Speed and velocity are basic physics terms with a real, non-trivial distinction — one is a plain number, the other includes direction, and the difference matters the moment you need to combine two motions.
Speed: Just a Number
Speed is a scalar quantity — a rate of motion with a magnitude but no direction. “The ship’s speed is 12 knots” tells you how fast, but nothing about which way.
Velocity: Speed Plus Direction
Velocity is a vector quantity — speed combined with a specific direction. “The ship’s velocity is 12 knots on a heading of 090°” tells you both how fast and which way, a genuinely more complete description of motion.
Why This Distinction Actually Matters in Navigation
The practical relevance shows up whenever you need to combine two motions — most commonly, a vessel’s own speed through water combined with a current’s speed and direction, to determine the resulting speed and course over ground.
You cannot correctly add “12 knots” (the ship’s speed) and “2 knots” (the current’s speed) as plain numbers to get “14 knots” unless they’re moving in exactly the same direction — if the current runs at an angle to the ship’s course, the combined effect requires vector addition (accounting for both magnitude and direction), not simple arithmetic addition. This is exactly the underlying principle behind set-and-drift calculations.
Where “Speed” Is Used Loosely
In everyday maritime speech, “speed” is used casually even when direction is implied by context (“full speed ahead” clearly implies a direction from context) — the more precise speed/velocity distinction matters specifically in calculations combining multiple motions, not in routine conversational use.
Exam Relevance
The speed/velocity distinction, and its application in vector-based navigation calculations (set and drift, relative motion in collision avoidance), is a foundational Navigation topic in deck officer competency exams.