Marine Machinery Oral Exam Questions: Coolers, Compressors, Pumps & Steering
Only what the full lessons do not teach — the leftover drills examiners use when the basics are already proved.
Key Principles at a Glance 6 points
- This file holds leftovers only — cooler rounds, compressor overhaul, pump rebuilds and steering tests live in their full lessons, linked below.
- Throttle the seawater outlet, never the inlet: starved tubes run half-full and differential aeration starts corrosion.
- Nylon beats copper mesh in intake filters because a fatigued copper strand breaks free and lodges in a valve or clearance.
- A centrifugal pump is a momentum machine — air builds barely 12 mm water gauge of suction, so it never self-primes.
- Balance holes kill axial thrust by equalising vane pressures; impeller thrust otherwise loads the bearings.
- Steering answers prove redundancy: split-half survival, four alarms read in order, float-switch logic before opening the system.
How to Answer — Mechanism First, Number Last
Idea in one line: the examiner passes the candidate who says what moves or blocks first — the figure comes last as the proof, never first as the guess.
Every drill below follows that order. Where a topic already has a full lesson — cooler cleaning and leak tests, compressor overhaul, pump rebuilds, steering tests — the drill points there instead of repeating it. For shell-and-tube cleaning, retubing and leak detection, plate-exchanger gaskets and tightening, compressor operation and overhaul, compressor safeties and receivers, centrifugal pump overhaul, gear and screw pumps and steering-gear tests and purging, revise there first.
1. Cooler Leftovers — Throttle Side, Growth and Chemistry
Idea in one line: rounds, fouling, leak direction, halo test, anodes and plate pack discipline already live in the cooler lessons — what survives here is the throttle-side trap, the air-lock call, growth prevention and water chemistry.
Shell-side ΔT reads high in cold seas — which valve moves, and why that one?
Throttle the seawater outlet slowly back to book ΔT. Outlet throttling keeps tubes flooded; throttling the inlet starves tubes into partial fill, and the wet–dry boundary sets up differential aeration that starts corrosion.
Performance fell suddenly, not over weeks?
Gradual shrink is fouling; a step change is an air lock — purge at the cock, then reassess before opening anything.
How is marine growth prevented in seawater spaces?
Biocide dosing — commonly a hypochlorite solution for about an hour a day — so growth never gets a footing instead of being scraped off later.
What protects the freshwater side, where there are no anodes?
Inhibitor chemicals that lay a passive protective film over the metal — chemistry, not sacrificial metal, because freshwater will not carry a galvanic circuit.
Opened cooler, anodes look brand new — good news?
Bad news — an unwasted anode carries no current: wrong alloy, bad fastening, or a passive film insulating it from its stud. Check the metallic continuity strips too, since the circuit is only as good as its wiring.
For rounds discipline, backflushing, retubing, pressure testing, the halo test and plate-pack overhaul, see Shell & Tube Heat Exchangers and Plate Heat Exchangers.
2. Compressor Leftovers — Filters, Siting, Oil Level and Valve Craft
Idea in one line: plate-valve overhaul, bumping clearance, strip-down order and safeties live in the compressor lessons — what survives here is the dirt path into the machine, where it sits, how its oil reads, and the small craft that keeps valves alive.
Why does a dirty intake filter end in hot discharge air?
Abrasive dust mixes with oil into a paste that gums the plate valves open — leaking valves overwork the machine, so discharge temperature climbs and carbon forms. That is why filters are cleaned on hours, not on looks.
Why nylon mesh instead of copper mesh?
Pulsating flow flexes copper strands until one fatigues free and lodges in a valve or a piston clearance — a first-stage failure from the filter itself. Nylon is resilient enough to survive the flexing, and oil-wetted viscous-impingement types suit oil-free control-air duty.
Where should the compressor sit, and how is ship vibration handled?
Dust-free, lit, ventilated, with working space, close to the point of use to cut line losses — on anti-vibration rubber mounts with flexible discharge and cooling-water connections, since a ship offers no vibration-free foundation.
Oil above H? Below L?
Above H the balance weights fling excess oil into carry-over, gumming valves; below L the running gear starves and wears. Hold between the marks.
Small valve craft the examiner loves: cushion plates, mirror finish, contoured ports, jig, locknut?
Cushion plates soften shock loading on thin plates; a mirror lap cuts flow drag that would heat the air; contoured ports avoid turbulence that throttles flow; the fixing jig prevents vice-strain that fails plates; self-locking nuts with split pins survive vibration that would slacken plain nuts.
Who sonic-gauges the receiver, and why gauge at all?
Shore specialists with electro-sonic gear — corrosion wastes shell metal invisibly, and only measured thickness proves the vessel still holds its pressure.
For overhaul sequence, bumping clearance, valve lapping, relief valves, bursting discs, fusible plugs and receiver inspection, see Air Compressors — Operation & Overhaul and Compressor Safety Devices & Receivers.
3. Pump Leftovers — Thrust, Momentum and the Follow-Up Checks
Idea in one line: relief protection, backlash bands, isolation and rebuilds live in the pump lessons — what survives here is why thrust is cancelled, why air defeats a centrifugal pump, and the follow-up checks behind insufficient delivery.
Why do impellers carry balance holes?
Pressure acts on both vane faces unequally and the difference shoves the rotor axially into its bearings — holes through the back vane equalise front-to-back pressure and cancel that thrust at its source.
Why can a centrifugal pump never self-prime?
It is a momentum machine, not a displacement machine: suction comes from centrifugal force m·ω²·r, and air has barely any mass — an empty casing pulls about 12 mm water gauge, so the casing must be flooded and air pockets vented before starting.
Delivery is weak but the pump runs — the examiner’s follow-up list?
Wrong rotation first, then the suction side: choked suction filter, clogged or leaking foot valve, air pockets in the suction line, weeping shaft seal drawing air, worn wear rings short-circuiting discharge back to suction.
What does a pump round actually record?
Suction against delivery pressure, abnormal noise or vibration, and gland or mechanical-seal tightness — pressure pair first, because everything else is commentary on it.
For isolation, dismantling, wear-ring limits, seals, backlash measurement and fault tables, see Centrifugal Pumps, Gear & Screw Pumps and Reciprocating Pumps.
4. Steering Leftovers — Redundancy, Alarms and Changeover
Idea in one line: ram circuits, departure tests, filters and purging live in the steering lesson — what survives here is the spoken proof of redundancy: the criterion, the changeover, the four alarms and the sensor-before-system rule.
State the single-failure criterion with an example.
No single failure may disable steering or leave the rudder free — e.g. a burst line auto-isolates one hydraulic half while the healthy half keeps steering.
How do you change to emergency steering?
Match the trick wheel to the rudder’s actual angle before transferring control local — or, by system, hand-drive the solenoid directional valves. Alignment first, transfer second, or the gear fights itself.
Phase, overload, control-power, main-power alarms — what does each blame?
Lost supply phase; overloaded motor or electrics; dead solenoid circuit; dead starter supply — check in that order, since each stage feeds the next.
Low-oil alarm but the level is fine?
The float switch itself — prove the sensor before opening the system.
Noisy pump? Sluggish rudder?
Noise is air, clogged filter, worn shaft seal or coupling. Sluggish is internal actuator leak, sticky main valve, false indicator, air, clogged filter or a tired pump — change pumps over with the bridge informed and compare response.
For circuits, 12-hour tests, filter service, oil charging and air purging, see Steering Gear — Tests, Filters & Purging.