Rationale behind the sloop rig
No design is perfect for all conditions; sloops are designed to optimize upwind sailing. However, sloops also offer an excellent overall compromise acceptable, if not optimal, to all points of sail. It is clear that the most difficult direction to sail is to the windward; this requires some specific design features. The sail should be as vertical as possible to optimize the energy of the wind.

Sloop rig

Photo: marine-concepts.com

Two forces act on a vessel to push it from vertical: the weight of the rig itself will tend to heel the boat, and the sideways force of the wind on the sails. The sloop is a light rig with fewer lines and spars, and the sails on a sloop tend to be flat which minimizes sideways force when well trimmed. The heeling forces are also counterbalanced by the keel, which uses weight and hydrodynamics to offset the forces from the rigging and sails.

When sailing upwind, it is also important to minimize the drag of the wind on the sail and rig. A major cause of drag of the sail is a vortex of turbulent air generated by the top of the mast and sail. Secondary causes are non-optimal aerodynamic shapes of masts, stays and control lines. The sloop minimizes the drag of the tip-vortex with a high and narrow sail design, maximizing the amount of sail for a given tip-vortex compared to a square-rigged or gaff-rigged ship. Also, the simplicity of the rig reduces the drag induced by control lines, masts and spars.