How to Build a Boat Gas Tank

Gas tanks are made to install below the deck.

Typical boat gas tanks are made from steel, aluminum and fiberglass. Steel tanks are vulnerable to rust and corrosion from moisture. Fiberglass tanks are lightweight, but ethanol in the fuel erodes the material over time. Aluminum gas tanks are lightweight, resist rust and corrosion and resist erosion created by ethanol. It is possible to build an aluminum boat gas tank in a workshop using aluminum alloy.

Things You'll Need

• 3/16-inch aluminum alloy plate
• Sawhorses
• 3/4-inch plywood
• Chalk line
• Circular saw with metal cutting blade
• Mallet
• Framing square
• Welding equipment
• Fuel filler fitting
• Fuel gauge
• Tank vent fitting
• Electric drill
• Hole saws

Instructions

1. Establish the length, width and height of the gas tank. Select 1/8-inch thick aluminum plate with a minimum alloy content of 3003 to allow welding of the material. Set up two sawhorses with a sheet of 3/4-inch plywood as a worktable.

2. Place the aluminum plate on the plywood. Measure along the longest edge of the plate from one corner. Mark a point that equals twice the width and twice the height of the gas tank combined. If the tank is 14 inches high and 12 inches wide, mark the edge of the plate at 52 inches. Make a corresponding mark along the opposite edge of the plate and snap a chalk line between the marks. Wear safety glasses and use a circular saw with a metal cutting blade to make this cut.

3. Measure across one end of the plate and mark the length of the tank. Measure across the opposite end of the plate and make a corresponding mark. Chalk a line between these marks. Cut the plate along the chalk line.

4. Measure along the longest edges of the plate and make alternate marks beginning with the height, then width, then height of the tank again. The remaining section of the plate should equal the width dimension. Chalk lines across the plate at each of the three marks.

5. Bend the aluminum plate 90 degrees at each of the marks across the plate, using a mallet to do so. Check the corners with a framing square as you go. Depending on your measurements, this forms the square or rectangular shape of the gas tank. Weld the edges of the plate together where they meet by making a continuous weld to ensure the seam is leak-proof.

6. Mark the top of the tank at locations for the fuel filler fitting, fuel gauge fitting and tank vent fitting. Measure the diameter of each fitting. Wear safety glasses and use an electric drill with the correct diameter of hole saw for drilling each of the holes.

7. Refer to the installation instructions and install the fuel filler fitting, fuel gauge fitting and tank vent fitting in the top of the gas tank. Some fittings have a nylon or neoprene gasket that seats against the inside of the tank. Secure the fittings as described in the instructions.

8. Measure the openings at the ends of the gas tank for end pieces. Mark two pieces of aluminum plate to these measurements and chalk the four lines that outline each piece. Wear safety glasses and cut the pieces with the circular saw and metal cutting blade.

9. Weld the end pieces to the tank. Make continuous welds to ensure all seams are leak-proof. Allow the welds to cool.

10. Fill the gas tank with fresh water and check the seams for leaks. Mark any spots that need additional welding. Empty the freshwater from the gas tank. Use a siphon hose to empty the water from a large tank. Weld any spots that were marked and allow the welds to cool. Repeat the leak test. Allow the tank to dry out completely.

Tips & Warnings

• Install aluminum baffles or a fuel bladder to limit sloshing fuel.

References

• Global Spec: Aluminum and Aluminum Alloys Specifications
• United Aluminum: Aluminum Specifications Cross Reference
• Marine Fuel: Types of Boat Fuel Tanks

Resources

• Read this Article in Spanish
• Street Rodder: Skip Welding vs. Continuous Welding Seams