The Downhaul

The downhaul is a line that pulls downward on a sail, spar, or fitting. On a spinnaker pole, the downhaul prevents the pole from rising under the upward pull of the spinnaker's lift. On a Laser dinghy, the cunningham doubles as the downhaul. The term is general — any line whose function is to pull a component down against forces that would lift it. The downhaul works in opposition to the topping lift, which holds things up. Together, the downhaul and topping lift define a vertical window within which the controlled element operates.

Guy wires on a radio tower serve the downhaul function laterally. The tower wants to lean under wind load. Guy wires radiating from the tower to anchor points on the ground resist this tendency, pulling the tower back toward vertical. Each guy wire is a downhaul in a different direction — not pulling downward per se, but pulling against the direction of the force that would displace the tower. The wires do not hold the tower up. They hold it still. The tower supports its own weight; the guys prevent lateral displacement.

In personal finance, an automatic savings deduction serves the downhaul function on spending. Income enters the household, and spending forces tend to lift expenditure toward the total available income — Parkinson's law of expenses. The automatic deduction pulls money out of the spending stream before it can be spent, preventing the household's expenditure from rising to meet its income. The deduction does not increase income or reduce desire. It pulls a portion of the flow downward into savings, against the upward pressure of consumption. Like the downhaul, it works by opposition — applying a constant countervailing force against the natural tendency of the system.

The downhaul is the principle of countervailing restraint — a force applied in direct opposition to a system's natural tendency to drift. The downhaul does not control the system's direction or speed. It controls only one axis of displacement, and always in the direction opposite to the drift. The downhaul acknowledges that the forces causing the drift will not stop, and therefore the restraint must be continuous. Remove the downhaul and the pole rises, the tower leans, the spending expands. The downhaul is the steady pull that keeps a system within its operating range, not by eliminating the displacing force but by matching it.