Replacing a Built-In Boat Fuel Tank: What's Involved and Why It's Usually a Pro Job

Fuel smells also come from hoses, the fill, the vent, the sender gasket and the engine. BoatUS describes how a technician confirms a tank leak: drain it, pressurize it slightly with air and watch for a pressure drop. Have that test done before you spend money on a tank, and ask the tech to check the hoses and fittings at the same time (see the fuel smell diagnosis guide).

Aluminum is the most common material for built-in tanks, and it usually fails from the outside. BoatUS explains that many builders poured flotation foam around the tank, and moisture trapped between the foam and the tank walls can eventually corrode through the metal. Federal rules for foam-encased metal tanks (33 CFR 183.552) say water must not collect between the foam and the tank or be held against it, but on older boats it often is. BoatUS also notes aluminum tanks are rarely repairable: once corrosion causes a leak, the tank is replaced. Welding or epoxy patches are not a fix.

Before anything is removed, photograph:

• The tank label. Federal rules (33 CFR 183.514) require the maker's name and address, month and year built, capacity in US gallons, material, the test pressure, the model number if any, and the statement "This tank has been tested under 33 CFR 183.510(a)."
• The tank's position, the fill, vent and feed hose routing, the sender wiring, the ground wire, the anti-siphon valve and the straps or chocks.
• The compartment's measurements, and the size of any hatch or access plate above it.

This helps you order the right replacement and check that everything goes back correctly.

Be realistic before you start. BoatUS points out that some builders include large access plates for tank removal, but more often replacing a tank means cutting the deck or the side and then doing extensive fiberglass repair. On an older, low-value boat the bill can approach or pass what the boat is worth. Get at least two written quotes that spell out: tank, hoses, sender, fiberglass work, deck refinishing, and what happens if they find rotten stringers or soaked foam when the deck comes up.

• Aluminum (custom or stock): can be built to the exact shape of the old one. To last, it needs to sit so it can drain and dry (step 8).
• Rotomolded polyethylene (plastic): BoatUS notes these have become popular because they resist corrosion and leaks, which makes them a good choice for older boats, though they have their own drawbacks, such as limited shapes and sizes. Makers such as Moeller sell molded replacement tanks in standard sizes for below-deck installation. Yachting Magazine adds that plastic tanks can swell slightly, so the mounting must leave room for that; that wide molded baffles can make a plastic tank hold less than an aluminum tank of the same outside size; and that choices get scarce for very large tanks, where custom aluminum is often the only fit.
• Steel: federal rules (33 CFR 183.512) ban terneplate tanks, ban black iron or carbon steel tanks unless galvanized inside and out, and ban ferrous tanks encased in foam or fiberglass.
• Fiberglass: rarely used for replacements; older fiberglass tanks can have ethanol compatibility problems (see the ethanol fuel care guide). BoatUS's lab tests on gasoline tanks from the 1960s and 1970s found they had lost up to about 40 percent of their strength after ethanol blends arrived, with resin leaching into the fuel and leaving black deposits on intake valves; most reports involved fiberglass gas tanks built before the mid-1980s. BoatUS also reported that resin makers said simply coating the inside of an old tank is unlikely to work, and that tank replacement was not covered by insurance. If your boat has an original fiberglass gas tank, that's a strong reason to plan for replacement.

For gasoline, buy a tank with a complete federal label. Size it to fit the compartment with room for the fittings, pads and straps, not just the old gallon number.

For gasoline boats with installed tanks, the EPA's evaporative emission rules (40 CFR 1060) phased in low-permeation fuel lines from 2009, diurnal (venting) controls such as a carbon canister from July 31, 2011, and low-permeation tanks from model year 2012. If your boat's original tank was certified to those rules, 40 CFR 1060.601 makes the replacement tank and lines subject to the same requirements. In plain terms: on a newer boat, buy a certified replacement and keep the vent system and canister as designed. Your installer and tank supplier can confirm what applies to your boat's model year.

