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The boat will accelerate until the force pushing the boat forward is balanced by the drag force pulling the boat back, and then the boat will travel at a constant speed. Labels Fluid dynamics Force and Motion sailing.
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Making homemade ice cream. We've since updated this article to include the science behind vegan ice cream. The further out the leeward foil arm is canted � essentially more raised � the closer the AC75 flies to surface and, crucially, the more righting moment is generated as the hull and rest of the boat gets further from the lifting surface of the foil.
There is another positive to this: as the lifting foil is angled, it produces lift to windward, which can force the boat more towards the wind than the angle it is sailing.
As the foils work to provide stability to the boat when it is stationary both foils are dropped all the way down to stop it tipping over and to provide massive amounts of righting moment, they are incredibly heavy. A pair of foil wings and flaps excluding the one-design foil arm which attaches them to the boat and lifts them up and down weigh kg. To put that into perspective, the entire boat itself with all equipment but without the crew weighs between kg and kg. It is partly due to this that you will see some teams with bulbs on their foils.
If you decide to go for a skinny foil wing which would be low drag and so faster then there will not be enough volume to cram sufficient material in to make the foil weigh enough. So some teams have decided to add a bulb in order to make it weigh enough but to also keep a less draggy, slimmer foil shape.
As with everything on the AC75, the mainsail is a relatively new concept. It consists of two mainsails which are attached to both corners of a D-shaped mast tube.
This has the effect of creating a profile similar to a wing. But there are drawbacks with a wing: they cannot be lowered if something goes wrong and require a significant amount of manpower and a crane to put it on or take it off a boat. One reason a wing makes for such a powerful sail is that the shape can be manipulated from top to bottom fairly easily with the right controls.
With the AC75 the designers wanted a sail that could have some of this manipulation, produce similar power but could also be dropped while out on the water. In addition to the usual sail controls, within the rules, the teams are allowed to develop systems for controlling the top 2m of the mainsail and the bottom 1.
What this means is that the teams are able to manipulate their mainsail in a number of different ways to develop power and control where that power is produced in the sail.
But it also means that they have the ability to invert the head of the sail. The advantage here is that instead of trying to tip the boat to leeward, the very top of the sail will be trying to push the boat upright and so creating even more righting moment.
In this case, the forward sweep would have somewhat of a canceling effect on the increased upwash due to taper. Raking the mast back increases sweep and will cause additional upwash on the top of the sail, necessitating more twist to the sail. Genoas and jibs are very tapered and swept. Those two features, combined with the already twisted apparent wind, cause significant upwash toward the head of the sail.
Each sail by itself is much simpler than the combination of a foresail and mainsail as in the sloop rig. The sails are operating so close to each other that they both have significant interaction with the other. The most interesting feature of this is that the two sails together produce more force to pull the boat than the sum of their forces if they were each alone.
Earlier, upwash was identified as the increase in flow angle immediately upstream of a wing. There is also a corresponding change in angle, called downwash, just behind a wing, where the flow leaving the wing has been turned to an angle lower than the original flow.
The mainsail of a sloop rig operates in the downwash of the forward sail, causing the flow angle approaching the mainsail to be significantly reduced from what it would be otherwise. This decreases the amount of force that the mainsail produces. The foresail of a sloop rig operates in the upwash of the mainsail.
The wind as far upstream as the luff of a genoa is influenced by the upwash created by the mainsail. Hence, a jib or genoa in front of a mainsail has a higher flow angle than it otherwise would have by itself, causing an increase in the amount of force that the forward sail produces. So, while the mainsail is experiencing detrimental interference from the foresail, the foresail benefits from the interference of the mainsail.
Notice that more air is directed around the curved leeward side of the foresail. This causes higher velocity lower pressure and more force.
The net result is that the total force of the two-sail system is increased, with the foresail gaining more than the mainsail loses. This is the same phenomenon from which a foresail of a sloop rig benefits. This angle represents the difference in upwash on the foresail and downwash on the mainsail due to each other. On a masthead rig, where the forestay is attached to the top of the mast and both sails taper to basically zero chordlength at their heads in a similar fashion, the interference effects of the sails on each other are similar along the entire height of the mast.
A fractional rig has the more complicated characteristic that the top of foresail is not as high as the top of the mainsail. This means that the top of the foresail is very close to the front of the mainsail at a height where there is still an ample amount of chordlength in the mainsail. As the foresail luff approaches the mainsail luff, the upwash on the foresail due to the mainsail increases, because the low pressure behind the mainsail has more affect the closer the flow gets to it.
The top of the main on a fractional rig extends well above the foresail, leaving the upper portion of the mainsail free to experience the apparent wind without the downwash interference of the foresail.
