PRC Naval Architecture & Marine Engineering common mistakes
PRC Naval Architecture & Marine Engineering · Understand the traps and how to avoid them.
Treating the hydrostatics answer as a single-point calculation and reporting displacement or GM from the design draft only, ignoring that the question specifies an inclined or light condition.
Candidates memorise one worked example at the design waterline and then apply the same displacement, KB, BM, or KM values to every condition; the exam deliberately varies draft, trim, and density to test whether the candidate re-derives from the actual loading condition.
What to do instead
Before computing, write down the stated condition (draft, trim, density, loading) and confirm each quantity is looked up or integrated at that condition; if a hydrostatic table is implied, state the interpolation between drafts explicitly and carry units through to the final answer.
Computing transverse stability without checking the free-surface correction, so GM is overstated whenever a partly filled tank is present.
Free surface effect is taught as a separate topic and candidates fail to connect it to the intact-stability question in front of them; the loss of GM is small on paper and easy to omit under time pressure.
What to do instead
Adopt a fixed checklist for any GM question: KG from moments, free-surface correction FSC = irho_liquid/(displacementrho_water) summed over slack tanks, then GM = KM - KG - FSC. State 'no slack tanks' explicitly if none are given.
Mixing intact-stability and damage-stability criteria, applying the intact GZ curve area requirements to a flooded-case question or vice versa.
The two sets of criteria share vocabulary (area under GZ curve, range of stability, angle of maximum GZ) but have different thresholds and different assumptions about permeability and flooding; candidates recall the numbers without the context.
What to do instead
Tag each stability question as intact or damage before selecting criteria, then recall the governing threshold with its condition attached (e.g. area to 30 degrees, area to 40 degrees, maximum GZ angle) and the assumed permeability for the damaged compartment.
Sizing primary structure by rule scantling formulae alone without checking buckling, fatigue, or the design load case that actually governs.
Candidates learn that classification-society rules give minimum thickness and section modulus and stop there, forgetting that rules are a floor and that buckling and fatigue can govern for long panels, high-stress details, or novel arrangements.
What to do instead
For each structural question, identify the governing limit state first (yield, buckling, fatigue, ultimate strength), then show the rule minimum and the separate check; if the arrangement departs from rule assumptions, say so and justify the direct calculation.
Answering a ship-design question with a single concept design point and no iteration or trade-off, as if length, beam, and power were independent choices.
Candidates treat design as a formula-recall exercise; the spiral nature of design (displacement drives dimensions, dimensions drive resistance, resistance drives power and fuel, fuel drives displacement) is understood in principle but not applied in the written answer.
What to do instead
Present the design as a sequence: state the requirement, choose the driving parameter, estimate consequences, and show at least one iteration or sensitivity check; name the trade-off being resolved (e.g. beam for stability versus resistance) rather than asserting one value.
Confusing the engine's rated output with the power delivered to the propeller, omitting shaft losses and the propeller-hull interaction coefficients in propulsion calculations.
Brake power, shaft power, delivered power, and effective power are distinct and candidates collapse them; the wake fraction and thrust deduction are learned as definitions but not applied in the chain from engine to thrust.
What to do instead
Always write the full chain with symbols: brake power to shaft power via transmission efficiency, to delivered power, to thrust power via open-water efficiency, to effective power via hull efficiency; check that each conversion factor is in the correct direction.
Diagnosing a machinery fault by naming the most familiar cause without working through the symptom pattern, e.g. attributing all exhaust smoke to fuel quality.
Fault-finding is often revised as a list of causes rather than a decision tree; candidates pattern-match on one symptom and skip the discriminating checks (load, timing, scavenge air, injector condition) that separate causes.
What to do instead
Structure the answer as symptom, possible causes ranked, and the specific test that discriminates each; state what a confirming or excluding reading would look like before concluding.
Describing a repair or welding procedure without addressing distortion control, preheat, or post-weld inspection for the material and thickness involved.
Candidates reproduce a generic sequence (cut out, fit, weld, test) and omit the variables that make the procedure acceptable: heat input, restraint, sequence, and NDT method matched to the joint.
What to do instead
For any repair question, state material and thickness, then give joint preparation, welding process and consumable, preheat/interpass limits, sequence to control distortion, and the inspection method and acceptance basis; note any hot work and confined-space controls.
Answering a regulatory or ethics scenario by citing the 'right' outcome without identifying the applicable requirement or the duty owed to whom.
Professional-conduct questions feel like common sense, so candidates give a moral conclusion and skip the framework: which rule applies, who is the duty holder, what must be recorded, and what the escalation path is.
What to do instead
Use a fixed structure: identify the governing requirement or code, name the duty holder and the party owed the duty, state the immediate action, then the reporting and record-keeping step and the escalation route if overruled.
Presenting calculations without units, sign conventions, or a stated assumption set, so a numerically correct answer is marked down or cannot be followed.
Under time pressure candidates write only the final number; in naval architecture sign conventions (GM positive, GZ positive, trim by stern) and unit systems (kN versus tonnes-force, density in t/m3 versus kg/m3) carry meaning and markers award method marks for them.
What to do instead
Write the formula, substitute with units, carry the sign convention explicitly, and state assumptions (seawater density, g value, permeability) at the top of the working; box the final answer with its unit and physical meaning.