Somebody has run a tap

A reading light in a guest cabin dims for a moment, three compartments from anything that could have caused it. Somebody has run a tap, and the fresh water pump has started. Whoever is sent to look at the light will find a light in good order, and say so.

A refit almost never adds a way of making power. It almost always adds load. Screens, actuators, a drawer refrigerator, lighting where there was none, a pump for a new head, a charger for whatever is carried aboard now — each small, each priced on its own merits, and each hung off distribution drawn before any of them existed, for a boat used differently.

A panel can be renewed and a bank enlarged without either of them touching the mismatch. Both are answers, and the question has not been written down. The question is a load list that is true: what is actually on each way, and what draws at the same time as it. Until that sheet exists, a yacht DC system refit prices hardware against a boat nobody has described.

What a breaker says, and what a drawing said

A breaker is a protection limit, not a statement of demand. It says where the way will be stopped and nothing about what is asked of it, which is the first thing a load list has to establish.

What a circuit was designed to carry was written in one place only: the drawing made when she was built. It is seldom still aboard, and where it is, three yards have superseded it in pencil without consulting each other. What is left is a board of legends, and a legend names a destination, not a demand.

So a load list cannot be copied off anything. It is walked. Every way is traced to whatever is on the end of it today, which is seldom what the legend claims and almost never one thing.

The run is the component nobody renews

Direct current aboard has one property a specification rarely allows for: the further a run goes and the harder it is worked, the less of what left the board arrives at the other end. A heavy load starting pulls the same down for a moment across everything sharing a feed with it — which is how a tap run in one compartment reaches a reading light in another. Voltage drop is not something that goes wrong. It is a property of every run, noticed first at the end of one both long and worked hard.

So a fitting gets reported, and a fitting gets renewed. A pump that starts hot. A light at the end of the longest run never quite the colour of the identical fitting amidships. An actuator that answers on the first attempt in the morning and not on the third. The replacement, handed the same run, reports the same thing in its turn.

A long run is the one most likely to have been lengthened, each extension reasonable on its own afternoon.

Nothing aboard runs at once

The arithmetic that defeats a refit is not addition. Add up every direct current load in a specification and the total describes a vessel that never existed: every light burning, both heads in use, the anchor coming up as the tender is launched.

The honest question is narrower: what runs together. Not what exists, but what is drawing at the same moment, on a still night in August, at anchor, with guests aboard and nobody minding the board. A true list names four things against each way:

• what is on that circuit now, counted at the fitting and not off a drawing
• what has been added since, the winter's fitting included, and by whose hand
• what runs at the same time as it, in which season, and for how long
• what the run is, how far it goes, and what else shares it

Written that way the list stops being an inventory and becomes the description of an evening. It is also the only form in which an added fitting can be judged, because what it draws alone was never the question — what it draws alongside everything running is.

What a quiet night is actually made of

The DC side is what the crew live on in the hours when nothing else aboard is turning. The cord is out, the sets are down — what remains is the battery bank and the distribution hanging off it. Almost everything an owner means by a quiet night at anchor is on that side: the lights, the fans, the cold boxes, the pump that answers a tap in the small hours.

Which makes it the system that decides whether the arrangement works, and the least likely to be written down. A scope names the new fittings and the new board; it rarely says what the night is supposed to be, and the night is the specification.

The bank itself, and the protection around it, is calculated before it is built, and a chemistry decision carries the name of whoever calculated it. The circuit beyond it is what is open, and what gets walked.

Most of what fails is a joint

A long run is one way of losing what left the board. A joint is the other, and the commoner, because it puts a long run's whole loss into one termination. Very little of what goes wrong on a DC installation is electronic. It is mechanical. A crimp made with the wrong die. A lug on a stud never pulled up, or pulled onto a washer since relaxed. A terminal carrying two cables because a stud was free one afternoon. A run resting on a frame edge, moving every time she works in a sea.

A poor joint is a small resistance in series with everything beyond it, and it does not stay small: it warms, warmth makes it worse, and worse makes more warmth. None of it is visible from the front of a panel, which is why a check of a DC installation is tactile before instrumental. Runs get pulled on, studs get checked, chafe points are found by a hand rather than by a meter — by the fitter standing where no later fitter will stand.

What the bench can do that the file cannot

The electrical shop is one of the eleven here, and the part of its work this turns on happens at a bench, not aboard: a marine switchboard made up, proved and commissioned before it goes in, each way sized against what the walk found on the end of it. None of it is done out of a file. Yacht rewiring that ends in a true drawing starts from a list somebody took off the boat.

A circuit drawn correctly can be isolated correctly, which is a fitter's problem, not an archivist's: an isolation taken off a drawing that disagrees with the boat leaves something live that the sheet calls dead. A fault on a true drawing is found by elimination; on a false one it is found by lifting panels until it appears. A way nobody can name is a way nobody will isolate, which is why what feeds the bilge pumps is a board's first question and its last.

This winter the whole of it is in sight at once, and by spring it is behind joinery that a shop built once.

Written at the yard in Tuzla. A cable size, a protection rating for a named circuit and anything to do with battery chemistry are absent from the piece above, and deliberately: each is arithmetic done against one arrangement by one person, and it comes back as a signature on a drawing rather than as a sentence in a journal. What a yard answers for is the part nobody signs: the list taken off her, and the drawing redrawn after the work rather than before it.