blade

In marine work, a blade is the submerged flat or foil-section appendage that controls direction or stability by generating hydrodynamic force against water flow. The three main types are the rudder blade, which yaws the vessel; the daggerboard or centerboard, which resists leeway and provides lateral hold; and the keel blade, which combines ballast with directional stiffness. Each operates on the same principle: relative motion between blade and water creates pressure differential across its surfaces.

A rudder blade is typically hung from a stock (vertical shaft) and rotates 15 to 35 degrees either side of centerline; beyond that angle, stalling occurs and steering authority collapses. Daggerboards are raised and lowered vertically through a slot in the hull and are common on centerboard boats and traditional gaff-rigged vessels. Centerboards pivot on a pintle rather than sliding, offering quicker deployment and less water resistance when retracted. Keel blades are fixed, cast or fabricated as one with the structural keel or bolted to its lower edge.

Rudder blades in commercial vessels are built from cast iron, fabricated steel plate, or composite materials. Fiberglass blades are lighter and corrosion-resistant but demand careful design to avoid cavitation erosion at high speeds. The blade section must generate lift at small angles of attack yet resist damage from grounding, ice, or floating debris. Thickness varies from 10 to 25 millimeters depending on span and vessel size; a 30,000 ton bulk carrier's rudder blade may be 2 to 3 meters in chord.

Failure modes include fatigue cracking at the rudder stock junction, cavitation erosion on the suction face at sustained high speeds, and structural damage from sudden hard-over commands or collision. Corrosion proceeds rapidly at the waterline and weld seams on steel blades unless protected by cathodic protection or paint. Galling and seizure of centerboard slots requires regular cleaning and greasing, especially in salt water where galvanic couples form between dissimilar metals.

The term blade is borrowed from the appearance of a sword or oar edge: narrow, flat, and edged for cutting through the medium. In shipyards, blade is used interchangeably with fin, plate, or surface, though blade is preferred when emphasizing its control function rather than its structural role. Blade area, measured in square meters, is critical: too small and the vessel cannot be steered in heavy weather; too large and steering becomes sluggish and wave-induced vibration increases.