Sizing a marine diesel generator is fundamentally different from sizing a generator for a building or a factory. The vessel is a closed electrical island — there is no utility grid to fall back on. The load profile shifts dramatically between port and sea passage. Hotel, propulsion auxiliary, and emergency loads each have their own duty cycles, applicable rules, and class society review focus. Get the sizing wrong and the consequences are not theoretical: an undersized genset trips during heavy AC compressor or thruster start, an oversized one runs at low load for years and risks "wet stacking" — incomplete combustion that can foul injectors, cylinders, and exhaust components.
This guide is written from a manufacturer's perspective. ASO Genset has delivered marine generators across vessel types and class society regimes — from a Norwegian offshore supply vessel project using 480 kVA class-approved main generator frames (DNV with +ICE-1A notation) to a 12-vessel Belt & Road fishing fleet (150 kVA per vessel, CCS) operating from Mauritania. We will walk through the actual sizing methodology — hotel/propulsion/emergency load classification, kW-to-kVA conversion with marine power factor, class society review factors, SOLAS Chapter II-1 emergency power requirements, and vessel type-specific considerations — and then share the top 6 sizing mistakes we see in real marine generator RFPs.
Table of Contents
- Why Marine Generators Are Different from Land Backup
- Hotel Load vs Propulsion Auxiliary Load vs Emergency Load
- Calculating Total Marine Electrical Load (Step by Step)
- Class Society Review Factors and Sizing Margins
- SOLAS Chapter II-1 Emergency Generator Requirements
- Vessel Type-Specific Sizing (Yacht / Fishing / Cargo / Offshore)
- Top 6 Sizing Mistakes We See in Marine Generator RFPs
- ASO Marine Reference Cases (Norwegian + Mauritania)
- Specification Checklist for Marine Generator RFQs
- Frequently Asked Questions
Quick Answer
Marine diesel generator sizing follows a specific sequence: (1) list all electrical equipment by category — hotel, propulsion auxiliary, mission-specific, emergency — with running power (kW) and duty cycle; (2) apply diversity and simultaneity factors per vessel operating profile; (3) convert total continuous kW to kVA using the marine power factor (typically 0.8 lagging); (4) add a margin for motor-starting transients (typically 20-25%, higher for vessels with large thrusters or AC compressors); (5) layer a documented operating margin on top, sized against the applicable class society's review focus rather than a universal fixed percentage. Where possible, keep continuous load in the 50-80% range of nameplate, while not operating below the engine OEM's minimum recommended load. For a 60-meter offshore supply vessel with approximately 300 kW of realistic sea-mode continuous load, the typical sizing lands around 480 kVA per main genset in a 2-unit redundant configuration, with the SOLAS emergency generator sized separately from the mandatory emergency load list.
Why Marine Generators Are Different from Land Backup
A land backup generator is sized against utility failure: it runs for hours during an outage, then sits idle for weeks until the next event. A marine generator is the primary power source for the vessel's entire electrical system — it runs continuously while at sea, often for weeks or months between port stops, with no possibility of falling back to a utility grid. The sizing implications are substantial on every dimension.
The Vessel Is a Closed Electrical Island
There is no utility grid. If the genset trips, the vessel goes dark — and on a moving ship, going dark can mean losing steering assistance, navigation, communication, essential auxiliaries, and in worst cases propulsion-related control functions. The sizing margin must therefore handle every realistic transient with headroom to spare, not just average load.
The Load Profile Shifts Dramatically
A passenger vessel in port runs hotel load (lighting, HVAC, galley, accommodation power) at 60-80% of installed capacity. The same vessel at sea may run propulsion auxiliary load (steering gear, fuel pumps, cooling water pumps, separators) at 40-60%, with hotel load reduced. A cargo ship at anchor runs essentially zero load. A fishing vessel hauling nets has periodic high-power refrigeration and processing surges. Sizing must address the highest realistic continuous load, not the average.
Low-Load Operation Has Its Own Risks
Marine diesel engines are generally specified to operate continuously near their rated load range. Sustained operation below the engine OEM's minimum recommended load can lead to incomplete combustion — unburnt fuel and lubricating oil can collect in the exhaust manifold and turbocharger, forming the residue commonly known as "wet stacking." Over time this can cause injector fouling, cylinder glaze, exhaust component degradation, and progressive loss of rated power. The risk is cumulative. Significantly oversizing a marine generator is not a "safe" choice — it carries its own long-term reliability cost.
Class Society Compliance Adds Review Requirements
Every classified vessel must comply with the rules of its chosen class society — ABS, DNV, CCS, LR, BV, or another IACS member. The acceptable sizing margin is generally established through the class society's review of the vessel's load balance, essential-service continuity, redundancy philosophy, motor-start performance, and FAT/sea-trial verification. For background on the five major class societies, see our guide to ABS, DNV, and CCS marine generator classification.
