A centerboard is a retractable, pivoted fin or plate located in a slot (centerboard trunk) within a sailboat’s hull, used to reduce leeward drift (sideways sliding) when sailing upwind. It acts as a movable keel, providing stability and lateral resistance, and can be raised for shallow water, docking, or reducing drag.

Key Features and Functions:
Retractable: It pivots upward and backward into a slot, allowing for easy shallow-water navigation, beaching, or trailer transport.

Lateral Resistance: Like a fixed keel, it acts as a hydrofoil to counteract the wind's force on the sails, preventing the boat from drifting sideways.

Pivot System: Unlike a daggerboard (which slides vertically), a centerboard hinges on a pivot pin (often at the forward end), allowing it to swing up.

Stability: Improves handling and stability when down, particularly for shallow-draft vessels, explains dictionary.com.

Usage: Typically lowered when sailing upwind (close-hauled) and raised to reduce drag when sailing downwind.

Centerboards are common in small sailboats, day sailors, and some larger "shoal draft" cruising boats designed to enter shallow waters.