The current is built from a stream function, so the flow is divergence-free, the way real water is. Gyres, races and slack water all fall out of the same mechanism rather than being placed by hand. The narrows are read at two scales, so a wide strait registers as well as a gap between rocks.

It is the same on every machine, and no water ever crosses the network. The field is a pure function of the world seed, the position, the world clock and the season, so two players a thousand metres apart compute the same sea without exchanging a byte of it. What they do have to agree on is the tuning, and the server publishes that (see MULTIPLAYER below).

A hull under way feels the current as drag relative to the water, which is what a current is. That is the same for every hull, so what a headwater costs you never depends on how small a boat's engine is.

A boat left drifting settles at the water's own speed. A karve was measured doing exactly that on the shipping game, with and without Njord and Sailing loaded, and a karve and a longship measured together earlier both came out near the water's speed, though their hulls damp very differently. The sideways resistance is not imposed by Undertow: it falls out of Valheim's own per-hull physics, so it differs from hull to hull.

Why unmanned boats are left alone. Vanilla already damps an unmanned hull almost to a stop, and a moored longship wandering off while you are away is not a feature. A crew that moors properly and wants the sea to punish the ones who do not can raise UnattendedDriftFactor.

One coastal point was read again and again through a tide with wake field: the stream along the shore reversed between ebb and flood, under an open-water drift that never turns, and the peak-flood reading was predicted before it was taken.

A full tide takes two in-game days by default. Ragnarok's Wrath is entirely optional: without it Undertow logs its absence once and the sea runs regardless.

Flotsam is the server's job, and it was watched on a dedicated server: items spawned up to the cap and the cap held; each one was reclaimed on its timer, including items kilometres from the only player; the server's whole object table moved by exactly one per item; and an empty server produced nothing and still cleaned up. In single player and on a hosted game, the host's own player counts like everyone else.

The items come from lists you can change. They must be items that float in Valheim, or they sink out of reach, and wake floats lists them. Floating is Valheim's loss-prevention marker rather than buoyancy, so an iron shield floats and ore does not: choose for the story, not for physics.

If a player can be held offshore by the current until they drown, the feature is wrong rather than mistuned, so the cap is enforced across every value the config permits. It was seen engaged in a race running a full metre per second, with the drift factor at the top of its range: a swimmer heading upstream still made headway, at their swim speed minus the cap.

The foam moves at the water's own speed and rides the swell. The streaks are symmetric end to end, so a glance gives you the line of the flow and only watching gives you its direction. There is no arrow, no number and no screen element.

Slack water keeps a scattering of faint flecks on purpose, so an empty-looking sea can be told apart from a mod that is not running: wake lines answers that. Set DriftLineSlackFloor to 0 if you want slack water strictly bare.

Night, fog, distance and a big sea dim the foam on their own, and DriftLineNightFloor sets how much of its daytime brightness it keeps at midnight. It is drawn by each player's own game, cosmetic only: it changes nothing about how a boat or swimmer moves, is never networked or saved, and a dedicated server ignores it. It keeps to a per-frame cost budget of its own, and thins itself out and says so in the log if it goes over. Two players on one deck see the same set, density and speed, but not the same individual streaks.

A current has no sound and no icon, so the log is how you watch the mod work. Turn on VerboseLogging and the log carries a survey of your seed's ocean at start-up, plus a running readout of the drift while you sail.

If you are an admin, edit a synced value on your own client while connected. It goes to the server, which saves it to its own config and tells every player. If you are not an admin, the server decides, using vanilla's own admin check rather than your client's word for it, and your edit stays in your own file.

Type wake floats to see which items float. Put the ones you want in FlotsamCommon and FlotsamRare, comma separated, on the machine that spawns the flotsam: the server, or the host.

When a release changes a default, your file is read before anything else, a copy lands beside it as .vN.bak before any value is touched, and only values still at an old shipped default are moved. Anything you set yourself is kept, and named in the log so you can see it was read and left alone. The [0 - Meta] section stamps which layout your file was written for; leave it be. wake status reports it.