SOLAS Mandates a Separate Emergency Power Source
Under SOLAS Chapter II-1, SOLAS-applicable passenger ships and cargo ships (generally of 500 GT and above engaged on international voyages) must carry an independent emergency source of electrical power — typically a dedicated emergency generator, separate from the main generators, located outside the main machinery space, and capable of supplying emergency services within the required time period. The sizing of this emergency unit is governed by the approved SOLAS emergency load list and the applicable class society / Administration, and is calculated independently of the main generator capacity.
Hotel Load vs Propulsion Auxiliary Load vs Emergency Load
Marine electrical loads are not a single number. They fall into three categories that have different duty cycles, different sizing methodologies, and different class society treatment.
Hotel Load
The "hotel load" is everything required to keep the vessel comfortable and functional as a place where people live and work: accommodation lighting, HVAC, galley equipment, refrigeration, laundry, sewage treatment, fresh water generation, entertainment systems, and crew/passenger services. On a yacht, hotel load can dominate. On a cargo ship in transit, hotel load is small. On a passenger ferry, hotel load can be the single largest demand.
Hotel load is generally continuous with predictable diurnal variation (higher during meal preparation, evening hours; lower at night). The diversity factor — the ratio of actual maximum simultaneous load to total connected load — is typically in the 0.6-0.8 range for hotel applications, subject to the project's specific load list and operating profile.
Propulsion Auxiliary Load
This is everything required to keep the vessel moving and operating safely at sea (but not the main propulsion itself, which is usually driven directly by the main engines, not the generators). It includes steering gear, fuel transfer and treatment pumps, lubricating oil pumps, cooling water pumps, ventilation fans, separators, deck machinery, bridge electronics, and engine room auxiliary services.
Propulsion auxiliary load varies significantly between port and sea modes. The same vessel may need 250 kW of auxiliary load at sea and only 50 kW at anchor. Diversity factor is typically in the 0.7-0.9 range, depending on the specific project.
Mission-Specific Load
Some vessel types add a third category that does not fit neatly into hotel or propulsion auxiliary:
- Fishing vessels: refrigeration compressors, processing equipment, net haulers, winches
- Offshore supply vessels: deck cranes, mud pumps, dynamic positioning thrusters
- Cargo ships: cargo handling equipment, reefer container outlets, ballast pumps
- Yachts: stabilizer hydraulics, water toy electrical, high-power AV systems
Mission-specific load often has high peak demand with low duty cycle (e.g., crane lifts, net hauls), which drives transient sizing margins rather than continuous sizing.
Emergency Load (SOLAS Required)
SOLAS Chapter II-1 defines mandatory emergency loads that must be powered by a separate emergency electrical source. These typically include emergency lighting throughout the vessel, navigation lights, internal communication, fire detection and alarm systems, GMDSS radio equipment, and (where required) emergency steering, bilge, and fire pumps. The exact list and autonomy period depend on vessel type, tonnage, and Administration / class society requirements. Emergency load is sized independently of the main generators and is covered in detail in the SOLAS section below.
Why Marine Generators Are Different from Land Backup
A land backup generator is sized against utility failure: it runs for hours during an outage, then sits idle for weeks until the next event. A marine generator is the primary power source for the vessel's entire electrical system — it runs continuously while at sea, often for weeks or months between port stops, with no possibility of falling back to a utility grid. The sizing implications are substantial on every dimension.
The Vessel Is a Closed Electrical Island
There is no utility grid. If the genset trips, the vessel goes dark — and on a moving ship, going dark can mean losing steering assistance, navigation, communication, essential auxiliaries, and in worst cases propulsion-related control functions. The sizing margin must therefore handle every realistic transient with headroom to spare, not just average load.
The Load Profile Shifts Dramatically
A passenger vessel in port runs hotel load (lighting, HVAC, galley, accommodation power) at 60-80% of installed capacity. The same vessel at sea may run propulsion auxiliary load (steering gear, fuel pumps, cooling water pumps, separators) at 40-60%, with hotel load reduced. A cargo ship at anchor runs essentially zero load. A fishing vessel hauling nets has periodic high-power refrigeration and processing surges. Sizing must address the highest realistic continuous load, not the average.
Low-Load Operation Has Its Own Risks
Marine diesel engines are generally specified to operate continuously near their rated load range. Sustained operation below the engine OEM's minimum recommended load can lead to incomplete combustion — unburnt fuel and lubricating oil can collect in the exhaust manifold and turbocharger, forming the residue commonly known as "wet stacking." Over time this can cause injector fouling, cylinder glaze, exhaust component degradation, and progressive loss of rated power. The risk is cumulative. Significantly oversizing a marine generator is not a "safe" choice — it carries its own long-term reliability cost.
