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.
Understanding Ships and Submarines: Speed Through Water
A pressure log has an opening that faces the incoming flow and separate openings that sense the surrounding water pressure. Water is slowed near the forward opening, so its pressure rises. The instrument compares this higher pressure with the undisturbed pressure at the other openings.
From that difference, it calculates a speed. The relationship is not linear.
If the vessel moves twice as fast, the pressure difference becomes about four times as large. This makes the sensor useful at normal cruising speed, but small pressure readings at very low speed can be harder to measure accurately.
The hull changes the water flow before it reaches any sensor. A layer of slower water forms close to the hull because of friction. This boundary layer can make a log read slightly differently from the true flow farther away.
Sensor position matters for this reason. Turbulence from propellers, thrusters, stabilizers, or the bow can disturb the reading. Marine growth, paint, mud, and trapped air bubbles can cause further errors.
Crews calibrate a log by comparing it with a trusted distance run over a measured course, usually at several different speeds. The correction may be stored in the ship's navigation system.
A Doppler log works by sending sound into the water and listening for a returning echo. Tiny suspended particles, plankton, or bubbles scatter some of the sound back. Motion changes the received frequency by a small amount.
The electronics measure that change and calculate the component of speed along each sound beam. Most systems use several beams aimed in different directions. Combining their results gives forward speed and can reveal sideways motion.
In shallower water, a Doppler log may receive echoes from the seabed. This bottom tracking mode measures motion relative to the bottom.
In deep water, it normally relies on echoes from moving water instead. These two modes answer different navigation needs.
Students often meet the same ideas in river crossings, aircraft flying in wind, or a swimmer moving through a flowing pool. The important habit is to state the reference frame before doing any calculation. A current has both size and direction, so it must be treated as a vector.
A current from the side changes the track, not merely the reported speed. Navigation computers combine log readings, compass direction, depth data, and satellite position to estimate the current. They still need human checks.
A sudden disagreement between instruments may mean a changing current, a fouled sensor, poor acoustic echoes, or a system fault. Good navigation depends on understanding what each instrument actually measures.
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.