Marine Generator Sizing for Fishing and Work Boats

The five engine ratings on the table

The marine power generation table on this site lists five engines: the 4BTA3.9-GM47 at 47 kW, GM55 at 55 kW and GM65 at 65 kW on 3.9 litres, and the 6BT5.9-GM80 at 80 kW and GM83 at 83 kW on 5.9 litres, at speeds of 1,500–1,800 rpm. They are collected on the marine diesel engines page and the marine generators page. Read those five numbers for what they are: engine ratings for marine generation engines. They are not the electrical output of a complete generator set — that figure depends on the alternator paired with the engine and on losses, and it is the figure your load list must be matched against. Sizing therefore runs in two steps: build the load list to fix the electrical output your set must deliver, then have the complete unit's rated electrical output — for the exact model, at your frequency — confirmed in the quote. The frequency question comes first, because the GM80 and GM83 are listed at 1,500 rpm: a 60 Hz vessel confirms its rating in the enquiry instead of borrowing the 50 Hz figure.

Build the load list by circuit

Walk the vessel and write down every electrical consumer, in four groups. Hotel loads: lighting, galley, accommodation, navigation and communications — modest individually, always on. Refrigeration: fish-hold or cargo refrigeration compressors, with starting currents several times their running draw — the group that most often breaks an undersized installation. Deck machinery: winches, pumps, net haulers and hydraulics power packs, intermittent but heavy. Services: battery charging, engine-room ventilation, bilge and fire pumps. For each item, record running kilowatts and whether it starts under load.

Now build the worst credible hour: the combination that actually occurs together — typically refrigeration cycling, deck machinery working, hotel loads on, and a compressor starting into that picture. Sum the running loads of that hour, then note the largest starting event separately: whether a set absorbs it without collapsing the voltage for everything else is a transient-capability figure for the complete unit, stated per model — have it confirmed for the model you are quoted. That hour, not the nameplate tour of the vessel, is what the generator must carry. Fishing vessels feel this sharply: the hour the hold is being pulled down to temperature while the deck is working is the design point, and it arrives on the busiest day of the season.

From the load list to a confirmed set rating

The load list fixes the electrical output your complete set must be rated for. The GM table then tells you which engine class the conversation starts in — nothing more. An engine's mechanical rating and a set's electrical output are different numbers: the alternator's efficiency and the set's losses sit between them, and the starting events in your worst hour are carried by the complete unit's transient capability, not by the engine's nameplate figure. So use the list this way:

• State the required electrical output — your worst-hour running sum, with the duty margin your practice requires — and name the largest starting event in the same document.
• Ask for the complete unit's rated electrical output for the exact model quoted, not the engine rating, and check it against the requirement.
• Have the starting capability for that unit confirmed against your largest motor start, in writing.
• At 60 Hz, have every figure confirmed for 1,800 rpm: the GM80 and GM83 engine listings are 1,500 rpm listings, and their figures do not transfer.
• List above the GM span: the marine category lists larger marine models (a 6CTA8.3 marine listing among them) — enquire by designation with the load list attached.

Duty pattern matters as much as the peak

Two vessels with the same peak can need different answers. A genset that runs near its rating for long working days wants margin and honest continuous-duty figures; a set that peaks briefly and idles long wants the opposite conversation — chronic light-load running is its own wear mechanism, and a smaller set worked properly outlasts an oversized one nursed along. Describe the pattern in the enquiry (hours per day at what fraction of load) and have the quoted model's rating basis stated for that pattern. Have the listed complete-unit terms — delivery 15–30 days, warranty 1 year or 1,200 hours — restated for your model in the quote.

Close the job the way the generator buying guide closes a land installation: model, rating and basis, scope of supply, price and delivery in one written document, checked line by line before the order. A vessel is a worse place than most to discover a specification error, because the correction has to come by sea or wait for port.

Worked example: a mid-size fishing vessel

Run the method on a representative load list. Hotel and navigation loads: 8 kW, continuous. Refrigeration: two compressors at 9 kW running each, one starting at a time with a heavy starting draw. Deck machinery: a winch and hydraulic pack at 14 kW when working. Services and charging: 4 kW. The worst credible hour — hold being pulled down, deck working, everything on — sums to roughly 8 + 18 + 14 + 4 = 44 kW of running load, with a compressor start inside it. That 44 kW is an electrical requirement, not an engine selection: the set that carries it must be rated for at least that electrical output as a complete unit, with the compressor start checked against the unit's transient capability. The enquiry asks for a unit whose confirmed electrical output and starting capability carry the list, with a cautious operator weighing growth plans or a tired refrigeration plant toward the larger unit. The GM engine ratings enter that conversation as engine figures, not as set outputs — no engine rating, on its own, selects the set. The reasoning is the deliverable: any supplier given this list can check it, challenge it and quote against it — which is precisely what a single guessed kilowatt never allows.

Redundancy and the second set question

Some vessels split the load across two smaller sets instead of one larger one, so that a single failure darkens half the boat rather than all of it, and so that harbour hours run one small set efficiently instead of one large one idling. It is an operator's doctrine decision, not a sizing formula, and the listed GM figures — engine ratings, not set outputs — cannot settle it. If your doctrine is redundancy, say so in the enquiry: state the electrical output, starting capability and redundancy the load list requires, and have candidate configurations quoted on actual genset data before choosing between them; if it is simplicity, one set specified by the worst-hour method above is the defensible answer. Either way, the load list is the document both doctrines are priced from, and it belongs in the vessel's file next to the marine engine records for the life of the installation.

The bottom line

Size by the worst credible hour, settle the frequency before the size, turn the load list into the electrical output, starting and redundancy requirements a complete set must meet, and write it down where the vessel keeps its records. A generator chosen that way can be defended to an owner, a surveyor and a crew — and, more importantly, it will still be the right size on the hardest day of the season, which is the only day sizing is ever really tested. When the list outgrows the GM span, the same document carries the conversation to the larger marine models listed in the marine category without starting again.

Finally, keep the sizing document alive. Load lists drift: a vessel gains a freezer, a bigger winch, a new crew comfort load, and the set that was right at installation becomes marginal three seasons later without anything breaking. Revisit the list at every refit, compare the new worst hour against the installed set's rated electrical output, and let the arithmetic — not a blackout during the season's best week — tell you when the installation has outgrown its generator.