Speed Log in Navigation: Types, Principle, Working,
Advantages and Disadvantages
Introduction
A speed log is an important navigational instrument used on ships
to measure the speed of a vessel and calculate the distance travelled. It is
useful in marine navigation, voyage planning, position estimation and safe
operation of vessels.
Different types of speed logs work using different principles. These
include mechanical rotation, pressure difference, electromagnetic induction,
Doppler effect and satellite navigation.
This article explains the types of speed logs, their principles, working, uses, advantages and disadvantages in simple language for B.F.Sc., fisheries science and marine navigation students.
What is a Speed Log?
A speed log is a navigational instrument used to:
- Measure the speed of a vessel.
- Calculate the distance travelled
by the vessel.
Uses of a Speed Log
Speed logs are useful for:
- Navigation
- Dead reckoning
- Voyage planning
- Position estimation
- Fuel and voyage management
- Safe operation of vessels
Easy way to remember
Speed Log = Speed + Distance
Speed Through Water (STW) and Speed Over Ground (SOG)
Two important terms associated with vessel speed are Speed Through Water (STW) and Speed Over Ground (SOG).
1. Speed Through Water (STW)
Speed Through Water is the speed of a vessel relative to the surrounding water.
It is useful in:
• Marine navigation
• Assessing the effect of water movement
• Calculating current-related drift
• Collision avoidance and navigation systems
2. Speed Over Ground (SOG)
Speed Over Ground is the actual speed of a vessel relative to the Earth's surface.
It is useful for:
• Chart positioning
• ECDIS navigation
• Passage planning
• Monitoring the vessel's actual movement over the ground
Difference between STW and SOG
Easy memory
|
Speed Through Water (STW) |
Speed Over Ground (SOG) |
|
Relative to surrounding water |
Relative to the Earth's surface |
|
Indicates movement through water |
Indicates actual movement over ground |
|
Influenced by water movement when compared with ground speed |
Represents movement relative to the ground |
|
Important for understanding vessel movement through water |
Useful for chart positioning and passage monitoring |
Units Used in Speed Measurement
Speed
The standard unit used for vessel speed is the knot (kn).
1 knot = 1 nautical mile per hour
Also:
1 knot = 1.852 km/h
Distance
Distance at sea is commonly expressed in nautical miles (NM).
Therefore:
Speed → Knot
Distance → Nautical Mile (NM)
Types of Speed Logs
The major types of speed logs are:
- Mechanical or Patent Speed Log
- Pitometer Log
- Electromagnetic Speed Log
- Doppler Speed Log
- GPS Speed Log
A simple way to remember them is:
M – P – E – D – G
Mechanical → Pitometer → Electromagnetic → Doppler → GPS
1. Mechanical or Patent Speed Log
The mechanical or patent speed log is one of the traditional types
of speed logs. It was mainly used on small vessels and older ships.
Construction
The main components include:
- Rotating impeller
- Towing line
- Gear mechanism
- Recording counter
Working
When the vessel moves through the water, the water causes the impeller
to rotate.
The rotation of the impeller is transmitted through the towing line
to a gear mechanism. The gear mechanism converts the rotation into a
measurement of the vessel's speed and distance travelled.
Working sequence
Water movement → Impeller → Towing line → Gear mechanism → Counter
Easy memory
Mechanical = Rotation
2. Pitometer Log
A pitometer log measures the speed of a vessel based on the difference
in water pressure.
Principle
The principle is based on the relationship between water pressure and
vessel speed.
A Pitot tube is fitted below the hull of the vessel. It has openings that
allow the measurement of dynamic and static pressure.
Working
When the vessel moves through the water:
- The forward-facing opening
measures dynamic pressure.
- The static opening measures static
pressure.
- The difference between these
pressures is measured.
- This pressure difference is
related to the vessel's speed.
- The speed is displayed by the
instrument.
Working sequence
Dynamic pressure + Static pressure measurement → Pressure difference →
Speed
Simple representation
SHIP
┌──────────────┐
│ │
└──────┬───────┘
│
PITOT TUBE
↓
Dynamic pressure
+
Static pressure
↓
Pressure difference
↓
SPEED
Easy memory
Pitometer = Pressure
3. Electromagnetic Speed Log
An electromagnetic speed log works on the principle of Faraday's
law of electromagnetic induction.
Principle
When seawater moves through a magnetic field, an electrical voltage is
induced. The induced voltage is related to the speed of the vessel.
Working
- A magnetic field is produced
around the sensor.
- Seawater moves through the
magnetic field as the vessel travels.
- An electrical voltage is induced.
- The electronic system processes
the signal.
- The vessel's speed is displayed.
Working sequence
Magnetic field → Seawater movement → Induced voltage → Electronic
processing → Speed
Exam keyword
Faraday's Law of Electromagnetic Induction
Easy memory
Electromagnetic = Voltage
4. Doppler Speed Log
A Doppler speed log (DSL) uses the Doppler effect to
determine vessel speed.
It is used on various types of vessels where accurate speed measurement
is required.
Principle
The principle is based on the Doppler effect, which involves a
change in the observed frequency of a wave due to relative motion.
Working
A transducer sends ultrasonic waves into the water.
