Auxiliary power on a small working boat

Auxiliary power aboard a small working boat runs the refrigeration, the electric winches, the deck machinery and the navigation electronics, and it usually does so while the main engine is idling or stopped. On most vessels under about 24 m the auxiliary set is a small diesel genset, often 10 to 30 kVA, that carries the hotel and deck loads the propulsion engine cannot supply at low rpm. Fuel discipline decides whether that set starts on the third turn of the key after a week of weather, and a shoreside supply decides how much of the working day is spent on that discipline rather than on the catch.

The pressures the French Atlantic fleet works under, and what changes when those pressures change, are documented in French by the campaign on common dolphin strandings, which sets out the gear, the seasonality and the estimated bycatch figures behind the rules the fleet now operates within. That context matters here only in one respect: any change to gear, season or area changes the electrical and fuel load the boat has to carry.

A small trawler alongside a concrete quay at dawn, fuel hose running from a road tanker to a deck fill, open engine-room hatch showing a compact diesel genset, low side light and wet deck planks. — Engine-room plant, refrigeration and winches: the loads the auxiliary set carries at sea.

What does auxiliary power do aboard a working fishing boat?

A fishing boat has two electrical regimes, and they are not interchangeable.

Under way, the main engine turns an alternator that supplies navigation, radio, pumps and basic lighting. At trawling or hauling speed the main engine is often at 1,200 to 1,600 rpm, which is enough to hold the alternator at voltage but leaves little margin. The auxiliary set takes the loads that would otherwise pull the bus down: the hydraulic power pack for the winches, the deck floodlights, the fish hold refrigeration, the autopilot, the plotter and sounder, the watermaker if one is fitted.

On a small trawler or netter, the split typically looks like this:

Refrigeration: 2 to 6 kW continuous for a hold kept at 0 to 2 degrees C, higher during pull-down after loading.
Electric or electro-hydraulic winches: 5 to 15 kW intermittent, with inrush on start that a small set must absorb without tripping.
Deck machinery: net drum, power block, conveyor, wash pump, 3 to 10 kW depending on size.
Accommodation and electronics: 1 to 3 kW, steady.

That is a peak load well above the continuous rating of a 20 kVA set, which is why the set is sized on the largest motor start plus the standing load, not on the sum of nameplates. ISO 8528 defines the ratings that apply: prime power for a set that runs continuously with a variable load, and limited-time running power for the overload a winch start imposes. A set rated at prime power and asked to take repeated winch starts outside that rating will not fail immediately; it will soot its exhaust, glaze its bores and lose the margin it needs at the next start.

How is fuel managed on a small vessel?

Fuel management on a small boat is a set of habits rather than a system, and the habits are what keep the auxiliary set reliable.

Tanks are usually two: a main tank of 500 to 2,000 litres and a small day tank of 50 to 150 litres feeding the engines. The day tank exists so that water and sediment settle before the fuel reaches an injector, and so that a contaminated main tank does not stop the boat. Fuel is drawn from the main tank through a separator and a coarse filter, and the day tank is topped up at intervals rather than continuously.

Three practices matter more than the equipment:

Water removal. Diesel holds water in suspension and releases it when cold. A boat that fishes in the Gulf of Biscay in February will find water in the bottom of the day tank after a cold night. Draining the tank sump before the first start is a two-minute job that prevents a fuel-pump failure at sea.
Filter discipline. Primary and secondary filters are changed on hours, not on symptoms. A filter that is nearly blocked will pass fuel at idle and starve the engine at load, which is exactly when the winch is running.
Consumption accounting. A 20 kVA auxiliary set burns roughly 3 to 5 litres per hour at 75 percent load. Over a 20-hour trip that is 60 to 100 litres, which is a real fraction of a small boat's bunker. Recording hours and litres per trip shows a fouled injector or a leaking return line before it becomes a tow.

ISO 3046 sets the reference conditions for the declared power and fuel consumption of reciprocating engines: 100 kPa ambient pressure, 25 degrees C air temperature, 30 percent relative humidity. A boat working in cold air at sea level gets slightly more than the declared output; a boat working in a hot engine room with a blocked vent gets less, and the fuel consumption per kWh rises with it.

What does a shoreside supply change for a boat alongside?

A shoreside supply changes three things: time, fuel quality and the state of the tanks.

Time is the obvious one. Filling a 1,000-litre tank from cans takes an hour or more of crew labour and produces a deck full of empty containers. A road tanker alongside delivers the same volume in fifteen minutes through a hose, with a meter reading both sides agree on. On a boat that lands and sails on the same tide, that hour is the difference between a full bunker and a partial one.

Fuel quality is the second. Marine gas oil delivered in bulk from a supplier who tests for water and particulate is more consistent than fuel carried in cans that have stood on a quay. The specification that matters is the sulphur and cetane content declared on the delivery note, plus a water content figure. A boat that takes fuel from a single supplier can compare batches; a boat that takes whatever is available cannot.

Tank state is the third, and the least discussed. A shoreside supply is the moment to dip the tanks, pump the bottoms, check the vent and the fill cap gasket, and confirm the day tank drain works. It is also the moment to check the auxiliary set's own fuel lines, which are the ones most often overlooked because they are short and hidden.

What the auxiliary set has to do when the working pattern changes

Gear rules, closed areas and seasonal closures change how a boat fishes, and each change moves the electrical load.

A vessel that switches from pelagic trawling to a different gear may run its winches differently: shorter pulls, more starts per hour, a higher duty cycle on the hydraulic pack. A vessel that shifts its season to avoid a closure may fish in warmer water, which raises the refrigeration load and the pull-down time after each haul. A vessel that spends more time in port between trips needs a set that starts reliably after standing, which is a battery and fuel-system question rather than a power question.

The set itself does not know why the load changed. It only sees the starts, the hours and the fuel. Sizing it on the worst case, keeping it inside its ISO 8528 rating, and feeding it clean, water-free fuel is what makes the difference between a boat that can take a new pattern and one that cannot.

What to check before the next trip

What the set sees

A short list, in the order that matters:

Day tank drained, no water in the sample.
Primary and secondary filters within their hour interval.
Auxiliary set fuel lines and return line sound, no weeping at unions.
Battery voltage and charge state, especially after a cold spell in port.
Coolant level and raw-water strainer clear; a set that overheats derates and then trips.
Load test at the dock: refrigeration, winch and lights together, for long enough to see the exhaust run clean.

None of this is specific to one fishery. It is what a small working boat needs from its auxiliary plant regardless of where it fishes, and it is the part of the operation the crew controls directly.

Sources

Ifremer