I85PREP
⌘ K
DGS GP Rating Exit Exam study materials
DGS GP Rating Exit Exam study resources

DGS GP Rating Exit Exam practice questions

One question at a time. Build your DGS GP Rating Exit Exam understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Nautical Science and Navigation

Under IALA Region A, a port-hand buoy is painted red and has which shape?

  1. Red conical
  2. Red cylindrical Correct answer.
  3. Green conical
  4. Green cylindrical

Explanation

In IALA Region A, port-hand marks are red can (cylindrical) buoys and starboard-hand marks are green conical buoys. A vessel entering port keeps the red cans to port and the green cones to starboard, so the shape and colour together identify the side of the channel.

Question 2Nautical Science and Navigation

A north cardinal mark carries two black cones as topmarks. How are the two cones arranged?

  1. Both cones point downwards
  2. The upper cone points down and the lower cone points up
  3. The upper cone points up and the lower cone points down
  4. Both cones point upwards Correct answer.

Explanation

Cardinal topmarks indicate the direction of safe water by the way the cones point: north has both cones pointing up, south has both pointing down, east points up over down and west points down over up. A mariner can therefore identify the quadrant of safe water from the topmark alone, even at night by the light character.

Question 3Nautical Science and Navigation

Which knot forms a temporary loop at the end of a rope that will not slip or jam under load, and can be used to pass a line around a person?

  1. Clove hitch
  2. Bowline Correct answer.
  3. Round turn and two half hitches
  4. Reef knot

Explanation

The bowline makes a fixed loop at the end of a line that neither slips nor jams, which is why it is used for rescue lines, heaving lines and mooring eyes. It can be tied quickly even when the rope is stiff or wet, and it is easily broken once the load is taken off.

Question 4Nautical Science and Navigation

The angle between the magnetic meridian and the compass meridian on board a particular ship, caused by the ship's own magnetic field, is called:

  1. Leeway
  2. Deviation Correct answer.
  3. Dip
  4. Variation

Explanation

Deviation is the compass error produced by the ship's own magnetism acting on the compass needle, and it changes with the ship's heading and with cargo. Variation, by contrast, is the angle between the true and magnetic meridians and depends only on geographic position.

Question 5Nautical Science and Navigation

Two power-driven vessels are crossing so as to involve risk of collision and the other vessel is on your starboard side. Which action complies with the Steering and Sailing Rules?

  1. Sound one short blast and continue on the same course and speed
  2. Maintain course and speed because you are the stand-on vessel
  3. Alter course to starboard and pass astern of the other vessel Correct answer.
  4. Alter course to port to pass ahead of the other vessel

Explanation

In a crossing situation the vessel that has the other on her own starboard side is the give-way vessel, and she must keep out of the way, normally by altering course to starboard and passing astern. Altering to port towards a crossing vessel is prohibited because it turns into the other ship's path and increases the risk of collision.

Question 6Nautical Science and Navigation

You span a distance on a navigational chart and transfer it to the latitude scale, where it reads 4 minutes of latitude. What distance is this?

  1. 4 cables
  2. 4 nautical miles Correct answer.
  3. 4 statute miles
  4. 4 kilometres

Explanation

One minute of arc of latitude equals one nautical mile, so four minutes on the latitude scale represent four nautical miles. Because the latitude scale is the only scale graduated in nautical miles, distances are always measured on the latitude scale abreast of the positions concerned.

Question 7Nautical Science and Navigation

Spring tides, which give the greatest tidal range, occur when:

  1. The Moon is at apogee, the farthest point of its orbit from Earth
  2. The Moon's declination is at its greatest north or south
  3. The Moon is at first or third quarter
  4. The Sun and Moon pull in line at new and full moon Correct answer.

Explanation

Spring tides occur near new and full moon, when the solar and lunar tide-raising forces act along the same line and add together, giving the greatest range. At the quarters the two forces act at right angles and partly cancel, producing neap tides with the smallest range.

