TL;DR — Rotating machinery must be guarded; stern tube bearing rebushing is triggered by specific measured clearances set in regulation; diesel and turbine maintenance both demand isolation before internal work, correct clearances on reassembly, and continuous monitoring of key condition indicators.
What the rule says
Machinery Guards
Federal regulation is unambiguous: gears, couplings, flywheels, and all rotating machinery capable of injuring personnel must be provided with adequate covers or guards. 46 CFR §58.01-20 This is not a recommendation — it is a mandatory requirement on inspected vessels. The marine inspector will look for missing or defeated guards as a deficiency.
Stern Tube Bearing Weardown and Rebushing
The tailshaft clearance rule at 46 CFR §61.20-23 establishes precise rebushing thresholds for water-lubricated stern tube bearings. The thresholds differ based on shaft diameter and machinery location.
For water-lubricated bearings other than rubber, where propelling machinery is located amidship, the after stern tube bearing must be rebushed when weardown reaches:
- 6.4 mm (0.25 in) clearance — shafts 229 mm (9 in) or less in diameter
- 7.95 mm (0.3125 in) clearance — shafts exceeding 229 mm (9 in) but not exceeding 305 mm (12 in) in diameter
- 9.53 mm (0.375 in) clearance — shafts exceeding 305 mm (12 in) in diameter
Where propelling machinery is located aft, the rebushing threshold is 1.6 mm (0.0625 in) less than the applicable amidship clearance for each shaft size category.
For water-lubricated rubber bearings, rebushing is required when any water groove is worn to half its original depth.
For oil-lubricated bearings, rebushing is at the discretion of the Officer in Charge, Marine Inspection (OCMI), with the manufacturer's recommendation considered in making that determination.
Hydraulic and Pneumatic System Piping Materials
Materials used in tubing, pipes, valves, flanges, and fittings for hydraulic and pneumatic systems must be selected from the specifications in table 1 to 46 CFR §56.60-1 or table 1 to §56.60-2, or from the material specifications of Section I or Section VIII of the ASME Boiler and Pressure Vessel Code. 46 CFR §58.30-15 All tubing and pipe materials must be suitable for the hydraulic fluid used and must remain ductile at the lowest operating temperature.
Fluid power motors and pumps installed on inspected vessels must be certified by the manufacturer as suitable for the intended use, and that suitability must be demonstrated by operational tests conducted aboard the vessel, witnessed by a marine inspector.
Diesel Engine Maintenance and Condition Monitoring
Reliable diesel operation depends on systematic monitoring of core condition indicators: compression pressure and firing (peak combustion) pressure per cylinder, exhaust temperature per cylinder, jacket-water and lube-oil temperatures and pressures, turbocharger speed, and fuel consumption. NAVEDTRA 14075 §3-1
Low compression pressure on a cylinder indicates leakage past worn rings, a burned or poorly seating valve, or a scored liner. A cylinder running hotter or colder on exhaust than its neighbors points to an injector fault, an intake or exhaust problem, or uneven load sharing. Rising fuel consumption for the same power output, falling firing pressures, and increasing smoke together signal wear or fouling.
Planned maintenance follows the manufacturer's running-hour schedule and includes: renewing fuel and lube-oil filters; cleaning or renewing injectors; checking and adjusting valve clearances (tappets); testing injection timing; inspecting and cleaning the turbocharger and air cooler; taking crank web deflections to check bearing and alignment condition; and periodically drawing pistons to inspect rings, liners, and bearings.
Two maintenance disciplines are paramount: absolute cleanliness — dirt is the enemy of injectors, bearings, and close-clearance parts — and correct torque and clearance on reassembly, because an over- or under-tightened bearing or head fails quickly.
Before any internal work, the engine must be stopped, cooled, starting air and fuel isolated, and the turning gear engaged so the engine cannot roll.
Steam Turbine Components, Lubrication, and Operation
The main parts of a steam turbine are the casing (which contains the steam and holds the fixed nozzles and blades), the rotor (which carries the moving blades), the bearings (which support and position the rotor), and the glands (which seal the shaft where it passes through the casing). NAVEDTRA 14104 §5-2
Journal bearings carry the rotor's weight; a thrust bearing holds the rotor in its correct axial position against the steam's push, maintaining the small clearances between moving and fixed blades.
Loss of lube-oil pressure is a trip-level casualty because a bearing will fail in seconds. Turbines therefore have oil-pressure alarms and low-oil trips, and often a gravity tank or an attached pump to keep oil flowing during coast-down.
Shaft glands are sealed with labyrinth packing and a gland-sealing steam system that keeps steam from leaking out at the high-pressure end and keeps air from leaking into the vacuum at the low-pressure end.
A cold turbine must be warmed slowly and rolled to heat it evenly; uneven heating bows the rotor and causes destructive rubbing and vibration. A turning gear (jacking gear) keeps the rotor rotating slowly whenever it is hot but not in use.
Other protective devices include the overspeed trip (shuts steam if the rotor exceeds safe speed), low-vacuum and high-exhaust-temperature alarms, and vibration monitoring. Condensate must be drained from steam lines before admitting steam.
