Introduction
Sizing a marine generator (genset) for a fishing trawler isn’t just “add up the watts and call it done.” What usually makes or breaks the system comes down to two things:
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When loads happen (steady freezing vs hauling vs transit vs dock)
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How motors start (the brief starting surge that can pull voltage/frequency down and trip controls)
If you remember one line: trawler generator sizing rarely fails on total running power—it fails on the starting moment.
For most trawlers, the critical loads are:
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Refrigeration: compressors cycle and restart; starting surge can be significant
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Winches / Hydraulic Power Unit (HPU): large motors, heavy starts, often under an already-loaded electrical system
This guide gives a practical workflow: do a nameplate-based first pass, then validate with real measurements so you’re not guessing at sea.
Fishing Trawler Generator Selection Workflow
(1) Confirm your electrical standard first: voltage, phase, frequency
Before you calculate anything, confirm:
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Frequency: 50 Hz or 60 Hz
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Phase: single-phase or three-phase (three-phase is generally better for motor performance)
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Compatibility: make sure the genset output matches onboard equipment standards (motors, drives, protection settings)
This sounds basic, but it prevents the most expensive kind of mistake: buying a “great generator” that doesn’t actually fit the boat’s electrical ecosystem.
(2) Build a load inventory (grouped the way a trawler really uses power)
Group loads into:
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Continuous / hotel loads: lighting, electronics, galley, chargers
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Refrigeration loads: compressors, circulation pumps, controls
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Deck loads: winch motor(s) or HPU motor(s), related pumps/fans
A simple inventory table is enough to start:
| Load | Qty | Running kW (or HP) | Start method | Notes |
|---|---|---|---|---|
| Refrigeration compressor | DOL / Soft start / VFD | LRA/FLA helps a lot | ||
| Winch or HPU motor | DOL / Soft start / VFD | Often the governing surge | ||
| Seawater / circulation pump | ||||
| Battery charger / inverter-charger | Can be “peaky” | |||
| Lighting & electronics | Typically steady |
You don’t need perfection on day one—capture the big loads first.
(3) Size by load cases (because your trawler doesn’t live in one scenario)
At minimum, define four load cases:
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Freezing / RSW steady: refrigeration + basic services
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Hauling: winch/HPU active + refrigeration + pumps
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Transit: lighter continuous loads
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Dock: charging + basic services
| Load case | What’s happening | Loads ON (example) | What you must determine |
|---|---|---|---|
| Freezing steady | cooling | compressors + hotel | steady running power |
| Hauling | deck work | winch/HPU + refrigeration + pumps | peak running + worst start event |
| Transit | underway | hotel + small pumps | light-load level |
| Dock | charging | charger + hotel | low-load duration |
Treat hauling as the main battlefield—most “it looked fine on paper” problems show up there.
4) Calculate peak running kW, then do a first-pass genset rating
A practical first-pass approach is to keep headroom so the genset isn’t living at the limit.
First-pass screen:
- Rated kW ≥ (Peak running kW in your worst load case) ÷ 0.8
Then check the other end of the spectrum: your genset shouldn’t spend long periods at very low load. Common industry guidance warns against chronic low-load operation and often points to a healthier mid-load band (many operators target something like “avoid very low load; aim for mid-range loading whenever practical”).
(5) Treat motor starting surge as a hard constraint (especially winch/HPU + compressors)
Many setups that “meet the running kW” still fail in real life because motor starting causes:
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noticeable voltage/frequency sag
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compressor control trips
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VFD undervoltage faults
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nuisance resets of sensitive gear
For fishing trawlers, the most important scenario to check is usually:
- The winch/HPU starts while refrigeration is already running.
The start method is often the deciding factor:
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DOL (direct-on-line): typically the highest starting surge
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Soft starter / VFD: usually far more manageable
In many projects, improving the start method (soft start or VFD) is a cleaner solution than simply upsizing the genset.
(6) One genset vs two gensets: decide based on load variability + refrigeration criticality
One larger genset
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simpler installation and fewer parts
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must survive heavy hauling demand and avoid unhealthy low-load operation during transit/dock
Two gensets (common on working vessels)
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run a smaller set efficiently during light loads
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redundancy to keep refrigeration and essential services running
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more flexibility to stay in a healthier loading range
A simple rule of thumb: if a refrigeration outage is operationally disastrous, redundancy stops being a “nice-to-have.”