The waves are reflected back from the seabed or suspended particles in
the water. Because of the movement of the vessel, there is a change in the
frequency of the reflected signal.
The instrument measures this frequency difference and uses it to
calculate the vessel's speed.
Working sequence
Ultrasonic waves → Reflection → Frequency change → Processing → Vessel
speed
Important exam keywords
- Ultrasonic waves
- Doppler effect
- Reflected waves
- Frequency difference
- Vessel speed
Easy memory
Doppler = Frequency
5. GPS Speed Log
A GPS speed log uses satellite navigation to determine the speed
of a vessel.
Unlike speed logs that use underwater sensors, GPS determines vessel
movement using satellite-derived position information.
Working
- GPS receives signals from
satellites.
- The vessel's position is
determined.
- The change in position is
monitored with time.
- Speed is calculated from the
change in position over time.
- The resulting speed represents
the vessel's Speed Over Ground (SOG).
Working sequence
Satellite signals → Vessel position → Position change with time → Speed →
SOG
Important point
GPS speed measurement gives Speed Over Ground (SOG).
Easy memory
GPS = Position
|
Type of Speed Log |
Main Principle |
Main Quantity/Effect |
|
Mechanical |
Mechanical rotation |
Impeller rotation |
|
Pitometer |
Pressure difference |
Water pressure |
|
Electromagnetic |
Faraday's law |
Induced voltage |
|
Doppler |
Doppler effect |
Frequency change |
|
GPS |
Satellite navigation |
Position change |
Five-word memory trick
Mechanical = ROTATION
Pitometer = PRESSURE
Electromagnetic = VOLTAGE
Doppler = FREQUENCY
GPS = POSITION
Remember these five words and the basic principle of each speed log
becomes easier to recall.
Advantages of Speed Logs
Speed logs provide several benefits during vessel operation and
navigation.
- They provide continuous speed
information.
- They improve navigation accuracy.
- They help estimate vessel
position.
- They support automatic navigation
systems.
- They can help in voyage and fuel
management.
- They improve operational safety.
- They reduce dependence on manual
speed estimation.
- They are easy to monitor during a
voyage.
Easy grouping
Navigation: Accuracy, position estimation and automatic navigation
Safety: Better monitoring and reduced human error
Operation: Continuous speed information and voyage/fuel management
Disadvantages of Speed Logs
Despite their advantages, speed logs also have certain limitations.
- Periodic calibration may be
required.
- Underwater sensors may be
affected by marine fouling.
- Modern electronic systems can be
expensive.
- Electrical failures can interrupt
operation.
- Water currents can affect the
relationship between speed through water and speed over ground.
- Regular inspection and
maintenance are required.
Easy memory
Calibration → Fouling → Cost → Electrical failure → Current → Maintenance
Speed Log:
A speed log is a navigational instrument used to measure the speed
of a vessel and calculate the distance travelled. It is useful in marine
navigation, voyage planning and position estimation.
The major types of speed logs are:
- Mechanical or Patent Speed Log
- Pitometer Log
- Electromagnetic Speed Log
- Doppler Speed Log
- GPS Speed Log
Different speed logs operate using different principles. The mechanical
log uses impeller rotation, the pitometer log uses pressure difference, the
electromagnetic log uses electromagnetic induction, the Doppler log uses the
Doppler effect and the GPS system uses satellite-derived position changes.
Basic principles
- Mechanical: Impeller rotation
- Pitometer: Pressure difference
- Electromagnetic: Faraday's law
- Doppler: Doppler effect
- GPS: Satellite navigation
Speed logs provide continuous speed information, improve navigation and
support safe vessel operation. However, they may require calibration and
maintenance, and some systems can be affected by marine fouling, electrical
problems and water conditions.
Frequently Asked Questions (FAQs)
What is a speed log?
A speed log is a navigational instrument used to measure the speed of a
vessel and determine the distance travelled.
What are the main types of speed logs?
The main types are mechanical, pitometer, electromagnetic, Doppler and
GPS speed logs.
What principle does an electromagnetic
speed log use?
An electromagnetic speed log works based on Faraday's law of
electromagnetic induction.
Which speed log uses the Doppler
effect?
The Doppler speed log uses the Doppler effect and measures changes
in the frequency of reflected ultrasonic signals.
What is the principle of a pitometer
log?
A pitometer log determines vessel speed using the relationship between water
pressure difference and vessel movement.
Does a GPS speed log measure STW or
SOG?
A GPS-based speed measurement provides Speed Over Ground (SOG).
What is the unit of ship speed?
Ship speed is commonly expressed in knots (kn).
How many nautical miles are travelled
at 1 knot in one hour?
At 1 knot, a vessel travels 1 nautical mile in one hour.
Quick Revision Table
|
Speed Log |
Remember This |
|
Mechanical |
Rotation |
|
Pitometer |
Pressure |
|
Electromagnetic |
Voltage |
|
Doppler |
Frequency |
|
GPS |
Position |
Final memory trick
M – P – E – D – G
Mechanical → Rotation
Pitometer → Pressure
Electromagnetic → Voltage
Doppler → Frequency
GPS → Position
This simple sequence can help students quickly recall the major types and
principles of speed logs during examinations.
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