Class Society Compliance Adds Review Requirements
Every classified vessel must comply with the rules of its chosen class society — ABS, DNV, CCS, LR, BV, or another IACS member. The acceptable sizing margin is generally established through the class society's review of the vessel's load balance, essential-service continuity, redundancy philosophy, motor-start performance, and FAT/sea-trial verification. For background on the five major class societies, see our guide to ABS, DNV, and CCS marine generator classification.
SOLAS Mandates a Separate Emergency Power Source
Under SOLAS Chapter II-1, SOLAS-applicable passenger ships and cargo ships (generally of 500 GT and above engaged on international voyages) must carry an independent emergency source of electrical power — typically a dedicated emergency generator, separate from the main generators, located outside the main machinery space, and capable of supplying emergency services within the required time period. The sizing of this emergency unit is governed by the approved SOLAS emergency load list and the applicable class society / Administration, and is calculated independently of the main generator capacity.
Calculating Total Marine Electrical Load (Step by Step)
Here is the full main-generator sizing calculation. All numerical ranges below are typical industry approximations; final values for any specific project depend on the OEM derating curves, the alternator family selected, the vessel's operating profile, and the class society's sizing methodology.
Step 1: List All Connected Loads by Category
Create a load list that includes, for each piece of electrical equipment: (1) connected power (kW or kVA per nameplate), (2) operating power (kW actually drawn under normal duty), (3) starting current factor (for motors), (4) duty cycle (continuous, intermittent, occasional), and (5) category (hotel, propulsion auxiliary, mission-specific, emergency).
This load list is the single most important document in marine generator sizing. Sloppy load lists are the root cause of most sizing failures.
Step 2: Apply Diversity and Simultaneity Factors
Not all equipment runs at full power simultaneously. The diversity factor (typically 0.6-0.9 depending on category) accounts for the fact that the total operating load is less than the sum of nameplate loads. The simultaneity factor (also called "load coincidence") accounts for which equipment is realistically running at the same time during peak operating modes.
For a vessel at sea: hotel load typically runs at around 0.7 diversity, propulsion auxiliary at around 0.85, mission-specific at around 0.5 (depending on operating mode).
Example: A 60-meter offshore supply vessel, after detailed load analysis, has a realistic continuous sea-mode load of approximately 300 kW (hotel + propulsion auxiliary + low-duty mission-specific).
Step 3: Convert kW to kVA Using Marine Power Factor
Marine alternators are typically rated at 0.8 lagging power factor — same as land applications. The conversion is:
Required kVA = Total continuous kW ÷ 0.8
Example (continued): 300 kW ÷ 0.8 = 375 kVA base demand.
Step 4: Add Transient Margin (Motor Starting, AC Compressor, Thruster)
Marine loads include significant motor-start surges. A large AC compressor or watermaker can draw 5-7× its running current at startup. Bow thrusters, deck cranes, and propulsion auxiliary motors create momentary peaks that can be 2-3× steady-state demand.
Typical industry transient margin: 20-25% above continuous demand for general vessels, 30-40% for vessels with large thrusters or refrigeration plant.
Example (continued): 375 kVA × 1.20 = 450 kVA design demand.
Step 5: Apply Documented Operating Margin per Class Society Review
On top of the engineering calculation, designers typically maintain a documented operating margin above calculated demand. This margin is not a universal fixed percentage required by class — see the next section for how each major class society approaches sizing review. In practice, a documented operating margin in the 5-15% range above design demand is commonly applied, with the exact value depending on vessel type, operating profile, and the approved load analysis.
Example (continued): 450 kVA × 1.05-1.10 documented operating margin = approximately 470-495 kVA. Selecting the next standard frame size up gives 480 kVA per main genset.
Step 6: Apply Redundancy
For most commercial classifications, the main generator configuration must allow full sea-mode operation with one unit out of service for maintenance. The 480 kVA per main genset in the example above was sized so that one unit alone can carry the full realistic sea-mode load, meaning a 2-unit main configuration provides true single-failure redundancy.
Worked Example Summary
For a 60-meter offshore supply vessel with approximately 300 kW of realistic sea-mode continuous load: 300 kW ÷ 0.8 PF = 375 kVA base → ×1.20 transient margin = 450 kVA design → ×1.05-1.10 documented operating margin = approximately 470-495 kVA → 2 main gensets of 480 kVA each, where one unit alone can carry the full sea-mode load while the other is unavailable for maintenance. The SOLAS emergency generator is treated as a separate compliance item, with capacity determined by the approved emergency load list rather than by the main generator rating.
Class Society Review Factors and Sizing Margins
Class societies usually do not approve a generator size by applying a universal fixed percentage. Instead, they review the vessel's load balance, essential-service continuity, redundancy philosophy, motor-start performance, emergency power independence, and applicable class notations. In practice, designers often keep a documented operating margin of roughly 10-15% above calculated demand, but the acceptable margin depends on the vessel type, operating profile, selected class rules, and the approved load analysis.
