Ships and submarines need to know how fast they are moving through the surrounding water, not just how fast they are moving over the seafloor. This matters for navigation, fuel use, steering, collision avoidance, and mission planning. A vessel can have a high speed through water while making little progress over ground if it is fighting a strong current.
Marine speed logs are instruments designed to measure this motion in a reliable way.
Key Facts
- Speed through water is the vessel speed relative to the water around the hull.
- Speed over ground is the vessel speed relative to Earth or the seafloor.
- If current is along the track, v_ground = v_water + v_current.
- A pitot log estimates speed from pressure difference: ΔP = 1/2 ρv^2.
- A Doppler log uses frequency shift: Δf = 2fv/c for motion directly along the beam.
- Distance traveled through water can be estimated by d = vt when speed is constant.
Vocabulary
- Speed through water
- The speed of a ship or submarine relative to the water immediately around it.
- Speed over ground
- The speed of a vessel relative to Earth, usually measured by GPS or bottom tracking.
- Paddle log
- A speed log that uses a small rotating wheel turned by water flow past the hull.
- Pitot log
- A speed log that estimates speed by comparing dynamic pressure from moving water with static pressure.
- Doppler log
- A speed log that measures speed from the frequency shift of sound reflected by water particles or the seafloor.
Common Mistakes to Avoid
- Confusing speed through water with speed over ground is wrong because currents can make these values different in both size and direction.
- Ignoring current direction is wrong because a 2 knot current helps the vessel if it flows with the track and slows it if it flows against the track.
- Using pitot pressure without squaring the speed relationship is wrong because dynamic pressure increases with v^2, not directly with v.
- Assuming a Doppler log always measures ground speed is wrong because it may track water particles in deep water or the seafloor in bottom-track mode.
Practice Questions
- 1 A ship moves at 12 knots through the water while a 3 knot current flows in the same direction. What is its speed over ground?
- 2 A submarine travels 8.0 m/s through the water for 45 minutes. How far does it travel through the water in meters and kilometers?
- 3 A vessel's paddle log reads 10 knots, but GPS shows only 7 knots along the same heading. Explain what the current is doing and estimate its speed.