Before we can put the two sails together, we need first to look at the mainsail alone. Like the jib it has its own flow pattern and area of circulation. For it to be an efficient foil, the flow of air must be attached to the sail at all times, and the Kutta Condition must be met for the air coming off the leech of the sail. In fact, the flow of air around a mainsail is much the same as that around the head-sail, although in the case of the main there is a large mast at the leading edge, and not a small headstay. The mast does have some effect, but as you will see later in this section, it's not a big one. Again the boundary layer at the aft edge attempts to make the sharp turn around the corner, but boundary layers being boundary layers, the air does not make the turn gracefully, and small vortices are created, which begin a circulation around the mainsail. Once again, this circulation increases the speed (and reduces pressure) on the lee side of the mainsail and decreases speed (increases pressure) on the windward side. Note, however, that if we add a headsail, the two areas of circulation overlap in the gap between the back end of the headsail and the area to leeward of the mast. This is the exact area where we used to think there was an increase in flow, but as you can see in Figure 15.18, the two circulations work against each other to dramatically reduce the flow.
THEART AND SCIENCE OF SAI LS
H=Streamline through headstay Sm=Main stagnation streamline
With the mainsail only, the stagnation point (Sm) connects with the sail on the windward side just aft of the mast.
In some ways this is a good thing. The problem with the old theory of the increased flow revitalising the air on the back side of the main was that the sudden increase in speed would soon have to face a sudden decrease in speed toward the back end of the sail to meet the Kutta Condition. As we discussed, the boundary layer does not like sudden decreases in speed and they tend to make it separate from the surface, which is exactly what we do not want to have happen. Fortunately no sudden increase in flow occurs and therefore no sudden decrease is necessary, and the boundary layer remains attached.
Let's look at Figures 15.19 and 15.20. These diagrams were generated using an analog field plotter by the aforementioned Arvel Gentry. Since his work is considered the best possible explanation of flow and lift, we will use his diagrams to illustrate what happens. With the mainsail only (see Figure 15.19), the stagnation point (Sm) connects with the sail on the windward side just aft of the mast. With the bulk of the mast in the way, the upwash, i.e., the air that is forced around the front of the foil, has to really speed up to make it around the corner, which now includes the mast. The result is fast-flowing air moving down the backside of the sail that will suddenly have to slow down to meet the Kutta Condition. The rapid slowing will likely result in separation. This is why a mainsail alone is not a very good foil for sailing to windward. I know that there are some yacht designers who believe that a mainsail alone is the way to go, but I have a hard time believing that it's true. Among other things, if this was the case America's Cup yachts could leave their headsails on the dock to save weight, but you don't see them doing that. It's generally accepted that it's important to have a headsail as well, so let's add one now.
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