What Each "Big 5" Class Society Tends to Review
| Class Society | Typical Review Focus |
|---|---|
| ABS | Emphasis on documented load analysis, essential services, redundancy, and witnessed testing scope. |
| DNV | Strong focus on system integration, redundancy philosophy, and notation-specific requirements such as DP or ice-class operation. |
| CCS | Practical approval pathway for Chinese-built or China-financed tonnage; documentation and type approval remain project-specific. |
| LR | Detailed review of electrical load balance, switchboard design, non-linear loads, and project-specific notation requirements. |
| BV | Commonly used for ferries, fishing vessels, offshore support, and short-sea shipping; approval scope depends on vessel type and notation. |
The review focus descriptions above are general observations from project experience; they are not formal statements of class society rules. Always confirm against the current applicable class society rules and the specific vessel notation before procurement.
IACS Acceptance and Real-World Class Selection
All five class societies above are IACS members and are widely accepted for international commercial shipping. However, the required class society for a specific project is normally determined by the owner, flag state, charterer, financing party, shipyard contract, and any class-transfer or project-specific approval requirements — not by a universal "any IACS member works" rule.
The Redundancy Rule You Cannot Skip
For most commercial vessel classifications, the main generator configuration must allow full sea-mode operation with one unit out of service for maintenance. This means sizing is not "total demand ÷ number of units" — it is "total sea-mode demand carried by the remaining N-1 units, with each remaining unit sized for its share of that load."
For a 2-generator vessel with 480 kVA sea-mode demand, each generator must be rated at 480 kVA (not 240 kVA). For a 3-generator vessel, each must handle 240 kVA (so any 2 carry the full load). This is the marine equivalent of N+1 redundancy and is non-negotiable for classed vessels.
SOLAS Chapter II-1 Emergency Generator Requirements
SOLAS (the International Convention for the Safety of Life at Sea) Chapter II-1 mandates an independent emergency electrical power source on SOLAS-applicable commercial vessels. The emergency generator is sized and located independently of the main generators, and its requirements are non-negotiable for international voyages.
What SOLAS Actually Requires
Under SOLAS Chapter II-1, Regulation 42 for passenger ships and Regulation 43 for cargo ships, SOLAS-applicable commercial vessels generally require an independent emergency source of electrical power. For cargo ships, this normally applies to ships of 500 GT and above; smaller cargo ships may be subject to special consideration by the Administration or class society. The emergency source is typically:
- Arranged separately from the main source of electrical power and located outside the main machinery space, in accordance with the applicable SOLAS / class requirements
- Provided with its own switchboard, starting system, and fuel supply
- Capable of supplying the required emergency services within the required time period — commonly within 45 seconds after loss of the main source
- Capable of running on its own fuel supply for the autonomy period required by vessel type — commonly 18 hours for cargo ships and 36 hours for passenger ships, subject to vessel type, voyage profile, Administration approval, and applicable class rules
Mandatory Emergency Loads
The list varies by vessel type and tonnage but generally includes:
- Emergency lighting throughout the vessel (engine room, bridge, accommodation, escape routes, embarkation stations)
- Navigation lights
- Internal communication systems (PA, sound-powered phones, alarm systems)
- Fire detection, alarm, and (where required) one emergency fire pump
- Emergency bilge pump (on some vessel classes)
- Steering gear (where required by vessel type)
- Watertight door operating systems (passenger ships)
- GMDSS radio equipment
- Vessel propulsion control and monitoring (limited)
Indicative Emergency Generator Ranges — Not Design Values
The ranges below are early-stage reference values only. Final emergency generator capacity must be calculated from the approved SOLAS emergency load list and confirmed by the applicable class society / Administration. Vessel type, tonnage, emergency fire pump requirements, steering gear philosophy, communication systems, watertight doors, and passenger-vessel requirements can materially change the final capacity.
| Vessel Type | Indicative Early-Stage Reference (Not Design Value) |
|---|---|
| Small commercial vessel / workboat, subject to local Administration requirements | 15-40 kVA |
| Mid-size cargo / fishing | 50-150 kVA |
| Large cargo / container / bulker | 150-300 kVA |
| Passenger ships / large ferries | 300-800+ kVA |
| Offshore platforms / FPSO | 300-1,000+ kVA |
The emergency generator is sized by working through the approved emergency load list with the applicable class society / Administration, applying realistic simultaneity (most emergency loads run together during an actual emergency), and selecting a frame size that accommodates the required services for the required autonomy period.
Vessel Type-Specific Sizing (Yacht / Fishing / Cargo / Offshore)
Beyond the general methodology above, each vessel type has its own sizing nuances driven by typical load profile, operating environment, and class society treatment.