This is where the pro earns the money:

• The tank is pumped out completely into approved containers, the battery is disconnected and the area is ventilated before any tool comes near it. Sealed, explosion-proof lights only.
• Hoses, the sender wire and the ground wire are disconnected and capped.
• If there's no big enough hatch, the deck is cut along a planned line so the piece can be reused or replaced and glassed back in.
• Foam stuck to the tank is cut away. Yachting Magazine notes that if a foamed-in tank has been leaking, the foam is probably soaked with fuel; it's hazardous and has to come out with the tank. On many boats this is also the first chance in years to look at the stringers and the foam. If either is wet or rotten, it changes the job (see the transom and stringer rot guide and the waterlogged flotation foam guide).

How the tank sits decides how long it lasts. The federal installation rules (33 CFR 183.550 and 183.552) include:

• The tank must not be part of the boat's structure, and must not support a deck or bulkhead.
• It must not move more than 1/4 inch in any direction at its mounting.
• Straps, chocks and supports that aren't part of a metal tank must be insulated from it by non-moisture-absorbing material.
• Water must drain off the top of a metal tank when the boat floats normally.
• Foam must not be the only support for a metal tank, and foam used around metal tanks must meet specific material standards (33 CFR 183.516).
• On foamed-in tanks, the connections, fittings and labels must stay accessible.

Yachting Magazine reports that most tank makers don't recommend foaming a new tank in place even if the boat was built that way, and some say doing so voids the tank warranty. Many technicians now set aluminum tanks on non-absorbent pads with straps, without foam, so air can circulate and water can't sit against the metal. That avoids the problem that killed the old tank. For a plastic tank, follow the tank maker's installation instructions on supports and clearance. Ask your installer which method they'll use and why. If foam is removed, your installer should check that the boat's flotation isn't affected (flotation foam serves a separate safety purpose on many small boats).

• Every opening into the tank must be at or above the top of the tank (33 CFR 183.518); no bottom drains.
• Every fuel fitting, joint and connection must be reachable for inspection and removal without removing permanent boat structure (33 CFR 183.554). If the deck was cut, ask for a deck plate or hatch over the tank fittings and sender so the next check doesn't need a saw.

Static electricity can ignite fuel vapor during fueling. Federal rules (33 CFR 183.572) require every metal part of the fuel fill system and tank that touches fuel to be grounded, with less than 100 ohms resistance to ground. In practice the tank's ground terminal and the metal deck fill are wired to the boat's bonding system. Make sure the ground wire is reconnected (or replaced if corroded) and ask the installer to check it with a meter (see the bonding system guide).

A new tank deserves new hoses; old ones are usually as tired as the tank. Federal rules include:

• Feed and pump-to-carburetor hose: USCG Type A1 where required (33 CFR 183.558), marked on the hose.
• Fill and vent hoses: A1 or A2, or B types only under specific conditions.
• Fill hose connections: a swaged sleeve, a sleeve with insert, or two adjacent metal hose clamps (33 CFR 183.564).
• All clamps: corrosion-resistant, placed past the bead or over the barbs, and not relying only on spring tension (33 CFR 183.532 and 183.560).
• Vent: must have a cleanable flame arrestor and must not let overflow fuel into the boat (33 CFR 183.520).
• Anti-siphon protection on the feed line where the line runs below the tank top (33 CFR 183.568; see the anti-siphon valve guide).

See the fill and vent hose and fuel line guides for details.

Use a new sender (with a new gasket) that matches your gauge's resistance range and the new tank's depth. A mismatched sender gives wrong readings (see the fuel gauge sender guide).

Before the deck is glassed back or hatches are screwed down, the installer should leak-test the tank connections and hoses following the tank maker's instructions. A common method, as Yachting Magazine describes, is to pressurize the tank slightly with air (to the low pressure the tank maker specifies, never more), brush soapy water on every connection and look for bubbles, then reseat and retest any that leak. Then:

• Add a small amount of fuel first, wait, and check every fitting, hose and seam with a dry rag and your nose.
• Run the blower for at least four minutes before starting, check the fume detector if you have one, and start the engine only when there's no smell.
• After the first few outings, look and sniff again.

File the invoice, the new tank's label photo, the hose markings and photos of the installation before the deck went back. Buyers, surveyors and insurers will ask, and BoatUS notes that a known tank issue is one of the first things buyers of older boats look at.