what a drag

what a drag

Most sailors are fully aware of friction drag: the resistance caused by the skin friction of a surface moving through a fluid. As the yacht (the surface) moves through the water (the fluid), it develops a boundary layer, or region of interrupted flow, surrounding the submerged surfaces. The water immediately touching the hull (the base of the boundary layer) is actually moving at the same speed as the hull. The hydrodynamic no-slip condition occurs all along the base of the boundary layer, due to the viscosity of the fluid. Further away from the hull surface, the water gradually returns to its normal flow, which occurs at the edge of the boundary layer. This transition of flow within the boundary layer causes inter-molecular shear stresses, which requires a certain force to overcome.

As the boat moves forward, it pushes water out of the way. Due to conservation of energy (remember that law from physics?), the moving boat must use some of its own energy to push the water out of the way. This loss of energy would tend to slow the boat down, which means that more energy is required to keep the boat moving at the original speed, in other words an increase in drag.