Yachts and Megayachts
Yacht generator sizing is dominated by hotel load — guest accommodation, galley, HVAC, water toys, stabilizers, and entertainment can easily exceed the propulsion auxiliary demand. A typical 30-meter motoryacht has 80-120 kW hotel load, 30-50 kW propulsion auxiliary. Sizing margin should accommodate peak entertainment scenarios (chef, full galley, all guests showering simultaneously).
Yacht generator selection in the 6-30 kVA range for smaller vessels has its own design considerations — noise, vibration, footprint, and engine hours. For detailed selection guidance in that range, see our yacht generator selection guide (6-30 kVA).
Fishing Vessels
Fishing vessel sizing is dominated by mission-specific load: refrigeration compressors that may need to chill many tons of catch, processing equipment, net haulers, winches, and on-board processing for larger trawlers. Hotel load is typically modest. The major sizing challenge is transient surges from refrigeration cycling and hauling operations.
Single vs twin generator decisions for fishing trawlers also have significant lifecycle cost implications — different motor start methods (DOL vs soft starter vs VFD) interact with generator sizing in non-obvious ways. For a detailed analysis of single vs twin generator configurations on fishing vessels, see our fishing trawler generator lifecycle cost guide.
Cargo Ships (Container / Bulk / General)
Cargo ship sizing is driven by propulsion auxiliary load at sea and cargo handling load in port. Container ships add reefer container outlet capacity — a vessel with substantial reefer plug capacity can have multiple MW of cargo electrical demand alone, often handled by dedicated reefer generators in addition to main and emergency units.
Bulk carriers have lower cargo electrical demand but require sizing for ballast pumps, cargo hold ventilation, and grab-bucket cranes (on geared vessels). General cargo vessels fall in between, with sizing driven by typical cargo handling equipment.
Offshore Supply / OSV / PSV / AHTS
Offshore supply vessel sizing is dominated by dynamic positioning thrusters (on DP-class vessels), deck cranes, mud pumps (on PSV), and large hotel/accommodation loads (these vessels carry significant crew on extended trips). Operating profile varies dramatically: at port, low load; in transit, moderate; on station with DP active, very high transient demand from thrusters responding to wind and current.
DP-class vessels (DP1, DP2, DP3) have specific class society requirements for generator redundancy — DP2 typically requires that any single failure cannot disable position-keeping capability, which drives multi-generator configurations with sophisticated power management.
For offshore vessels with custom generator cabin / module requirements (ABS/DNV approved, IECEx/ATEX hazardous area, skid-mounted), see our custom offshore generator cabin design guide.
Top 6 Sizing Mistakes We See in Marine Generator RFPs
These are the six sizing errors we see most often when reviewing marine generator RFPs and bid responses — drawn from competitive bid analysis on customer projects we have quoted and won, and from the projects we have been called in to diagnose after a competitor's specification underperformed in service.
1. Oversizing — Treating "Bigger" as "Safer"
Pattern: Owner or naval architect adds substantial margin "for safety" on top of the engineering calculation. Specifies a much larger frame than the documented load analysis supports.
Result: The vessel runs at a low fraction of nameplate for years. Sustained low-load operation can accelerate wet stacking, injector fouling, cylinder glaze, and exhaust component degradation. Over time the genset may produce less rated power than nameplate. Replacement or major overhaul may become necessary earlier than planned.
Fix: Size to land continuous load near the engine OEM's recommended operating range, typically in the 50-80% nameplate range where possible. Add transient margin (20-25%) on top, not 50-100%. Trust the engineering calculation and the documented load analysis.
2. Sloppy Load List (Missing Mission-Specific Equipment)
Pattern: Load list captures hotel and propulsion auxiliary but omits or under-estimates mission-specific load — fishing refrigeration, offshore DP thrusters, reefer outlets, cargo cranes.
Result: Generator trips on overload first time mission-specific equipment runs at peak. Owner blames the supplier; root cause was the original load list.
Fix: Build the load list with the operating crew or master, not from a naval architect spreadsheet alone. Include realistic peak operating modes.
3. Ignoring Motor-Starting Transients
Pattern: Sizing done on running kW only. Large AC compressor, watermaker, or bow thruster motor draws 5-7× starting current; generator voltage dips below limits and trips out.
Result: Blackout on first startup of major equipment. Embarrassing during sea trials; dangerous in service.
Fix: Add 25-40% margin for vessels with large motor loads. Consider soft starters or VFDs for the largest motors to reduce starting current.
4. Confusing kW and kVA in Bid Comparison
Pattern: Bid A quotes "500 kVA"; Bid B quotes "500 kW". Buyer treats them as equivalent.
