Pulling propeller
The propeller works ahead of the pod body in cleaner water, so the inflow is less disturbed than behind a boss, shaft brackets and rudder. The pod body then sits in the propeller race and can be shaped as a small foil.
01 :: ship as power station
What diesel-electric means on a cruise ship
Several engine-generator sets feed a common high-voltage bus. The bus does not care whether a megawatt is about to become propeller torque, chilled-water compression, galley heat, theatre lighting or a bow thruster. This is the architectural trick: the engines can be started, stopped and loaded near their efficient range while the ship's consumers take exactly the electrical power they need.
Single-line diagram
The animated pulses are active power. In the real plant there are protection relays, synchronising gear, bus-tie breakers, harmonic filters, excitation systems, PMS logic and blackout recovery layers between every box.
02 :: why azipods work
A pod is a propeller, motor and rudder in one submerged machine
An Azipod-style unit is not just a thruster bolted below the stern. It moves the propulsion motor into a steerable gondola, couples the propeller directly to the motor shaft, and uses a variable-frequency drive to set torque and rpm. That creates hydrodynamic, manoeuvring and machinery-layout advantages.
03 :: voyage load simulator
Slide the ship from hotel mode to full sea speed
Propulsive power rises roughly with the cube of speed near service conditions, while hotel load is large but comparatively steady. The result is why cruise itineraries are sensitive to speed: adding a few knots can demand tens of megawatts and another diesel on the board.
Power split over speed
Efficiency and losses
04 :: arithmetic underneath the romance
Megawatts, volts, amps and fuel
The most useful numbers are simple enough to do on a napkin. A 20 MW pod is about a thousand amps per phase at 11 kV. Drop the bus to 6.6 kV and the current jumps by two thirds, making copper, breakers and heat harder. That is why large cruise ships distribute propulsion power at medium voltage and only step down near hotel consumers.
Three-phase current
I = P / (√3 × V × pf)
For 20 MW at 11 kV, pf 0.95: . The same pod at 6.6 kV is . Real drives also have harmonic filters, cooling pumps and short-time overload ratings.
Speed-power cube
P₂ ≈ P₁ × (V₂/V₁)³
Cutting from 22 kn to 18 kn can nearly halve propulsion power, before weather and hull fouling. This is why slow steaming is powerful even on ships dominated by hotel loads.
Specific fuel model
fuel ≈ kWh × SFOC(load)
The simulator uses a representative medium-speed curve: poor below 30% load, best around 70–85%, then slightly worse at overload. It is a teaching model, not an engine guarantee.
Worked bus-current table
| Consumer | Power | 11 kV @ pf .95 | 6.6 kV @ pf .95 | 690 V @ pf .90 | Why it matters |
|---|
05 :: the hardware stack
From fuel rack to propeller disk
The words “diesel-electric” hide a chain of machines and control loops. Each stage is efficient in isolation, but at cruise-ship scale the small losses become megawatts of heat that must be removed by fresh-water, sea-water and ventilation systems.
Medium-speed diesel or dual-fuel engines
Four-stroke engines around 500–750 rpm are small enough to package in several genset rooms and large enough to produce 8–20 MW apiece. Multiple engine sizes are common: big sets cover sea speed, smaller sets cover port and night loads without wasting fuel.
Alternators and excitation
Each engine spins a synchronous generator. Automatic voltage regulators control excitation; governors and the PMS control real-power sharing. Before a breaker closes, voltage, frequency and phase angle must synchronise with the bus.
Medium-voltage switchboards
6.6 kV and 11 kV switchboards are typical at this scale. Bus sections, tie breakers and protective relays decide whether a fault is merely an alarm, a tripped feeder, a split bus, or a blackout recovery sequence.
Transformers
Propulsion transformers adapt the bus to drive input and add isolation. Ship-service transformers step down to 690 V motor control centres, then lower distribution for galleys, cabins, lighting, entertainment, HVAC controls and electronics.
Variable-frequency drives
Large marine drives rectify AC to a DC link and synthesize controlled-frequency AC for the motor. The propeller is fixed-pitch, so rpm and torque come from the inverter rather than from blade pitch.
Pod motor and bearings
Full-size cruise pods usually use large AC motors in the gondola, directly coupled to the propeller shaft. The pod needs shaft seals, thrust bearings, steering motors, cooling circuits and condition monitoring because maintenance access is expensive.
06 :: comparison matrix
Representative cruise-ship propulsion arrangements
Exact shipyard data varies by hull, year and refit. The table separates published Oasis-class examples from teaching cases used by the simulator.
| Case | Generating plant | Installed electric | Propulsion | Speed | Notes |
|---|
07 :: source notes
What the numbers are anchored to
This page keeps runtime network access at zero. The source trail used while building it included Wärtsilä 46F product data, public Oasis and Utopia class particulars, ABB/Azipod technical descriptions, and standard naval-architecture speed-power relationships. Where the page says “representative”, it is using a didactic model rather than a manufacturer-certified value.