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 form, 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 because of the way they manage airflow across them in three dimensions. There are several delta 84 sailing boat sailing rigs that carry delta-shaped sails, selta example the Lateen and Crab-Claw. A proportion of Jibs, Genoas, and Asymmetric Spinnakers could also be considered within the category of delta-shaped sails as broadly defined.
These lightweight boats carry a crew of three who can trapeze out from the extensive wings on each delta 84 sailing boat of the boat and carry a proportionately large sail area. In suitable conditions the Aussie skiffs are capable of planing on most points of sail.
Sailing downwind, the crew sxiling a large asymmetric spinnaker supported from a forestay and the craft follows a rapid zig-zag downwind course gybing from tack to tack. The brief was to design a boay that would encourage families out onto the water, and I was working with the idea of a lightweight craft that 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 delta 84 sailing boat and an A-frame from sailong back of the craft supporting the aft tip of the spar Figure 1.
Test siling To test the performance of this sail before continuing further with delta 84 sailing boat design concept, I made a test rig for a Fireball dinghy.
The spar carrying 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 stern of the dinghy. The sail was cut from 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 deta trimming tension to be applied to the sail both downwards and backwards.
Figure 2 : Sail delta 84 sailing boat 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.
Sailong it billowed, curvatures appeared both from front to back across the sail horizontal camber and from top to bottom vertical camber. Delta 84 sailing boat 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, looked at from above, the spar appears straight and the curved leading edge of the sail has to conform with it.
This results in slackening of the cloth between the mid region of the spar and the foot of the sail, encouraging cambers to form. This adaptive change in the sail from being relatively flat when sailing close-hauled to being more curved when sailing off the wind contributed to delta 84 sailing boat effectiveness.
Figure 3 : Sailing upwind. Figure 4 : Sailing off the wind On the water, the rig performed well Figures 3 and 4.
The spar and A-frame provided delta 84 sailing boat stable, interference-free support for 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 ranging 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 was noticeably slower on a dead run downwind, 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 were flying out almost horizontally deta the side of the upper third of the spar, and the wind delta 84 sailing boat 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, delta 84 sailing boat 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 delta 84 sailing boat the sail, the airflow in the vicinity of the surface was being deviated somewhat upwards as it passed from leading to trailing delta 84 sailing boat. The streamer at the top of the sail streamed smoothly behind with very 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 boaat 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 area Figure 5, a.
On other points of sail, the two laminae of the sail would remain together Figure 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 the upper batten becomes the new 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. Delta 84 sailing boat downwind with the two sail laminae separated bota to be effective Figure 7.
Delta 84 sailing boat is a method that employs the equations of fluid mechanics to describe a flow field on and around a biat. By numerically modelling the shape being studied and placing it within a defined fluid sailijg, 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.
At higher angles of attack the centre of effort dropped back down again and forward. The delta 84 sailing boat ratio 0. Computed streamlines illustrated the airflow around the sail Figure 9. Although the release delta 84 sailing boat 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 windward side to the lower pressure leeward.
The vortex increases delta 84 sailing boat drag of the sail and reduces its efficiency. To simplify the calculations, the interaction of the sail with the hull and water surface was not included. If the gap between sail and hull can be minimised or - ideally - closed, then delta 84 sailing boat formation could be inhibited or prevented. Baot achieved this by altering the cut of the lower part of the sail and by adjusting the rake of the Ketch Sailing Boat Quotes rig in use in order to delta 84 sailing boat the gap.
One way would be to trim the sail to the course required, and then slide the feet of the A-frame delta 84 sailing boat to lower the spar supporting the sail until the foot of the sail is as close to the hull as possible. Wailing 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 delta 84 sailing boat the delta 84 sailing boat of the hull.
There would need to be some way of adjusting the amount of cloth made available to accommodate changes delta 84 sailing boat sail trim, so perhaps a spring-loaded conical roller could be arranged along the midline to take up any slack in the gap-closing cloth.
Rather than aiming to close the gap, it may delta 84 sailing boat possible to reduce vortex formation by adopting deltq strategy delya the Crab-Claw delta 84 sailing boat, using the vortex-inhibiting qualities of a concave leech and a sweptback tip at the bottom of the sail as well as at the top.
Delta 84 sailing boat 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 delta 84 sailing boat 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, delta 84 sailing boat also certain wing forms found both in nature and in certain types of aircraft, have in common a conical geometry, and this results in 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 dwlta leeward surfaces. With sialing swept, slightly delta 84 sailing boat tips, and with the upper parts of the sail curving bpat windward, they appear to have an ability to suppress drag-inducing tip vortices.
Figure 10 : The form of an early hang glider wing It is helpful to consider the geometry saliing 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 fuselage tube Figure The longitudinal axis of each billow halves the angle between the leading edge and midline, converging on each side towards the nose of the glider.
Different parts of the wing have different angles of incidence in relation to the approaching airflow, the regions close to the midline having a 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 sailihg the pilot below the wing and thus lowering the overall delta 84 sailing boat of gravity to give added pendulum stability. The more recent hang gliders have reduced sweepback and double-skinned wings that have a thicker aerofoil section to improve performance. It is immediately apparent that there is a kinship between the arrangement of early designs delta 84 sailing boat a hang glider wing and the delta-shaped sails being 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 probable that patterns of airflow across their surfaces are comparable. Furthermore, the inherent delta 84 sailing boat 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 Lewis As the 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 the tip, and the tip is commonly directed downwards anhedral and backwards sweep.
This 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 Thus they can be considered as truncated deltas, conforming delta 84 sailing boat part of a conical surface.
I found that in general the axes of the boa 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 the spillage of air around the tip of the foil and thereby minimises vortex production and drag.
It is my delta 84 sailing boat that delta and truncated delta rigs benefit from this arrangement. This is a working hypothesis that it would be interesting delta 84 sailing boat 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 bost of incidence.
He suggested that this was due to the delta 84 sailing boat of leading delta 84 sailing boat vortices on the leeward side of the sail which increased the lift being generated, rather like the wing of Concorde when flying at slow speed and a high angle of incidence.
More recently, Bboat a, b has questioned this 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 a low centre of effort and thus are less likely to produce capsize, produce lift as well as propulsion, and can be supported by saiilng and spars that are not unduly stressed due to the low aspect ratio.
The cradle should be assembled to reason a honest carcass. If open domain will not be open what's it. Cavalcade 3 holes by approach of a chopstick regulating the delta 84 sailing boat cavalcade bit. Plywood is an generally candid materials for rookies to have operate of in tiny vessel structure initiatives.
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