Result: Bid A is actually a 400 kW unit (at 0.8 PF); Bid B is a 625 kVA unit (at 0.8 PF). The "cheaper" bid is significantly undersized.
Fix: Require every marine generator bid to state both kW and kVA, with rated PF, at the specified ambient temperature. For a structured way to evaluate competing marine generator bids, see our 25-point generator bid comparison checklist.
5. Forgetting Tropical / Coastal Site Conditions
Pattern: Sizing done at ISO 25°C reference, but vessel operates in Persian Gulf or Southeast Asian waters with 45-50°C engine room ambient and 80%+ humidity.
Result: Generator delivers a significantly reduced fraction of nameplate output in service. Single-failure redundancy effectively disappears. Saltwater corrosion accelerates on uncoated components.
Fix: Apply tropical derating to nameplate sizing per the OEM derating curve. Specify Class H alternators, anti-condensation heaters, and coastal-spec coating. For detailed tropical engineering requirements, see our tropical climate diesel generator selection guide.
6. Treating Emergency Generator as an Afterthought
Pattern: Main generators carefully sized; emergency generator specified at last minute, often undersized for the actual SOLAS emergency load list and class society autonomy requirement.
Result: Class society survey identifies non-compliance during plan approval; vessel cannot pass trials until corrected; expensive retrofit.
Fix: Size emergency generator from the mandatory SOLAS emergency load list, with documented calculations, and include in plan approval submission early.
Pattern Across All 6 Mistakes
Half of these mistakes lead to undersizing (load list, transients, kW/kVA confusion, tropical derating, emergency gen). The other half lead to oversizing (the "safety margin" mistake). Unlike land generators — where most problems are undersizing — marine generators carry meaningful risk in both directions. The discipline is: trust the engineering calculation, apply documented margins, and resist the temptation to "just go one size bigger" without supporting load analysis.
The long-term cost of oversizing shows up as wet stacking — incomplete combustion that fouls injectors, exhaust manifolds, and turbocharger housings on engines that spend their lives at low load. For the full diagnostic, prevention, and recovery framework, see our marine generator wet stacking prevention guide.
ASO Marine Reference Cases
The following two deployments illustrate how ASO Genset approaches marine generator sizing across different vessel types, class society regimes, and operating environments. Customer names are withheld for confidentiality; project parameters are described in terms agreed for public reference.
Hero Case — North Sea Offshore Supply Vessel (2024)
| Parameter | Specification |
|---|---|
| Vessel type | Offshore supply vessel, Norwegian sector |
| Main genset configuration | 2 × 480 kVA main generators, with each unit alone capable of carrying the realistic sea-mode load |
| Emergency genset configuration | Separately sized SOLAS emergency generator, with capacity determined by the approved emergency load list rather than by the main generator rating |
| Class society | DNV with +ICE-1A notation (light ice operation) |
| Sizing rationale | Approximately 300 kW realistic sea-mode continuous load ÷ 0.8 PF = 375 kVA base; +20% transient margin = 450 kVA design; +5-10% documented operating margin = approximately 470-495 kVA; selected 480 kVA standard frame per main unit, configured so one unit alone carries full sea-mode load while the other is available for maintenance. |
| Ice notation features | Arctic-grade marinization, low-temp lube oil, heated air intakes, cold-start verified at -25°C ambient |
| DNV approval | Modular approval reusing existing 480 kVA engine approval; only arctic-grade marinization package newly certified |
| Total DNV cost per unit | USD $22,000 (slightly higher than baseline due to +ICE notation) |
| Timeline | 14 weeks from order to delivery |
Key sizing decision: The main generators were selected so that one unit could carry the realistic sea-mode load while the other unit was unavailable for maintenance. The emergency generator was treated as a separate compliance item, with capacity determined by mandatory emergency services rather than by the main generator rating. The 480 kVA frame matched a well-supported DNV-approved engine family, which kept the modular approval cost and timeline favorable.
Supporting Case — Mauritania Belt & Road Fishing Fleet (2023)
| Parameter | Specification |
|---|---|
| Application | Belt & Road fishing fleet, Chinese-Mauritanian joint venture, operating from Nouakchott |
| Configuration | 12 vessels, one 150 kVA main genset package per vessel, with emergency / reserve power arranged according to the approved vessel electrical design |
| Class society | CCS classification (China EXIM Bank financing requirement) |
| Vessel type | Mid-size fishing trawlers, ~30-35m, with on-board refrigeration |
| Sizing rationale | Hotel ~25 kW + propulsion auxiliary ~30 kW + refrigeration (peak duty cycle) ~50 kW = ~105 kW continuous at peak operating mode; ÷0.8 PF = 131 kVA; + documented operating margin = ~150 kVA; selected 150 kVA standard CCS-approved frame |
| Marinization | Coastal salt-air protection, IP55 control panel, anti-condensation heaters, Class H alternator |
| CCS cost per unit | USD $9,200 |
| Timeline | 10 weeks from order to delivery (faster CCS approval cycle) |
| Fleet status | All 12 vessels in service since 2023; no unplanned generator downtime reported through current operating year |
Key sizing decision: Sizing per vessel rather than per fleet. The owner initially asked for "1,800 kVA total for the fleet" — which is the right total, but the practical configuration is one independent 150 kVA genset per vessel. Each vessel needed to operate independently with its own classed power source. The 150 kVA frame matched the CCS-approved engine family and kept per-unit cost low through volume.