Question 8Nautical Science and Navigation

In sight of one another, a power-driven vessel making way sounds three short blasts on her whistle. What does this signal mean?

  1. I am altering my course to starboard
  2. I am operating astern propulsion Correct answer.
  3. I am in doubt whether you are taking sufficient action
  4. I intend to overtake you on your port side

Explanation

Under the Steering and Sailing Rules, one short blast means altering to starboard, two mean altering to port and three mean the vessel is operating astern propulsion. The signal indicates the engines are going astern, which often means the vessel will lose steering way and should be given a wide berth.

Question 9Nautical Science and Navigation

Your true course is 090°(T), variation is 6° W and deviation is 3° E. What compass course should be steered?

  1. 099°(C)
  2. 087°(C)
  3. 093°(C) Correct answer.
  4. 081°(C)

Explanation

Going from true to compass, apply the errors in reverse: adding 6° W variation to 090°(T) gives 096°(M), and subtracting 3° E deviation gives 093°(C). The check is C→M→T: 093° plus 3° E gives 096°(M), then minus 6° W gives 090°(T).

Question 10Nautical Science and Navigation

In the Northern Hemisphere, the dangerous semicircle of a tropical cyclone is the one in which:

  1. The vessel is nearest the storm centre and therefore always meets hurricane force winds
  2. The wind circulation and the storm's forward movement combine to give the strongest winds Correct answer.
  3. The wind blows against the direction of the storm's track, giving lighter winds
  4. The storm's track is curving away from the observer's position

Explanation

In the Northern Hemisphere the circulation is counter-clockwise, so on the right-hand side of the track the wind blows in roughly the same direction as the storm moves. The two speeds add, giving the strongest winds and highest seas, which is why that side is called the dangerous semicircle; the left-hand side is the navigable semicircle.

Question 11Nautical Science and Navigation

Under the IALA buoyage system, which mark indicates that there is navigable water all around it and is normally painted with red and white vertical stripes?

  1. Isolated danger mark
  2. Cardinal mark
  3. Safe water mark Correct answer.
  4. Special mark

Explanation

A safe water mark is painted with red and white vertical stripes and carries a single red spherical topmark. It shows that safe navigable water lies all around the mark, such as at a fairway entrance, and it may be passed on either side.

Question 12Nautical Science and Navigation

Two power-driven vessels are meeting on reciprocal or nearly reciprocal courses so as to involve risk of collision. What action should each vessel take under the Collision Regulations?

  1. Both vessels alter course to port and pass starboard-to-starboard
  2. The overtaking vessel must keep clear
  3. Both vessels alter course to starboard and pass port-to-port Correct answer.
  4. The vessel with the other on its port side must give way

Explanation

Rule 14 requires both power-driven vessels in a head-on situation to alter course to starboard so that they pass port-to-port. A head-on situation exists when each vessel sees the other ahead on a reciprocal or nearly reciprocal course and risk of collision is present.

Question 13Nautical Science and Navigation

A vessel is anchoring in good holding ground where the depth of water is 10 metres. To obtain a scope of about 5 to 7 times the depth, approximately how much cable should be paid out?

  1. 15 to 20 metres
  2. 100 to 140 metres
  3. 25 to 30 metres
  4. 50 to 70 metres Correct answer.

Explanation

Scope is the ratio of the length of cable paid out to the depth of water. For a depth of 10 metres, a scope of 5 to 7 means paying out 50 to 70 metres of cable, which gives the anchor a good horizontal pull in good holding ground.

Question 14Nautical Science and Navigation

What is the main purpose of a deviation card kept on a vessel's bridge?

  1. To show the gyrocompass error for each watch
  2. To record the residual deviation for each compass heading so that compass courses and bearings can be corrected Correct answer.
  3. To record the annual compass swing and any repairs to the compass
  4. To list the magnetic variation for each port visited

Explanation

A deviation card records the residual deviation remaining on each heading after the compass has been adjusted. It allows the navigator to convert between compass directions and magnetic or true directions when steering a course or plotting a bearing.