(7) Use an online sizing tool as a second opinion (quick sanity check)
Once you’ve done your inventory + load cases + margin, it’s useful to cross-check with an online calculator such as Cummins’ Marine Generator Sizing Tool
Treat it as a sanity check, not the final judge. If the tool’s recommendation differs significantly from your load-case result (for example, by ~15–25% or more), it usually means one of these is off:
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a major load was missed
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simultaneity assumptions are unrealistic
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motor start method assumptions don’t match reality (DOL vs soft/VFD)
(8) Close the loop with real measurement (before you commit)
Before purchase or final retrofit sign-off, measure under real conditions:
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peak running kW/kVA during hauling
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voltage/frequency behavior during HPU start and compressor restart
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how long the boat operates at low load during transit/dock
This is where guesswork turns into an engineering decision.
Worked example
Assume:
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Refrigeration: 2 × 7.5 kW compressors
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Winch/HPU: 1 × 30 kW motor
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Pumps/aux: 6 kW total
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Continuous hotel loads: 8 kW
Hauling peak running kW = 15 + 30 + 6 + 8 = 59 kW
First-pass genset rating: 59 ÷ 0.8 = 73.75 kW → consider a ~75 kW class genset
Next step: verify that the winch/HPU start method (DOL vs soft/VFD) keeps voltage/frequency stable during the worst start event (with refrigeration running).
Technical Appendix: Engineering Calculations & Acceptance Checks
This appendix turns “it runs” into “it’s deliverable”: calculation conventions, key parameters, and what to validate during commissioning.
(A) Use consistent terms: kW, kVA, and PF
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kW: real power
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kVA: apparent power
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PF: power factor
Relationship: kVA = kW / PF
Engineering reviews should track running kW and running kVA, then validate starting events against genset transient capability.
(B) Engineering load sheet (recommended fields)
| Load | Qty | Running kW | PF | Running kVA | Simultaneity | Start method | Start kVA basis |
|---|---|---|---|---|---|---|---|
| Compressor | DOL/Soft/VFD | LRA / vendor data | |||||
| HPU motor | DOL/Soft/VFD | curve / test / assumption | |||||
| Pumps | |||||||
| Chargers/rectifiers | |||||||
| Continuous hotel loads |
Per load case:
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Peak running kW = Σ(kW × qty × simultaneity)
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Steady running kVA = Σ(kVA × qty × simultaneity)
Worst-case start event (recommended convention):
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Worst-case start kVA = (steady running kVA in that case) + (incremental motor starting kVA)
Where possible, confirm using genset manufacturer motor-start/transient curves and real measurements.
(C) Power quality: regulation, waveform, harmonics
If the vessel uses VFDs, chargers, or sensitive controls, specify and verify:
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voltage regulation (steady + transient)
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waveform quality
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harmonic distortion targets where appropriate
(D) Temperature rise, derating, and engine room reality
Engine-room temperature and ventilation can materially reduce effective output. Include:
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temperature rise expectations for the alternator
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manufacturer derating curves for high ambient conditions
(E) Acceptance testing (what to measure)
During commissioning/sea trial, log:
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voltage dip (%) and recovery during worst start events
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frequency dip and recovery
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steady load profile (avoid chronic low load)
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harmonics/THD if power electronics are significant
Further reading (system design perspective)
If you want a broader, systems-level reference that goes beyond pure sizing—covering selection and system design considerations for reliable power at sea—this long-form guide is a useful companion: The Complete Guide to Marine Generators: Sizing, Selection & System Design for Reliable Power at Sea
Contact
Do You Need a Generator Set?
Need a generator sizing plan you can actually purchase, retrofit, and sign off? Contact us. We support fishing trawlers end-to-end—from load inventory and genset selection to system design and on-board validation—focusing on the two issues that most often cause problems: motor starting surge (refrigeration + winch/HPU) and chronic low-load operation.
Send us your equipment list and we’ll come back with a recommended sizing range, key risks (worst-case start events), and a clear next-step checklist.
