What is the small black-and-red hose behind the bulkhead on my secondary diesel tank for?
My secondary diesel tank has a large black hose coming from the deck filler and a smaller black-and-red hose disappearing behind the bulkhead. What is the smaller hose for, and how can I check whether it is connected correctly?
Answer
The smaller black-and-red hose is most likely the tank breather, which lets air escape during filling and enter as diesel is drawn out. Trace it to confirm its purpose, because colour alone cannot distinguish a breather from a fuel supply, engine return or transfer hose.
Trace the hose
Start at the tank connection and follow the smaller hose behind the bulkhead with a torch and inspection mirror. Where it ends is the strongest clue. A connection on top of the tank proves little, because supply fittings can have pickup tubes extending down inside.
Before inspecting or disturbing fuel connections, stop the engine, switch off any fuel-transfer pump and keep flames and sparks away.
- Tank breather: normally connects to the upper part of the tank and runs to a dedicated vent fitting outside the accommodation, sometimes through a hull fitting or a hollow pushpit upright.
- Engine fuel supply: runs towards a shut-off valve, primary fuel filter or water separator, then the engine.
- Engine return: comes back from the engine and carries unused diesel into the tank.
- Transfer or balance connection: links the secondary tank to the main tank, often through valves or a transfer pump.
If the hose joins the filler assembly, inspect that too. Some installations incorporate the vent connection there instead of using a separate hull outlet.
Check the hose diameter
A deck-filler hose commonly has an internal diameter of around 38 or 51 mm. A separate diesel breather is often around 16 or 19 mm, while engine supply and return hoses are frequently 6 to 10 mm. Larger engines and some tank installations use other sizes.
These sizes are identification clues, not replacement specifications. Measure the hose or read its markings before ordering; the outside diameter is larger than the stated internal bore.
A red stripe may simply identify the hose manufacturer's product range. Look for a printed fuel-hose specification, bore size and manufacturer. Red is not a universal code for a return or vent.
On a secondary tank, establish which tank feeds the engine. Some auxiliary tanks only refill the main tank, so their smaller connections may serve a transfer circuit rather than the engine directly.