Specification Checklist for Marine Generator RFQs
Use this checklist when issuing an RFP for a marine generator project, or when reviewing competing bids. Each line item is one we have seen cause problems when omitted or vague.
| Category | Required Specification |
|---|---|
| 1. Vessel particulars | Vessel type, length, displacement, intended trade route, operating environment. |
| 2. Class society | ABS, DNV, CCS, LR, BV, or other IACS member. Required class notations (e.g., +ICE, +DP2, +ECO). |
| 3. Load list | Complete electrical load list by category (hotel / propulsion auxiliary / mission-specific / emergency) with connected and operating kW, duty cycle, and starting current. |
| 4. Required output | Required kVA AND kW at site ambient conditions, with rated PF stated. Single-unit redundancy assumption documented. |
| 5. Sizing margins | Transient margin (typically 20-25%), documented operating margin justified through load analysis, tropical derating where applicable. |
| 6. Alternator | Class H insulation, marinized windings, anti-condensation heaters, harmonic distortion tolerance per project's UPS / VFD load profile. |
| 7. Engine | Marine-rated engine with class society type approval. Specified OEM engine family. |
| 8. Cooling | Heat exchanger or keel cooling specification appropriate to vessel type and operating environment. |
| 9. Marinization | Coastal/salt-air protection, IP rating, coating system, fasteners (SS316 for marine). |
| 10. Controls / paralleling | Synchronization, load sharing, power management system for multi-genset installations. |
| 11. Emergency generator | SOLAS Chapter II-1 compliant configuration: located outside the main machinery space, separate fuel supply, autonomy per vessel type, mandatory emergency load list. |
| 12. Class approval scope | Type approval, plan approval, witnessed FAT, on-board survey scope clearly defined. |
| 13. Documentation | Load balance calculation, sizing margin justification, OEM data sheets, class society certificates, FAT report. |
| 14. Spares and service | Recommended spares list, global service network coverage for intended trade route. |
| 15. Warranty | Warranty terms specific to marine operation, including coverage at intended ambient and humidity conditions. |
Frequently Asked Questions
What is the 80% rule for marine generators?
The "80% rule" is a common practice of sizing a marine generator so that continuous operating load sits at no more than around 80% of rated capacity, leaving headroom for transient surges, future load growth, and documented operating margin. Combined with the practical 50% lower threshold (below which wet-stacking risk and combustion-related issues can increase), the target operating window for a properly sized marine generator is commonly described as 50-80% of nameplate where possible, while not operating below the engine OEM's minimum recommended load.
How do I size a marine generator step by step?
(1) Build a complete load list by category — hotel, propulsion auxiliary, mission-specific, emergency — with running kW and duty cycle for each item. (2) Apply diversity factors per category (typically around 0.7 hotel, 0.85 propulsion auxiliary, 0.5 mission-specific). (3) Convert total continuous kW to kVA dividing by the marine power factor (typically 0.8 lagging). (4) Add transient margin of 20-25% (higher for vessels with large thrusters or AC compressors). (5) Layer a documented operating margin justified through the vessel's load analysis and applicable class society review. (6) Configure for redundancy so any single unit out of service still allows full sea-mode operation. (7) Separately size the SOLAS emergency generator from the approved mandatory emergency load list.
What is the difference between hotel load and propulsion load on a marine generator?
Hotel load is everything required to keep the vessel comfortable and functional as a place where people live and work: accommodation lighting, HVAC, galley, refrigeration, fresh water, sewage treatment, entertainment. Propulsion auxiliary load is everything required to keep the vessel moving and operating safely at sea, but not the main propulsion itself: steering gear, fuel pumps, cooling water pumps, separators, ventilation, deck machinery, bridge electronics. Main propulsion engines are usually driven directly by their own engines, not by the generators — though some vessels (especially modern cruise ships and offshore platforms) use diesel-electric configurations where generators do supply propulsion power.
How do class societies treat marine generator sizing?
Class societies usually do not approve a generator size by applying a universal fixed percentage. Instead, they review the vessel's load balance, essential-service continuity, redundancy philosophy, motor-start performance, emergency power independence, and applicable class notations. In practice, designers often keep a documented operating margin of roughly 10-15% above calculated demand, but the acceptable margin depends on the vessel type, operating profile, selected class rules, and the approved load analysis. Class approval cost varies significantly by society, vessel type, and notation scope — see the related ABS vs DNV vs CCS classification guide for cost-related context.