Question 15Nautical Science and Navigation

Neap tides, which produce the smallest tidal range, occur when:

  1. the Moon is at perigee on the equator
  2. the Sun and Moon are at right angles to each other relative to the Earth Correct answer.
  3. the Sun, Moon and Earth are in a straight line
  4. the Moon is at its maximum declination

Explanation

Neap tides occur near the first and third quarters of the Moon, when the solar and lunar tidal forces act at right angles and partly cancel each other. The result is a smaller tidal range than at spring tides.

Question 16Nautical Science and Navigation

In the Northern Hemisphere, Buys Ballot's law states that with your back to the wind, the centre of low pressure lies:

  1. to your right and slightly behind
  2. to your left and slightly forward Correct answer.
  3. directly ahead of you
  4. directly behind you

Explanation

In the Northern Hemisphere the wind blows anticlockwise around a low-pressure centre, so with your back to the wind the low centre lies to your left and slightly forward. The opposite applies in the Southern Hemisphere.

Question 17Nautical Science and Navigation

On a standard navigational chart, depths and soundings are reduced to which reference level?

  1. Mean high water springs
  2. Mean sea level
  3. Chart datum, normally lowest astronomical tide Correct answer.
  4. Highest astronomical tide

Explanation

Chart datum is the level to which all soundings on the chart are reduced, and it is normally close to lowest astronomical tide so that charted depths are conservative. The actual depth of water is the charted depth plus the height of tide at that time.

Question 18Nautical Science and Navigation

You take simultaneous bearings of two fixed, charted objects and plot the position lines. What does their intersection represent?

  1. A visual fix Correct answer.
  2. A dead reckoning position
  3. A running fix
  4. An estimated position

Explanation

Two or more position lines obtained from simultaneous observations of fixed objects intersect at a fix, which is a reasonably accurate observed position. A running fix uses one object observed at different times, with the course and distance run applied between observations.

Question 19Nautical Science and Navigation

A vessel steams at 12 knots through the water on a course of 000°(T). A current sets 090°(T) at 3 knots. What is the vessel's approximate speed made good over the ground?

  1. 12.0 knots
  2. 15.0 knots
  3. 12.4 knots Correct answer.
  4. 9.0 knots

Explanation

The current is at right angles to the vessel's heading, so the speed made good is the vector sum of the two velocities: the square root of 12 squared plus 3 squared, which is about 12.4 knots. The course made good would also be altered toward the set of the current.

Question 20Nautical Science and Navigation

The gyrocompass bearing of a landmark is 045°(G). The true bearing taken from the chart is 048°(T). What is the gyro error?

  1. 3° high
  2. 3° E
  3. 93° E
  4. 3° W Correct answer.

Explanation

Gyro error is found by comparing the compass bearing with the true bearing: compass minus true gives 045° minus 048° = -3°, which is 3° W. To convert a gyro bearing to true, add 3°.

Question 21Nautical Science and Navigation

In restricted visibility, a power-driven vessel that is underway and making way through the water sounds which whistle signal?

  1. One prolonged blast at intervals not exceeding 2 minutes Correct answer.
  2. Two prolonged blasts at intervals not exceeding 2 minutes
  3. One prolonged blast followed by two short blasts every 2 minutes
  4. One short blast at intervals not exceeding one minute

Explanation

Rule 35(a) of the COLREGS requires a power-driven vessel making way in restricted visibility to sound one prolonged blast at intervals of not more than two minutes. The signal warns other vessels that the ship is moving through the water and cannot rely on stopping quickly.

Question 22Nautical Science and Navigation

A light is charted as Fl(2) R 10s. What does this description indicate?

