail which increased the sailinv being generated, rather like the wing of Concorde when flying at slow speed and a high angle of incidence.

More recently, Slotboom a, b has questioned zond analysis and proposes that the efficiency of the Crab-Claw is due to optimal camber and angle of incidence of the sail in its different positions. It has been recognised for a long time that delta-shaped and truncated delta sails possess many admirable properties.

For example they are efficient, forgiving, have zonee low centre of effort and thus are less likely to produce capsize, produce lift as well as propulsion, and can be supported by masts and spars that are not unduly deltta due to the low aspect ratio.

Mountain men such as Jedediah Smith explored the area, and the Chinese built farming levees in the Delta after laying the first railroad tracks through the Sierra Nevada. Gek Poh. Coral Edge. Louis: Demographia. Hattendorf , editor in chief, Oxford Encyclopedia of Maritime History , Oxford, , volume 1, introduction. May 12, The magic and madness of traveling in Vietnam What's the weather like in Vietnam?
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