JRC JLN-740N Doppler Log

Complete Marine Technical Guide for Accurate Vessel Speed Measurement

Product Identification

The JRC JLN-740N Doppler Log is an advanced marine navigation instrument designed to provide accurate vessel speed and distance information using Doppler ultrasonic technology. It measures Speed Through Water (STW) and supplies essential navigation data to ECDIS, AIS, Radar, Autopilot, VDR, and Integrated Bridge Systems (IBS).

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Product Introduction

Accurate vessel speed information is critical for safe navigation, route monitoring, collision avoidance, and voyage optimization. The JRC JLN-740N was developed as a modern Doppler speed log solution capable of delivering highly reliable speed data under various sea conditions.

The system utilizes ultrasonic signals transmitted from a hull-mounted transducer. By measuring the Doppler frequency shift of reflected signals, the JLN-740N calculates vessel speed relative to the surrounding water.

The equipment is commonly installed aboard:

• Container Ships
• Oil Tankers
• Chemical Tankers
• Bulk Carriers
• LNG/LPG Carriers
• Offshore Support Vessels
• Passenger Ships
• Naval Vessels

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Main Product Image

The image above shows the JRC JLN-740N display unit, which provides real-time speed, distance, and operational information to bridge personnel.

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What Is a Doppler Log?

A Doppler Log is a marine navigation instrument that measures vessel speed using the Doppler Effect.

The system emits ultrasonic signals into the water beneath the vessel. When these signals reflect from suspended particles or the seabed, the frequency shift is measured and converted into speed information.

Unlike GPS-based speed measurement, a Doppler Log provides:

• Speed Through Water (STW)
• Distance Run
• Longitudinal Speed
• Transverse Speed (depending on configuration)

This information is vital for accurate navigation and vessel maneuvering.

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Doppler Measurement Principle

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Main Features

High Accuracy Speed Measurement

Provides precise Speed Through Water information.

Ultrasonic Doppler Technology

Utilizes proven Doppler signal processing for reliable measurements.

Distance Counter

Automatically calculates cumulative distance traveled.

Digital Display Unit

Provides clear and user-friendly navigation data.

Self-Diagnostic Functions

Continuously monitors system performance and operational status.

Multiple Interface Outputs

Supports integration with:

• ECDIS
• Radar
• AIS
• VDR
• Autopilot
• Integrated Bridge Systems

Compact Bridge Installation

Designed for easy installation on modern navigation consoles.

IMO-Compliant Navigation Equipment

Suitable for SOLAS-regulated vessels.

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Display & Operator Interface

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Main Components

1. Display Unit

Located on the bridge.

Functions

• Speed Display
• Distance Monitoring
• Alarm Display
• System Configuration

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2. Signal Processing Unit

Processes ultrasonic signals received from the transducer.

Functions

• Doppler Calculations
• Signal Processing
• Data Output Generation

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3. Hull-Mounted Transducer

Installed below the waterline.

Functions

• Ultrasonic Transmission
• Signal Reception
• Speed Measurement

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4. Interface Module

Provides communication with onboard navigation systems.

Supports

• ECDIS
• AIS
• Radar
• VDR
• Autopilot

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5. Power Supply Module

Provides regulated power to the complete system.

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System Components

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Marine Applications

The JLN-740N is used for:

Speed Through Water Measurement

Provides accurate navigation speed information.

Voyage Monitoring

Supports route monitoring and voyage optimization.

ECDIS Integration

Supplies speed data for electronic navigation systems.

Autopilot Support

Improves steering system performance.

Dynamic Positioning Support

Provides vessel motion information.

Collision Avoidance

Enhances navigational awareness.

Voyage Data Recording

Supplies speed data to VDR systems.

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Common Problems & Troubleshooting

1. No Speed Indication

Possible Causes

• Transducer failure
• Power supply issue
• Signal processor fault

Corrective Actions

• Verify power supply
• Inspect transducer connections
• Check processor status

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2. Unstable Speed Reading

Possible Causes

• Air bubbles beneath hull
• Excessive turbulence
• Marine growth on transducer

Solution

• Inspect transducer location
• Clean transducer face
• Verify installation condition

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3. Incorrect Speed Display

Causes

• Calibration issue
• Sensor fault
• Signal interference

Solution

• Verify calibration settings
• Check signal processor
• Inspect communication wiring

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4. Communication Failure

Symptoms

• No speed data on ECDIS
• AIS or Radar not receiving speed input

Solution

• Verify NMEA outputs
• Check communication ports
• Inspect interface cables

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5. Distance Counter Not Updating

Causes

• Processor fault
• Software issue
• Configuration problem

Solution

• Restart system
• Verify configuration settings
• Check processor diagnostics

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Maintenance Tips

Proper maintenance ensures long-term operational reliability.

Daily Checks

• Verify speed indication
• Review alarm status
• Confirm display operation

Weekly Checks

• Inspect interface connections
• Verify communication outputs
• Review system diagnostics

Monthly Checks

• Verify calibration values
• Test navigation interfaces
• Inspect system logs

Dry Dock Maintenance

During dry dock periods:

• Clean transducer face
• Inspect transducer housing
• Verify mounting integrity
• Check cable condition

Annual Maintenance

Recommended:

• Functional testing
• Calibration verification
• Interface testing
• Performance diagnostics

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Why Ships Still Use Doppler Logs

Despite the availability of GPS speed information, Doppler Logs remain essential because they provide:

Speed Through Water (STW)

Critical for navigation calculations.

Independent Speed Source

Provides redundancy during GPS failures.

Improved Autopilot Performance

Supports more accurate steering control.

Better Maneuvering Accuracy

Important during docking and restricted-water navigation.

SOLAS Compliance

Required aboard many commercial vessels.

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Frequently Asked Questions (FAQ)

What does the JRC JLN-740N measure?

It measures vessel Speed Through Water (STW) and distance traveled using Doppler ultrasonic technology.

Is it different from GPS speed?

Yes. GPS measures Speed Over Ground (SOG), while the JLN-740N measures Speed Through Water (STW).

Can the JLN-740N connect to ECDIS?

Yes. The system provides navigation data outputs compatible with ECDIS and Integrated Bridge Systems.

What happens if the transducer becomes fouled?

Speed accuracy may decrease and unstable readings may occur.

Is calibration required?

Yes. Periodic verification and calibration checks are recommended during maintenance periods.

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Short Product Description

The JRC JLN-740N Doppler Log is a professional marine speed measurement system designed to provide accurate Speed Through Water and distance information using advanced Doppler ultrasonic technology. Featuring digital displays, self-diagnostics, navigation system integration, and reliable performance, it is widely used aboard commercial and offshore vessels worldwide.

SM MARINE SERVICE™

Marine Automation & Navigation Equipment Supplier

Available Condition:

• New Equipment • Used Equipment • Refurbished Equipment • Reconditioned Equipment

Stock Location:

• Bangladesh • China

Services:

• Worldwide Shipping • Technical Support • Spare Parts Supply • Marine Equipment Sourcing

Contact Information

SM MARINE SERVICE™
Samir Market, DT Road, Pahartali, beside CDA Market, Chattogram, Bangladesh

Mobile / WhatsApp / Viber: +88 01818 929292

Email: info@smmarineservice.com

Website: www.smmarineservice.com

 

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