  1. A red light flashing once every 10 seconds
  2. A fixed red light that remains visible for 10 seconds before being eclipsed
  3. A red light showing a group of two flashes in each period of 10 seconds Correct answer.
  4. An occulting red light with 10 seconds of darkness in each period

Explanation

In the abbreviation Fl(2) R 10s, 'Fl' means flashing, '(2)' gives the number of flashes in a group, 'R' gives the colour red, and 10s is the period of the light. The light therefore shows a group of two red flashes, followed by a longer eclipse, every ten seconds.

Question 23Nautical Science and Navigation

Which statement correctly defines the draught of a vessel?

  1. The vertical distance from the waterline to the keel Correct answer.
  2. The vertical distance from the uppermost continuous deck to the waterline
  3. The height of the hull above the waterline at the bow
  4. The maximum breadth of the hull measured at the loaded waterline

Explanation

Draught is the vertical distance from the waterline to the lowest point of the keel or hull, and it determines the minimum depth of water in which the ship can float. It increases with loading and decreases as fuel and stores are consumed.

Question 24Nautical Science and Navigation

The tidal range at a standard port is 4.8 m and high water occurs at 12:00. Using the rule of twelfths, what is the approximate height of tide at 15:00?

  1. 1.2 m
  2. 0.4 m
  3. 2.4 m Correct answer.
  4. 1.6 m

Explanation

The rule of twelfths divides the tidal range into twelve parts: 1/12 in the first hour, 2/12 in the second, 3/12 in the third, and so on. By three hours after high water, 6/12 of the range has fallen, so half of 4.8 m, or 2.4 m, remains above chart datum.

Question 25Nautical Science and Navigation

In coastal pilotage, what is a leading line?

  1. A line drawn on the chart to show the boundary of a prohibited area
  2. A position line obtained from a single compass bearing
  3. The track followed by a vessel between two waypoints
  4. A transit of two charted marks that indicates a safe track Correct answer.

Explanation

A leading line is formed by two charted objects that appear in line, or in transit, when viewed from the vessel. Steering along the transit keeps the vessel on the safe channel shown on the chart, clear of shoals or dangers.

Question 26Nautical Science and Navigation

A vessel proceeds at speed in shallow water and experiences squat. What is the typical effect?

  1. Draught decreases and the vessel becomes more responsive to the rudder
  2. The vessel is lifted bodily in the water and the rudder becomes noticeably more effective
  3. Under-keel clearance decreases and the vessel may become sluggish on the helm Correct answer.
  4. The stern is lifted and the bow is sucked down, improving steering

Explanation

In shallow water the flow around the hull is restricted, causing a drop in pressure and increased squat. This reduces under-keel clearance and can make the vessel sluggish on the helm; the effect increases with speed and in confined channels.

Question 27Nautical Science and Navigation

How is a running fix obtained?

  1. By advancing an earlier position line along the track and distance run to a later time Correct answer.
  2. By plotting two simultaneous bearings of two different objects
  3. By plotting a bearing and a radar range of one object at the same moment
  4. From a single bearing combined with a measured vertical angle

Explanation

A running fix is obtained when a position line taken at one time is transferred forward along the vessel's track and distance run to cross with a later position line. It is used when only one object or one type of bearing is available at a time.

Question 28Nautical Science and Navigation

What is the difference between course and heading?

  1. Course is the intended direction of travel; heading is the direction the bow points Correct answer.
  2. Course is the direction the bow points; heading is the direction made good over the ground
  3. Course is measured only in degrees magnetic; heading only in degrees true
  4. Course and heading are identical terms for the direction the vessel is steering

Explanation

Course is the direction in which the vessel is intended to travel, usually expressed as a three-figure bearing. Heading is the direction in which the bow is actually pointing at any moment, and it may differ from the course because of wind, current and steering error.

Question 29Nautical Science and Navigation

In the Northern Hemisphere, the corrected sextant altitude of Polaris is 47°. What is the approximate latitude of the observer?

  1. 47° S
  2. 43° N
  3. 47° N Correct answer.
  4. 53° N

Explanation

Polaris lies close to the north celestial pole, so its corrected altitude is approximately equal to the observer's latitude in the Northern Hemisphere. A measured altitude of 47° therefore gives a latitude of about 47° N.

