9-Mar-07 1 of 21

Section 3Section 3Speed Speed LogsLogs

9-Mar-07 2 of 21

Practically all ships carry a speed log or more, also there are..

Usual Nav Equipment requirements as per SOLAS Ch V, MO: 21 - complying with the relevant IMO Perf Standards.

• For all pax and other vessels ≥ 300GT:As a minimum – STW (speed thru water)

• If ≥ 50,000 GT– also SOG (spd over ground) and actual surface ‘attitude’ of the vessel

3.0 Introduction: the name - Log?/Knots?

9-Mar-07 3 of 21

3.0 Introduction: Log types

– Logs may be classified as• Towed log (virtually obsolete)• Bottom log

– Most common types:• Electromagnetic (Em)• Doppler

9-Mar-07 4 of 21

3.1.1 Principle of Operation:Principle is based on the….

•Faraday's Law of Induction • conductor (liquid) passes/cuts through

the magnetic fieldp.d. will be induced in the liquid

filmdetected by the electrodes

3.1 The Electromagnetic Log:It is a bottom log:♦Retractable, or♦fixed (using flush fitting sensors)

2.2

9-Mar-07 5 of 21

Electrodes

Wat

er fl

ow

Wat

er fl

ow+ive Current (EMF) -ive

S T W

Em Log Sensors

9-Mar-07 6 of 21

3.1.2 Marine Em Logrodmeter (sensor):•streamlined, flat, ellipsoidal housing •attached to the outer end of a strut projecting or flush

coil is mounted vertically inside the housingelectric current through the coil produces a horizontal magnetic field voltage in the water picked up by the outer electrodesvoltage is linearly proportional to the speed of relative water flow?

∴ a measure of the speed of the vessel through the water!

2.2

9-Mar-07 7 of 21

3.1.3 Em log: advantages

linear output throughout its operating range (typically 0 to 40 knots)high sensitivitycalibration stability over a long period of time (means what?)relative freedom from fouling.

however, ♦expensive and ♦speed over the ground can not be

determined.

9-Mar-07 8 of 21

The police speed gun – how does

it operate?

Other Log ?

9-Mar-07 9 of 21

3.2 The Doppler Log:uses the Doppler principle (Doppler shift???)claimed to be more accurate than other logsparticularly useful because:1. In shallow water it can measure speed over

the ground2. in deep water it measures speed through the

water (Why this preoccupation with SOG/STW?)

3. in medium to shallow depths• speed over the ground and/or • speed through the water

4. measure sideways speed

9-Mar-07 10 of 21

Stationary

Frequency is equal

Frequency is not equal

Fire truck moving towards

Doppler Effect

Doppler Shift

9-Mar-07 11 of 21

3.2.1 The Doppler Effect/Shift

• for a moving sound source→frequency difference depends on speed!

3.2.2 Principle: Doppler Log

fr = ft ⇒ stopped

fr > ft ⇒ moving forward

fr < ft ⇒ moving astern.(fr - ft) is a measure of the ship's speed!

2.6

9-Mar-07 12 of 21

Frequency transmitted ft

Frequency Receivedfr

•If fr higher than ft vessel going ahead

•If fr is lower than ft vessel going astern

Speed of vessel = fr - ft

3.2.2 Principle: Doppler Log

9-Mar-07 13 of 21

3.2.2.1 Janus configuration:two beams are used♦One pointing forward and one aft♦To eliminate some of the errors..

-due to the pitching of the vessel

Beam is about 60° to the horizontal, so that -

not too little reflected energy for the receiver

a big enough forward component of the beam

2.7

Doppler Log: Transducer Configurations

9-Mar-07 14 of 21

3.2.2.2 Crosstrack ConfigAn athwartships beam enables sideways or "Crosstrack" speed to be measured.

Crosstrack speed is useful as it provides much more accurate measurement of the ship's actual movement

9-Mar-07 15 of 21

3.2.3 Operating Modes

• Bottom Track/Lock: SOG• Water Track/Lock: STW

If SOG ≠ STW ⇒ current exists !

Features:• may be able to hold bottom lock/track down

to 200m • might use a water reference layer at

20/30m for water lock/track.

2.9

2.10

9-Mar-07 16 of 21

3.2.5 Sources of Error:

• Transducer alignmentthe beam should be exactly fore and aft, and/or P/S

• Ship motionsAffect instantaneous speeds

• Speed of sound variationAs discussed earlier - factors affecting propagation of acoustic energy!

9-Mar-07 17 of 21

3.4 IMO Performance Standards

Should show the forward speed up to the maximum design/engine speed of the vessel in >2mof depthShould comply with the above when

o rolling to max 10 degreesopitching to max 5 degrees

If STW or SOG- mode selection/indication givenError in speed in best conditions – not > 2% of the ship’s speed or 0.2 knotsError in Distance run in best conditions – not > 2% of the ship’s distance run in one hour or 0.2 knots per hourIf the sensor is extendable type – clear indications provided on the display

9-Mar-07 18 of 21

9-Mar-07 19 of 21

Docking Logs

9-Mar-07 20 of 21

The paddle wheel….!

9-Mar-07 21 of 21

Doppler Log Transducer – best sites