Apparent wind toward the top of the mast comes from a much higher angle, so the mainsail above the foresail experiences much higher wind angles than the lower portion of the mainsail where the genoa is causing substantial downwash. Reviewing all of the affects so far reveals that both sails experience increasing flow angle with height. The foresail operates in the twisted flow of the apparent wind, with upwash induced by itself due to taper and sweep, and in the upwash field of the mainsail.
The mainsail is operating in the same twisted apparent wind, with additional upwash caused by its taper, but somewhat lessened by its forward sweep. It is also flying in the downwash field of the foresail, which is probably twisted because the foresail flies in a twisted fashion. This is particularly exaggerated with a fractional rig. With the flow directions established, it is now useful to consider the ramifications of sail shape. Previously, it was stated that a sail section exists solely as.
Now it is interesting to explore the differences in camber that are possible and what would be most beneficial. Since a sail Small Wooden Sailing Boats Works is constructed of flexible material, its cambered shape is supported by the pressure difference that it generates. It follows that the leading edge entry angle of the sail must be reasonably aligned with the incoming flow angle. If the entry angle is too high the sail will luff, and if it is too low the sail will stall, since the flow would be required to turn an impossibly sharp corner around the luff.
It is also apparent that the entry angle should increase with height to match the twisted flowfield. There are two remaining issues. Where should the trailing edge be, which defines the angle of attack at each height, thus twist? What path to take to get there, or what should the specific cambered shape of the sail be? The trailing edge location in relation to the leading edge locations establishes the angle of attack of a particular section.
This would achieve the highest angle of attack and hopefully the most lift, but unfortunately the ability to trim a sail to unlimited angles of attack is not possible. Eventually at some angle of attack, sail sections and airfoils experience stall. This occurs when the air flowing around the leeward side of the sail no longer travels on the surface of the sail. The flow separates from the sail resulting in a large loss in lift.
Depending on the shape, stall can occur abruptly with a small increase in angle of attack, or more gradually with some indications that the flow is separating from the surface at specific locations first. This is easily seen using telltales tufts of yarn that swirl erratically when the flow departs from the desired direction instead of streaming aft when the flow is attached to the sail.
Separation occurs simply because the pressure gradient that the flow is trying to pass through is too extreme. Recall that lift is generated because the flow accelerates around the convexly curved leeward surface of the sail creating low pressure.
Eventually, as the flow approaches the back of the sail, the flow must slow down to near its original speed and pressure, since after it leaves the sail it will return to its original state when the sail is no longer there to influence it. This is referred to as pressure recovery. Another way to think about this is that when air flowing over the leeward side of the sail and air flowing over the windward side of the sail reach the trailing edge, they must have the same pressure, as there will not be anything in between anymore to enable maintaining different pressures.
It does not mean that two particles of air that start at the leading edge and travel along different sides of the sail will arrive at the trailing edge at the same time a common misconception. It simply means that the pressure of air flowing off the top right at the trailing edge of the sail will be equal to the pressure of air flowing off the bottom. This must be true since they are coincident there. That pressure is generally close to the original pressure of the flow prior to being disturbed by the sail.
So, accelerated flow around the leeward side slows down toward the leech in order to provide the necessary matching at the trailing edge as the air is returned toward its original conditions. Overall, air still travels much faster around the leeward side of the sail than the windward side. As the flow slows down from its accelerated state on the leeward side it yields a pressure gradient along the back of the sail that is increasing from very low pressure to produce the desired lift to a much higher pressure toward the leech.
The amount of initial acceleration dependent on angle of attack and shape and the length of the pressure recovery determine how steep this gradient is. When the increase in pressure that the flow is experiencing becomes too extreme, the flow no longer stays attached to the surface of the sail. It is pushed away by the higher pressure and stall occurs, yielding less lift. It is favorable to slow the flow in a smooth fashion over a longer distance so that there is no steep rise in pressure.
This happens most effectively over a long, straighter shape aft, lacking in curvature that would attempt to promote higher velocity. As the air flows around both sides of the sail, its pressure changes with the varying local velocity caused by the curvature of the sail. With the entry angle defined by the oncoming flow direction and the angle of attack governed by avoiding stall, there are still numerous sail shapes that can be established to connect the luff and the leech.
Since the purpose of the sail is to develop force to move the boat in a forward direction, it would be most effective to have as much of the sail as possible operating with the largest possible pressure difference across it. The way to achieve that is to accelerate the air quickly around the curved leading edge of the sail in order to generate low pressure on the leeward side close to the luff and then maintain it back over a significant portion of the sail.
This is achieved by imparting high curvature to the front of the sail. Once the flow is accelerated curvature can diminish and the flow will continue quickly around the leeward side of the sail.
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