Question 30Nautical Science and Navigation

In a river or narrow channel, where is the tidal stream usually strongest and the water deepest?

  1. Evenly across the whole width of the channel
  2. On the outside of the bend Correct answer.
  3. Exactly in mid-channel, regardless of the bend
  4. On the inside of the bend, where the water is shallowest

Explanation

In a bend, centrifugal force drives the water towards the outer bank, so the strongest stream and the deepest water are found on the outside of the bend. The inner bank is the shallow, slack side and is the usual place for shoaling.

Question 31Nautical Science and Navigation

In IALA buoyage Region A, which colour is used for a port-hand lateral mark?

  1. Red Correct answer.
  2. Yellow
  3. Green
  4. Black and yellow

Explanation

In Region A, port-hand lateral marks are red and are left to the port side when entering a harbour from seaward. Starboard-hand marks in the same region are green, so the colour identifies which side of the channel the mark belongs to.

Question 32Nautical Science and Navigation

In a semidiurnal tidal pattern, what is the approximate interval between successive high waters?

  1. About 18 hours 36 minutes
  2. About 12 hours 25 minutes Correct answer.
  3. About 6 hours 12 minutes
  4. About 24 hours 50 minutes

Explanation

A lunar day is about 24 hours 50 minutes, and a semidiurnal tide has two high waters in that period. Successive high waters therefore occur about 12 hours 25 minutes apart, which is why high water is roughly 50 minutes later each day.

Question 33Nautical Science and Navigation

Under the COLREGs, what does the term 'underway' mean for a vessel?

  1. The vessel has her main engines running and ready
  2. The vessel is moving through the water with steerage way
  3. Not at anchor, moored, or aground Correct answer.
  4. The vessel is drifting with no steerage way and no headway

Explanation

Under the COLREGs, a vessel is underway when she is not at anchor, not made fast to the shore, and not aground. A vessel does not have to be moving through the water to be underway, so a drifting vessel can still be underway.

Question 34Nautical Science and Navigation

Two power-driven vessels are crossing so as to involve risk of collision. Which vessel must keep out of the way?

  1. The faster vessel
  2. The vessel that has the other on her starboard side Correct answer.
  3. The vessel that is to leeward
  4. The vessel that has the other on her port side

Explanation

In a crossing situation, the give-way vessel is the one that has the other vessel on her starboard side. She must avoid crossing ahead and will normally pass astern of the stand-on vessel, which keeps her course and speed.

Question 35Nautical Science and Navigation

A north cardinal mark is sighted. On which side of the mark is the safest navigable water?

  1. To the south of the mark
  2. To the east of the mark
  3. To the west of the mark
  4. To the north of the mark Correct answer.

Explanation

A north cardinal mark shows that the safest navigable water lies to the north of the mark, so the vessel should pass on that side. Its topmark has both cones pointing upward, and its body is painted black above yellow.

Question 36Nautical Science and Navigation

What is the main purpose of a Notice to Mariners?

  1. To advise mariners of chart and publication changes Correct answer.
  2. To give long-range weather forecasts for coastal waters
  3. To list the crew and passenger numbers for a voyage
  4. To publish commercial freight rates for port arrivals

Explanation

Notices to Mariners inform mariners of changes that affect charts and navigational publications, such as new wrecks, altered buoys, or amended depths. They are used to keep charts and publications corrected before passage planning and during a voyage.

Question 37Nautical Science and Navigation

What is the difference between a tidal stream and a current?

  1. A tidal stream occurs only in the open sea, while a current occurs only in rivers and estuaries
  2. A tidal stream is horizontal flow caused by the tide; a current may have other causes Correct answer.
  3. A tidal stream is vertical water movement, while a current is horizontal water movement
  4. A tidal stream is caused mainly by wind, while a current is caused mainly by the moon and sun

Explanation

Tidal streams are the periodic horizontal flows associated with rising and falling tides. The term current is broader and includes wind-driven flow, river outflow, and ocean circulation, so not every current is a tidal stream.

