Auxiliary Machinery and Systems
Auxiliary machinery questions test whether you can explain how pumps, compressors, boilers, purifiers, steering gear and deck machinery work, why they fail, and what is required for the safety-critical ones.
Core concepts
- Centrifugal pumps: flow varies with speed, head with speed squared. Cavitation begins when NPSH available falls below NPSH required; throttling the discharge reduces flow, not the suction problem.
- Positive displacement pumps: always protect the discharge with a relief valve; never run against a closed valve.
- Boilers: water treatment controls hardness, alkalinity, dissolved oxygen and chloride. Blowing through the gauge glass proves the level is real.
- Steering gear: SOLAS II-1/29 requires the main gear to move the rudder from 35 degrees one side to 30 degrees the other within 28 s at maximum ahead service speed, with duplicate power units, changeover, and emergency steering drills.
Worked example
A centrifugal pump rated 100 m3/h at 1,450 rpm is slowed to 1,200 rpm. New flow = 100 x (1200/1450) = 82.8 m3/h; head falls to (1200/1450) squared = 0.685 of the original; shaft power falls to 0.566. Use these figures before deciding on a smaller motor or impeller trim.
Traps
Confusing NPSH available with NPSH required; assuming a centrifugal pump needs no relief protection during normal service; treating boiler chemistry as a dosing problem instead of finding the leak or ingress.