Fuel tanks: age, material and leaks

What to inspect · Power & sail

Built-in fuel tanks are hard to see and very hard to replace, because many boats were built around them. Aluminum tanks corrode where water sits, and the welded mild-steel (black iron) tanks in many 1970s-80s boats, Taiwan trawlers included, tend to leak eventually.

What the tanks are made of

• Aluminum has long been the most common tank material. It corrodes when water is held against it for long periods, for example where flotation foam was poured around the tank.
• Many 1970s-80s trawlers, including Taiwan-built ones, have welded mild-steel tanks, often called black iron, sometimes glassed over on the sides and bottom. One owner who replaced his wrote that most such tanks leak sooner or later.
• Rotomolded polyethylene tanks have become popular, including as replacements. They resist corrosion and leaks, though BoatUS notes they have drawbacks of their own.

How leaks start

• Sources differ on the main cause in older trawlers. PassageMaker (1998) blamed water from leaking deck fills rusting the tanks; Pacific Yachting (2022) blamed hull flex cracking tanks at the welded corners. Both can happen.
• Early leaks can be small: a smell of diesel, or a little fuel in the bilge after filling or in rough water, when fuel sloshes against a rusted top.

Checking tanks before you buy

• Use your nose in the engine room and look for diesel in the bilge, especially under the tanks. Absorbent pads tucked under a tank are a clue.
• Look at every part of each tank you can reach: tops, fill and vent fittings, seams and supports. Rust stains and weeping seams are warning signs.
• Take a fuel sample from the tank bottom or the filter; water in it needs explaining. Ask for records of any tank replacement, cleaning or fuel polishing.
• Find out how a tank would come out. Some boats have access plates; many would need a deck, a bulkhead or even the engine removed.

If the tanks need replacing

• Owners who have done it describe cutting the old tanks into pieces and fitting several smaller tanks that pass through existing hatches.
• One trawler owner's 2015 blog put the job at roughly $10,000 to $30,000 for 36- to 44-foot trawlers, based on other owners' reports. Treat that as a rough, dated guide; costs vary by region, year and boat, so get yard quotes.

Sources disagree on Main cause of leaking steel fuel tanks in older Taiwan trawlers — left out rather than guessed. Check your local authority.

General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.