Question 38Nautical Science and Navigation

What should be done first when a person is seen to fall overboard from a vessel underway?

  1. Raise the alarm, throw a lifebuoy, and mark the position Correct answer.
  2. Wait for the engine to be stopped before raising the alarm
  3. Alter course immediately toward the person without informing anyone
  4. Lower the rescue boat before marking the position

Explanation

The immediate priorities are to alert the bridge and crew, put floatation into the water, and mark the position so the person can be kept in sight. The recovery manoeuvre, such as a Williamson turn, follows once the alarm has been raised and the situation is understood.

Question 39Nautical Science and Navigation

Under the COLREGs, which daylight signal identifies a vessel constrained by her draught?

  1. A black cone pointing upward
  2. Two black balls in a vertical line
  3. A black ball
  4. A black cylinder Correct answer.

Explanation

A vessel constrained by her draught shows three all-round red lights in a vertical line at night and a cylinder by day. The cylinder is the daytime shape that distinguishes this status from other vessel statuses.

Question 40Nautical Science and Navigation

What is the standard light characteristic of a south cardinal mark?

  1. Six quick flashes followed by one long flash every 15 seconds Correct answer.
  2. Nine quick flashes followed by one long flash every 10 seconds
  3. Three quick flashes followed by one long flash every 10 seconds
  4. One quick flash every 5 seconds

Explanation

A south cardinal mark shows a group of six quick flashes followed by one long flash, with a period of 15 seconds, or the very quick equivalent with a 10-second period. The six-flash group is the identifying feature of the south cardinal mark.

Question 41Marine Engineering and Ship Machinery

On a motor ship, the main engine starting air receivers are normally maintained at a charging pressure of about:

  1. 10 bar
  2. 3 bar
  3. 100 bar
  4. 30 bar Correct answer.

Explanation

Starting air receivers for main engine starting are designed to be charged to roughly 2.5 to 3.0 MPa, that is about 30 bar, so that enough stored energy is available for the required number of consecutive starts without running the compressors. Air at this pressure is admitted through the starting air valve and distributor to turn the engine against compression.

Question 42Marine Engineering and Ship Machinery

Under MARPOL Annex I, machinery space bilge water may be discharged overboard through the ship's oily-water separating equipment only when its oil content does not exceed:

  1. 5 ppm
  2. 50 ppm
  3. 15 ppm Correct answer.
  4. 100 ppm

Explanation

MARPOL Annex I fixes 15 ppm as the maximum oil content of machinery space bilge water that may be discharged through approved oily-water separating equipment. A 15 ppm bilge alarm and an automatic stopping device must be fitted so that the discharge is cut off whenever the limit is exceeded.

Question 43Marine Engineering and Ship Machinery

In a large slow-speed two-stroke marine diesel engine, lubrication of the cylinder liner is provided by:

  1. a dedicated cylinder lubricator feeding quills Correct answer.
  2. splash from the rotating crankshaft
  3. oil mist carried in the scavenge air
  4. oil blended into the fuel oil supply

Explanation

Large two-stroke engines are of the crosshead type, so the crankcase and the combustion space are separated and crankcase oil never reaches the liner. Mechanical or electronic cylinder lubricators deliver metered alkaline cylinder oil through quills to the liner; this oil is a total-loss lubricant and is burned in the cylinder.

Question 44Marine Engineering and Ship Machinery

When heavy fuel oil is being burned, the viscosity at the inlet to the main engine fuel pumps is normally controlled to about:

  1. 2 cSt
  2. 380 cSt
  3. 12 cSt Correct answer.
  4. 100 cSt

Explanation

Heavy fuel oil must be heated so that its viscosity at the fuel pump inlet falls to roughly 10 to 15 cSt, which gives correct atomisation at the injector and clean combustion. The 380 cSt figure is the nominal viscosity of the fuel grade measured at 50 degrees Celsius, not the viscosity required at the engine.