Electrical Test Instruments
The multimeter (analog VOM or digital DMM) measures voltage (connected in parallel, high input resistance), current (connected in series or via clamp/CT, low insertion resistance), and resistance (on de-energized, isolated circuits only, using the meter's own source). DOE-HDBK-1011 Vol.4 §14-1
The wattmeter reads true power directly by combining a current coil and a voltage coil, accounting for power factor; on switchboards it is fed through current transformers (CTs) and potential transformers (PTs).
The megohmmeter (megger) applies a high DC test voltage — commonly 500–1000 V — to read insulation resistance in megohms. It is the primary test for motor, generator, cable, and switchgear insulation and for locating grounds. Equipment must be de-energized and discharged before and after the test. Readings are trended over time to catch gradual insulation deterioration.
Additional instruments include the clamp-on ammeter (reads load current without breaking a circuit), the phase-sequence indicator (confirms rotation before connecting motors or paralleling sources), and the synchroscope (matches an incoming generator to the bus).
Why it matters on the exam
QMED Junior Engineer exam questions on this topic cluster around four areas:
1. Stern tube bearing rebushing thresholds. Examiners test whether candidates know the specific clearance values and how they change with shaft diameter and machinery location. The 1.6 mm (0.0625 in) reduction for aft-located machinery is a frequent distractor — candidates confuse which location gets the tighter threshold. 46 CFR §61.20-23
2. Pre-work isolation procedures. Both diesel and turbine questions ask what must be done before internal work begins. For diesels: stop, cool, isolate starting air and fuel, engage turning gear. NAVEDTRA 14075 §3-1 For turbines: drain condensate from steam lines, use jacking gear whenever the rotor is hot but not running. NAVEDTRA 14104 §5-2
3. Instrument connection method. Voltage is measured in parallel; current in series (or via clamp). Resistance is measured only on de-energized circuits. Megger tests require the equipment to be de-energized and discharged both before and after. DOE-HDBK-1011 Vol.4 §14-1
4. Machinery guard requirements. The regulation covers gears, couplings, flywheels, and all rotating machinery capable of injuring personnel — not just one category. 46 CFR §58.01-20
5. Hydraulic system component certification. Fluid power motors and pumps must be manufacturer-certified and operationally tested aboard the vessel in the presence of a marine inspector. 46 CFR §58.30-15
Common pitfalls
Confusing amidship vs. aft machinery clearance thresholds. Aft-located machinery gets a smaller allowable clearance (tighter limit) by 1.6 mm (0.0625 in) compared to amidship. Candidates often invert this. 46 CFR §61.20-23
Applying the wrong bearing type rule. Rubber water-lubricated bearings are rebushed based on water groove depth (half original depth), not on a clearance measurement. Non-rubber water-lubricated bearings use the clearance thresholds. Oil-lubricated bearings are at OCMI discretion.
Megger test on energized equipment. The megohmmeter must only be used on de-energized and discharged equipment. Applying it to a live circuit will damage the instrument and create a safety hazard. DOE-HDBK-1011 Vol.4 §14-1
Skipping condensate drain before admitting steam to a turbine. Water induction into a turbine at speed causes catastrophic blade damage. Draining condensate from steam lines before admitting steam is a fundamental watchstanding requirement. NAVEDTRA 14104 §5-2
Treating lube-oil loss as a non-urgent turbine casualty. Loss of lube-oil pressure is a trip-level event — bearing failure occurs in seconds. The gravity tank or attached pump exists specifically to maintain oil flow during coast-down.
Overlooking the marine inspector witness requirement. Operational tests of fluid power motors and pumps on inspected vessels must be witnessed by a marine inspector, not merely documented by the crew. 46 CFR §58.30-15
Ignoring cleanliness as a maintenance discipline. Exam questions sometimes present cleanliness as optional good practice. The source material treats it as a primary maintenance discipline on equal footing with correct torque and clearance. NAVEDTRA 14075 §3-1
Quick check
At what clearance must the after stern tube bearing be rebushed on a vessel with amidship machinery and a shaft diameter of 250 mm (approximately 10 inches)?
7.95 mm (0.3125 in) — this shaft exceeds 229 mm (9 in) but does not exceed 305 mm (12 in), placing it in the middle diameter category. 46 CFR §61.20-23
How does the rebushing clearance threshold change when propelling machinery is located aft rather than amidship?
The threshold is reduced by 1.6 mm (0.0625 in) from the applicable amidship value — meaning the bearing must be rebushed sooner (at a smaller clearance).
What condition triggers rebushing of a water-lubricated rubber stern tube bearing?
When any water groove is worn to half its original depth.
What four actions must be taken before performing internal work on a diesel engine?
Stop the engine, allow it to cool, isolate starting air and fuel, and engage the turning gear so the engine cannot roll. NAVEDTRA 14075 §3-1
Why is loss of lube-oil pressure considered a trip-level casualty on a steam turbine?
Because bearing clearances are very small and speeds are very high — a bearing will fail within seconds of losing oil pressure. NAVEDTRA 14104 §5-2
How must a voltmeter be connected to a circuit, and why?
In parallel across the measurement points, with high input resistance, to avoid loading (drawing current from) the circuit being measured. DOE-HDBK-1011 Vol.4 §14-1
What test voltage does a megohmmeter typically apply, and what must be done to equipment before and after the test?
Commonly 500–1000 V DC. Equipment must be de-energized and discharged both before and after the test.