Diesel cooling, oil and raw water circuits

You bolted this on three phases ago and nobody told you what it was for. Here it is.

The sump is the tray at the bottom of the engine, and it is where all the oil lives when the engine is not running. Every drop that goes anywhere in this engine starts here and comes back here. There is nothing else — no separate tank, no reservoir somewhere else in the boat. What is in that tray is all the oil the engine has.

Sitting in the lowest part of it is the pickup strainer, a coarse mesh. The pump draws everything through it, so anything solid enough to matter is stopped before it can reach a bearing.

You will never see it, service it, or clean it afloat. It is welded into the bottom of a tray you cannot open without lifting the engine.

Which is exactly why oil changes matter. Oil wears out — it collects soot, water and metal, and it turns to sludge — and that sludge settles in the bottom of the sump, right where that mesh is. Change the oil on time and it never builds up. Leave it and one day the pump is trying to draw through a blocked screen, and the first you know is an alarm.