Question 45Marine Engineering and Ship Machinery

In a centrifugal fuel oil purifier, the gravity disc is used to:

  1. balance the bowl while it accelerates
  2. fix the position of the oil and water interface Correct answer.
  3. retain sludge in the sludge space
  4. set the throughput of the feed pump

Explanation

The gravity disc is fitted at the water outlet of the purifier bowl, and its internal diameter fixes the radius at which the oil and water interface settles inside the bowl. A disc of the wrong size forces the interface too far inward or outward, so that either oil is lost with the separated water or water remains in the cleaned oil.

Question 46Marine Engineering and Ship Machinery

Under SOLAS, the main steering gear must be able to put the rudder from 35 degrees on one side to 35 degrees on the other side, with the ship at maximum ahead service speed, within not more than:

  1. 45 seconds
  2. 60 seconds
  3. 15 seconds
  4. 28 seconds Correct answer.

Explanation

SOLAS requires the main steering gear to move the rudder from 35 degrees on one side to 35 degrees on the other at maximum ahead service speed, and from 35 degrees on either side to 30 degrees on the other side, within not more than 28 seconds. This performance is demonstrated on trials and is verified during surveys.

Question 47Marine Engineering and Ship Machinery

In a ship's vapour-compression refrigeration plant, the refrigerant entering the evaporator is in the state of:

  1. high-pressure superheated vapour
  2. low-pressure liquid-vapour mixture Correct answer.
  3. low-pressure superheated vapour
  4. high-pressure liquid

Explanation

The expansion valve throttles the refrigerant from condensing pressure down to evaporating pressure, and part of the liquid flashes off during the process. The refrigerant therefore enters the evaporator as a low-pressure liquid-vapour mixture, which boils at a low temperature and absorbs heat from the cargo or provision spaces.

Question 48Marine Engineering and Ship Machinery

In a slow-speed two-stroke engine with uniflow scavenging, the exhaust valve begins to open:

  1. well before the scavenge ports uncover Correct answer.
  2. just after the ports close on the upstroke
  3. at top dead centre of compression
  4. exactly when the scavenge ports uncover

Explanation

In uniflow scavenging the exhaust valve opens well before bottom dead centre, while the scavenge ports are still covered by the piston, so that the cylinder pressure can blow down towards scavenge pressure. Fresh air then enters when the ports are uncovered and sweeps the cylinder in one direction towards the exhaust valve.

Question 49Marine Engineering and Ship Machinery

Crank web deflection readings taken on one unit of a large diesel engine show an increased sagging (closed) value compared with the previous set. This indicates that:

  1. the main bearing below that crank throw has worn Correct answer.
  2. the cylinder liner of that unit is worn oval
  3. the connecting rod big-end bearing is too tight
  4. the crankshaft has cracked across the crankpin

Explanation

Crank web deflections measure how much the crankshaft bends between adjacent main bearings. A larger sagging, or closed, reading shows that the shaft has dropped at that position, which points to wear or settling of the main bearing under that crank throw rather than to a crack, a liner fault or a big-end bearing fault.

Question 50Marine Engineering and Ship Machinery

With engine load and fuel index unchanged, the scavenge air temperature measured after the charge air cooler rises steadily. The most likely immediate effect on the engine is:

  1. exhaust gas temperature falls and thermal load decreases
  2. fuel consumption falls because the air is less dense
  3. scavenge air pressure rises and turbocharger speed increases
  4. exhaust gas temperature rises and thermal load increases Correct answer.

Explanation

Warmer charge air is less dense, so each scavenge stroke delivers less oxygen to the cylinder for the same fuel index. The trapped air-fuel ratio therefore falls, combustion becomes slower and hotter, and the exhaust gas temperature and the thermal loading of the engine components rise.