Spinnaker - Operation

Operation

A spinnaker is extremely useful for sailing with the direction of the wind. A spinnaker is a type of airfoil and will generate lift if it is flown at a reaching angle. Since the lift and drag generated by the spinnaker both act to move the boat forward, lift to drag ratio is unimportant compared to their combined total. The goal then is to generate the maximum amount of lift possible with no consideration of drag. Because of this, running spinnakers have extreme amounts of camber, making them nearly hemispherical in form. The large camber maximizes the drag. Reaching spinnakers have less camber as they operate partially with an airflow that generates lift.

A well designed spinnaker will have taut leading edges when filled; leading edges that curl in will both reduce the lift and risk a collapse of the spinnaker. A well designed spinnaker will also have a smooth curve when filled, with no bubbles or depressions caused by inconsistent stretching of the sail fabric. Any deviations from a smooth curve will cause the airflow over the leeward side of the sail to separate causing a reduction in lift and reduced performance.

When running downwind in heavy weather or when hit by a gust, with or without a spinnaker, there may be a tendency for a roll of increasing amplitude to build up, known as the death roll. It has been shown that this is due to aerodynamic instability of bermuda rigs when running, which can be aggravated by gusts, waves, mainsail twist, daggerboard etc. too far down, hull form, and the sailing equivalent of pilot-induced oscillation. Excessive heel leads to loss of rudder effectiveness resulting in the boat slewing round uncontrollably in the direction opposite to the direction of heel. This is known as broaching. Aerodynamic instability when running can be countered by easing the pole forward slightly and over-sheeting the spinnaker somewhat to stop it swinging from side to side, by reducing mainsail twist using the boom vang, and by skillfully trimming the mainsheet. Luffing carefully onto a broad reach may help to retain control, as can moving everyone's weight as far aft as possible. Reducing sail should be considered.