How does sail and sign work

Since a sail has essentially no thickness, it exists only as camber. The flow over the convex leeward side has reduced pressure through accelerated flow and the flow over the concave windward side has increased pressure through decelerated flow. The difference in pressure across the sail holds the flexible sail into its cambered shape and produces force to pull the boat.

An airfoil developing lift causes the flow approaching it to bend upward. This is because the lower pressure on top of the airfoil pulls air up toward it. This upward change in flow angle is called upwash. In addition to the upwash that an airfoil causes on itself due to the lower pressure on top influencing more air to flow over it, additional upwash occurs due to changes in the planform of the wing.

This is because, just as the low pressure on top of the wing influences the air some distance upstream to move upward toward it, that low pressure also influences air a similar distance away in the spanwise direction to alter its direction. This causes variations in upwash along the span of the wing on adjacent sections. This is because an airfoil generates much more lift in its forward portion than it does aft, so using the quarter-chord line as a reference is a convenient manner to characterize the sweep of a wing.

Sweep has the effect of increasing the upwash on the outboard wing sections. As a wing is angled aft, flow over the outboard sections must pass by the low pressure on top of the wing sections immediately inboard and forward.

The close proximity of that low pressure to the air just outboard causes the outboard flow to turn upward more, resulting in higher upwash on the outboard wing. Taper is defined as the ratio of the chordlength of the tip divided by the chordlength of the root. For sails, where the head tapers to nearly a point, the taper is extreme zero , resulting in a triangular planform.

A tapered wing has a much shorter tip section than root section. As the wing tapers, lift produced by the shorter outboard sections is less because they have less surface area to support lift. Since the outboard sections are smaller than the inboard sections, they are significantly influenced by the larger wing just inboard.

Air approaching the outboard portion of the wing is deflected by the low pressure on top of the larger inboard wing that is still generating a large amount of lift only a short distance away. The close proximity of that low pressure to the outboard wing causes the flow to be pulled upward additionally over the outboard wing.

Hence, the smaller outboard sections operate with higher upwash. This enhances the amount of lift that they produce but does not make up for their loss of area.

Identifying the flow conditions that sails operate in is very useful for understanding how they work. The wind blows over the surface of the earth and, as with any fluid flowing over a surface, has friction with it.

This friction slows the air closest to the surface and through shear causes the air immediately above it to slow some, too. This effect continues upward until at some distance above the surface the air is all moving at a similar speed. This behavior is called the boundary layer. While it occurs at a very small scale in the water flowing along the surface of hulls and keels, it occurs at quite a large scale in the air flowing over the earth.

This means that the true wind speed is increasing up the entire height of a mast. Apparent wind is the wind velocity experienced by the sails on a moving boat. This is the wind speed and direction that can be directly measured felt from the boat while it is moving. It is a combination of the true wind and the wind generated by the motion of the boat. The figure shows how these two wind components are added to create the apparent wind.

Notice that the apparent wind vector at the bottom of the rig, where the true wind speed is slower, is shorter slower and angled from a more forward direction, than the apparent wind vector at the top of the rig, where the true wind speed is faster. This variation in true wind speed not only causes the variation of apparent wind speed with height, but also its variation in angle. This is because all of the mast and sail are moving at the same speed and in the same direction as the boat across the moving air.

Since the wind solely due to the movement of the boat is identical at all heights, the apparent wind speed and direction resulting from its addition to different true wind speeds at various heights is different. While in this example the true wind velocity only varied in strength with height, it is possible that a variation in true wind direction can occur with height.

In that situation, each tack will experience different apparent wind twist than the other. The increase of apparent wind angle with height is a factor that influences a sail to fly in a twisted manner, where the top is angled more offcenter from the boat than the bottom. Other factors affecting how much twist is appropriate are sweep and taper as they alter the amount of upwash along the span of the sail. A mainsail by itself cat rig is tapered, but if the mast is close to vertical is actually swept forward.

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 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.

Just follow directions and it will show your purchases. Important Note: A hold will be placed on the credit card or checking account debit card at the beginning of the cruise as a deposit for an onboard purchases. This will reduce the amount of available credit on the credit card or restrict the availability of cash in the checking account. The hold will be released and actual charges will be billed at the end of the cruise. You can mosey on over to the Front Desk anytime and inquire as to your balance The purposes of using credit cards instead of cash, and initiating automatic tipping, are to relieve the long lines at the Front Desk.

Who wants to spend minutes every day of their "vacation" standing in line at the "bank"? Also, your debit card must have a "VISA" or other logo on it, or it won't be accepted. Also, the ship may put a "hold" on the funds, based on what your credit limit is. Sorry, I misread the Carnival brochure I was looking at the cruise deposit, not the Sail and Sign deposit. I found the sign and sail card info on the carnival website We're planning to put up a cash deposit If I don't go over the amount I put down as a deposit I won't have to go to the front desk at all.

I often wondered if we were the only ones that did this Agree that they make cool souvenirs.. From your other thread that you started you indicated that you wanted to use cash. Now you say you want to use a Debit Card. That's even worse. Alot of things are not adding up here. Well, who does? Instead of the final charges going to the credit card on file, can I pay off our balance in cash instead?

We are light drinkers and we plan to book our excursions through private companies. And since we are Canadian, I can get cash at a better exchange rate then using our credit card. Pay close attention daily to your account balances. They Carnival sometimes make mistakes like all of us and put charges to the wrong accounts. Also when your balance gets low don't know what that magic number is they will page you to the pursers desk and request more funds be deposited.

If you have none available, they will close your account. If you still have funds available on the last night, go down to the pursers desk and ask to close out your account. They will provide you with either a check or with cash of your remaining balance. I'm also wondering about this. Since we also are Canadians, we would prefer spending all our US money while on the cruise, not bring some back, although we could use it on another trip. Carnival Cruise Lines. October 19, Log In Issues.

Prev 1 2 Next Page 1 of 2. Recommended Posts. Posted July 9, How does the Sail and Sign card work? Link to comment Share on other sites More sharing options SheriNtexas Posted July 9, Can you change your prepaid gratuity amount that is charged if you want to tip in cash instead??