Fifty hertz on the quay, sixty aboard

She comes alongside, the cable is made up, and the lights come on. Then the laundry takes longer than it did, a fan coil sounds a note nobody recognises, and a circulating pump nobody has thought about since she was built runs warmer than it ever has. Nothing has failed. She has been connected to a country whose grid runs at fifty hertz, and she was drawn around sixty.

Shore power for a yacht in a Turkish shipyard is not answered by whether the supply connects. It is answered over months: a yard period is the longest continuous stretch in her life when she is not making her own power, and duration turns a tolerance into an exposure.

What does not care about frequency

A good deal of what is aboard is indifferent. Resistive loads dissipate what the voltage gives them and take no notice of how often it reverses: galley heating elements, immersion heaters, towel rails, trace heating on a pipe run. Most lighting sits in the same column.

It is also the larger column, and the one anybody aboard experiences: light in a cabin, hot water, a socket that works. She can feel entirely alive on the half that never noticed.

Voltage is a separate question and a simpler one, and confusing the two is the usual error. A transformer will move her voltage all day; it will not move 50 Hz to 60.

What a motor does when the supply slows

An induction motor takes its speed from the frequency feeding it, so one designed for 60 Hz and fed at 50 turns roughly a sixth slower. What the driven load does with that varies. A centrifugal pump at five-sixths speed delivers roughly five-sixths of its flow and a little over two-thirds of its head. A compressor must still make the pressure it always made, so its torque demand does not fall at all.

The fan cooling that motor is on the same shaft, turning a sixth slower and moving appreciably less air than the design assumed. Unless the voltage comes down in proportion, the flux in the iron rises at the same time: heated from two directions, cooled less.

The honest description is not that a 60 Hz machine on a 50 Hz supply fails — nothing trips — but that its thermal and torque margins have been quietly spent, and insulation life runs against temperature on a curve steep enough that a modest, permanent excess halves it. A weekend does nothing a bearing would report; four months is another order of exposure, and the failure, when it comes, comes a season later at sea, where nobody connects it to a quay.

What has a motor in it is a longer list than a switchboard label suggests: circulating pumps and their compressors, pressure sets, bilge and sewage pumps, fan coils, engine room ventilation, the hydraulic power pack.

The question that is usually missing

An owner arrives with a good list of questions for a yard — the questions worth putting to a Mediterranean yard are well rehearsed. A power question is rarely among them, because power reads as a utility rather than a scope item, and what a quote actually models files it accordingly. It is a scope item: it decides which systems can be proved during the yard period and which only at the end.

Three ways she is carried through the months

There are only three, and they do not compare in money.

A converted supply puts a frequency converter, rotary or static, between quay and inlet, and everything aboard sees what it was built for. Its price is rating and single-point risk: it must carry her harbour load with margin, it needs an owner and a maintainer, and when it faults on a Sunday she is dark until it is attended.

A transformed supply solves voltage only, and so is not a way of running the motor half. It runs the half that does not care, deliberately: resistive and electronic loads live, motor loads stay off by decision rather than by accident. That works only if somebody writes down which systems sit in the dead column and the crew hold that line for months.

Her own generating sets are the third, and the reason the other two are often skipped: she makes 60 Hz because she was built to, and nothing has to be arranged in advance.

Nor is the third way available throughout. Her sets are cooled by the sea, so for the weeks she is out of the water that cooling is gone, and there are further windows, described from the crew's side in living aboard through a yard period, when her own distribution carries nothing. In those weeks the shore arrangement is her only supply, a poor thing to discover once she is on blocks.

What her own sets actually spend

The third answer is the expensive one, and the currency is not the yard's paper.

It is her fuel, burned at a load factor a set is not happy at: a machine carrying a small fraction of its rating for weeks never reaches temperature, glazes its bores and wets its exhaust. It is her running hours, accruing against intervals that fall due mid-refit, and her exhaust, next to open decks and people, for months. And it is the sets themselves, which as the machinery space scope has it are the hardest-worked machines aboard and the ones a cabin's comfort depends on.

Months alongside on her own sets spends exactly the service life the refit was bought to restore, and that account is settled in overhaul rather than on paper.

What has to be established before she arrives

This is work for the owner's team rather than a claim about a berth.

• Her shore inlet: the rating on its plate, and what her shore transformer, if she has one, does.
• A load list for the harbour condition rather than the seagoing one: sea chests dry, air conditioning and watermaker off.
• Which of the remaining loads are motors, named individually, plates read rather than assumed, and what derating their makers permit at 50 Hz.

The electrical trade here is one of eleven workshops on one payroll, so that conversation is had with the people who will terminate her temporary distribution rather than a subcontractor booked later.

The limit

None of this is answerable from a drawing. A plate, a load list and a maker's derating letter answer it. No yard can say anything useful about a quay in the abstract.

What gives it a date is that the three ways do not keep. A converter has to be found, rated and brought to the berth; a dead column has to be written down and held for months. Left open, the decision is still taken on the morning the cable is made up, and it will be her own sets for as long as arranging anything else takes.

So it is settled before she arrives, or it is settled for her: against a named vessel, a named inlet and a stated month.

Written at the yard in Tuzla. What a particular quay can offer a particular vessel is settled against her own inlet plate and with the supply authority, and the frequency her machinery will tolerate belongs to her makers and her class file rather than to us.