The spinnaker is very similar to an oversized, somewhat inefficient head-sail. The wind is attempting to attach to the sail on both sides, but because
of the large chord depth, the wind has a hard time making the turn, so
Wind flows onto the leading edge of the sail and attaches, but at the deepest part it cannot take the bend, and so it separates.
for the second half of its journey across the spinnaker it is separated from the surface.
Wind Direction attach to the sail on both sides, but because of the large chord depth of the spinnaker, the wind has a hard time making the turn, so for the second half of its journey across the spinnaker it is separated from the surface. This separated air causes the sail to drag the boat sideways rather than to provide lift, and if the sailor is trying to carry the spinnaker too close to the wind, the result will be massive leeway and very little lift. Therefore, the flatter the spinnaker, the more efficient it will be for reaching. You might even wonder why it's worth setting the spinnaker in the first place when it's such an inefficient sail for reaching. The reason is that the large increase in sail area still helps with boat speed unless you are trying to carry it too close to the wind and going sideways. Again, like many things in sailing, it's a delicate balance. Note that some of the wind's energy is being used simply to lift and fill the sail so the rest can be translated into boat speed. For this reason, as we will learn under sail trim, bigger is not always better, especially in light winds, since getting a sail to lift and fly away from the disturbed air at the back of the mainsail is key to good spinnaker trim.
As the wind comes aft, air still continues to flow across the sail, but now the amount of projected area becomes a bigger factor than attached flow. As shown in Figure 8.6, the elliptical shape of a spinnaker is hardly an efficient foil for providing lift, but once the wind comes aft that inefficiency becomes less of an issue. At some point the spinnaker is squared so far aft that it is no longer affected by the dead air in the lee of the mainsail. The point at which this happens differs from boat to boat, but usually around an apparent wind angle of 110 degrees for masthead sails, and 120 degrees for fractional sails, the spinnaker is working on its own, clear of the mainsail. This is the point where wind begins to enter the spinnaker from both sides and flow toward the center of the sail causing a pocket of dead air in the middle. This dead air is forced down and out the bottom of the sail by new air being sucked in at the edges. When you are running downwind with a spinnaker set you can often see the water below the sail rippling from the downdrafts of air. When the boat is sailing dead downwind there is no attachment on the leeward side of the sail. The angle at which the wind hits the edges of the sail is so abrupt that it cannot gain a hold on the surface, and it spins in a vacuum in a series of small vortices.
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