Fuel tanks and system

The fuel system begins at one or more diesel tanks and includes deck fill, fill hose, vent hose, shutoff, supply and return lines, a primary filter/water separator, the engine's secondary filter and sometimes a transfer or polishing pump. A pickup tube draws fuel above the tank bottom. A Racor-type primary filter removes water and solids before fuel reaches fine injection equipment. Inspection ports allow cleaning. Tanks may be aluminum, stainless, mild/black iron, molded polyethylene or fiberglass compatible with diesel.

Black iron means weldable mild steel, not cast iron. It can last for decades when dry outside but corrodes where water sits. Aluminum suffers pitting where wet or chemically attacked. Polyethylene avoids metal corrosion but needs support and compatible fittings.

Contaminated fuel commonly stops an otherwise healthy engine just when rough conditions stir tank sediment. A leaking supply line admits air and stalls the engine; a leaking tank creates fire, pollution and loss-of-range problems. Blocked vents can collapse a tank or starve the engine. Offshore reliability comes from clean tanks, reachable shutoffs, visible primary filtration, spare elements and the crew's ability to bleed the system.

Service means draining water, changing filters, cleaning bowls, sampling the tank bottom and confirming shutoff/vent flow. Polishing circulates fuel through filters; it can remove suspended water/solids but cannot scrape adherent sludge behind baffles or repair corrosion. Physical tank cleaning requires inspection access. Add a vacuum gauge or dual selectable filters only when the layout and operating plan justify them.

Repair accessible fittings, hoses and pickup/vent faults. Tank welding is specialist work after complete removal and gas-free preparation. Replacement may require cutting cabinetry; a custom tank's metal cost is often smaller than access labor. Portable jerry cans are reserve capacity, not a repair to an unsafe main tank, and must be secured and managed for UV, venting and transfer.

• Covered by a standard survey — accessible tank surfaces, supports, corrosion/leaks, fill/vent/supply hoses, shutoffs, filter bowl, fuel odor and general installation. The surveyor may take a visual sample but usually does not open or clean tanks.
• Not covered — internal tank inspection behind baffles, pressure testing, laboratory fuel analysis, polishing efficacy or removal of joinery. A fuel sample/technician visit ($300–$1,000) is worth doing before purchase if the boat sat for years, filters show sludge/water, the engine starves under load or the tank exterior is hidden and suspect.

• The broker or owner — “State each tank's material, capacity, location and access; send fabrication/cleaning invoices and photos of all sides.” “When was bottom fuel last sampled and what was found?” “Send dates for fill, vent and engine hoses and every filter change.” “Has the engine ever stopped from contaminated fuel or air ingress?”
• The surveyor — “Identify tank material and mounting, inspect every accessible face and low point, verify fill/vent routing and shutoffs, examine the primary-filter bowl, and obtain a bottom sample if access permits.”

Smell lockers before opening the engine space. Rub a white paper towel under tanks, fittings and filters. Look for wet foam, white aluminum crust, rust flakes, pink/green staining at copper-alloy fittings, soft hoses and unprotected chafe points. Shine a light through a clear filter bowl for water beads or dark debris. Open the deck fill and check the cap seal; inspect whether the vent can admit boarding seas. Take a bottom sample into a clear jar and let it settle.