What is wet stacking and how do I avoid it?
Wet stacking is the accumulation of unburnt fuel and lubricating oil in the exhaust system of a diesel engine running for extended periods below the engine OEM's minimum recommended load. The cool exhaust temperature can prevent complete combustion, leaving residue that can progressively foul injectors, glaze cylinder walls, and degrade turbocharger and exhaust components. Marine generators are susceptible because oversizing is common and continuous low-load operation is the normal duty cycle on some vessels. Practical mitigation: where possible, size the generator so continuous load sits in the 50-80% range; if operational reality requires lower load, run periodic high-load operation or load-bank testing in accordance with the engine OEM's recommendation.
Do I need a separate emergency generator on my vessel?
Under SOLAS Chapter II-1, SOLAS-applicable passenger ships and cargo ships generally must carry a separate emergency electrical power source. The exact requirement depends on vessel type, tonnage (for cargo ships, typically 500 GT and above), and Administration / class society. The emergency generator is typically located outside the main machinery space, has its own fuel supply and starting battery, and supplies mandatory emergency loads (emergency lighting, navigation lights, internal communication, fire detection, GMDSS, and others depending on vessel type) for a specified autonomy period — commonly 18 hours for cargo ships and 36 hours for passenger ships, subject to vessel type, voyage profile, Administration approval, and applicable class rules. Some smaller vessels and certain vessel types may be subject to special consideration, but for SOLAS-applicable international trade, a separate emergency generator is normally required.
How should I budget for a marine generator including class approval?
Marine generator package cost varies too widely to quote accurately by kVA alone. Engine OEM, alternator brand, heat-exchanger or keel-cooling arrangement, paralleling controls, class approval scope, coating system, documentation package, FAT witness requirements, and spares package can change the final price more than the base kVA rating. For early-stage budgeting, separate the estimate into hardware, marinization, class approval, testing, documentation, logistics, and commissioning rather than relying on a single $/kVA figure. The class approval portion specifically — type approval, plan approval, witnessed FAT, on-board survey — varies meaningfully between societies and is typically a separate line item on the project budget.
Free Download: Marine Generator Sizing Calculator (Excel)
Built from ASO marine sizing experience across class society regimes
- Hotel + propulsion auxiliary + mission-specific load worksheet with diversity factors
- kW → kVA conversion calculator with marine PF
- Transient margin selector by vessel type
- Class society review-focus reference (ABS / DNV / CCS / LR / BV)
- SOLAS Chapter II-1 emergency generator early-stage sizing worksheet
- 15-point RFQ specification checklist
No spam. Excel emailed instantly. Use it with any supplier, including ASO.
Related Reading
- ABS vs DNV vs CCS Marine Generator Classification — Detailed comparison of the "big 5" classification societies for shipowners and naval architects.
- Yacht Generator Selection Guide (6-30 kVA) — Smaller yacht-specific sizing in the 6-30 kVA range, including noise and footprint considerations.
- Fishing Trawler Generator Lifecycle Cost Guide — Single vs twin configuration analysis, DOL vs soft starter vs VFD for fishing vessels.
- Custom Offshore Generator Cabin Design Guide — ABS/DNV approved, IECEx/ATEX hazardous area, skid-mounted offshore generator packages.
- Data Center Backup Generator Sizing Guide — Parallel sizing methodology for land-based critical-load applications.
- Tropical Climate Diesel Generator Selection Guide — Engineering for high ambient and humidity environments common to tropical marine operations.
- 25-Point Diesel Generator Bid Comparison Checklist — Commercial framework for evaluating competing generator bids side-by-side.
Contact
Specifying or Reviewing a Marine Generator Project?
ASO Genset designs and manufactures marine diesel generators for commercial vessels across class society regimes — including the Norwegian sector offshore supply vessel and Mauritania Belt & Road fishing fleet projects referenced in this guide. We build to international marine specifications: marinized Class H alternators with anti-condensation heaters, class society-approved engine families (DNV, CCS, ABS, LR, BV), salt-air protection stacks, and SOLAS Chapter II-1 emergency generator configurations. Our quotes itemize class society scope, sizing margins, and marinization features as line items — so you can see exactly what you are paying for and compare us against any other bidder using the 15-point checklist above.
Whether you are scoping a new build, reviewing competitor bids on a vessel project, or troubleshooting an existing marine genset that is wet-stacking or tripping on transient load, send us your vessel particulars (type, size, trade route), load list, and class society requirement. We will respond with a sizing calculation, recommended specification, and line-item quotation — typically within two business days.
