A branch of the science of pneumatics which deals with air and other gases in motion and with their mechanical effects. In its maritime connection it can be used to explain how a wind produces forward motion in a sailing vessel even when it blows from before the vessel's beam.
When a wind strikes a surface at an angle, its force can be resolved into two components, one acting at right angles to the surface and the other along the surface. If this surface is the sail of a boat, the component blowing along the sail can be disregarded, as it is providing no force on the sail, but the component at right angles to the sail does exert a force. That component can now similarly be resolved into two more components, not in relation to the angle of the sail but to the fore-and-aft line of the boat. The larger of these two components exerts a force which tries to blow the boat directly to leeward, and the smaller of them, blowing along the fore-and-aft line, is all that is left of the wind to drive the boat forward. It is at this point that the boat's keel, or the centreboard in the case of dinghies, comes into play. It provides a lateral grip on the water which offers considerable resistance to the larger component and very little resistance to the smaller, so that the boat moves forward and makes only a small amount of leeway.
The aerodynamic forces on the sail, described above, arise from the flow of air over the sail and the changes in the pressures acting on the sail which this produces. In plain view, a section of a sail is roughly parabolic in form, with the steepest part of the curve at its luff. When an airstream strikes such a surface at an angle, it accelerates over the upper surface, thereby reducing pressure there. Accelerations are not so great over the lower surface so the overall effect is to create a force acting on the sail to leeward. Resolving this force into longitudinal and lateral components provides the driving and heeling/sideslipping forces mentioned above.
The greater the speed of flow over the upper surface of the sail, the greater the aerodynamic force, and the faster the yacht moves forward. With this in mind, modern sail plans increase the speed of the airflow by means of a foresail (or sails) so aligned to produce a slot or ‘funnel’ along the luff of the mainsail. The slot increases the flow by acting as a form of venturi (duct). In designs where the clew of the foresail overlaps the luff of the mainsail, the airflow is funneled with even greater speed over the steepest part of the mainsail's curve, thereby increasing the aerodynamic force.
Therefore, when a vessel is sailing close-hauled, with the wind blowing forward of the beam, the aerodynamic forces resulting from pressure changes over the sails allow it to move against the wind. These also cause heel, which is resisted by the crew (of a sailing dinghy) or the ballast keel (of a displacement yacht), and leeway which is resisted by hydrodynamic forces generated by the centreboard or keel.