Custom Aluminum Fuel Tanks for Boats

The aluminum fuel tank is one of the most critical components of your boat’s reliability and safety on the water. Most boat owners don’t even think about the fuel tank until the ugly end of life symptoms like gas vapors and water in the fuel start popping up.

We added a new product feature on our webstore where you can customize your aluminum tank order, get an instant quote, and order right away.

Click here to go to the fuel tank product page.

This post is intended as a supplement to that product page to explain all the details that go into designing, building, and installing a new aluminum boat fuel tank. Our goal is to build your aluminum tank that will last the rest of your boat’s lifetime. Of course, critical installation details are important, but we guarantee you will not find a better constructed tank than one from Halk Marine.

Build Standards and Testing

We build all our tanks to meet or exceed ABYC and USCG and federal regulations. The tank will be pressure tested and a secure tag welded to the tank to identify the specifications of the tank and proof of testing and manufacturing.

Halk Marine is ABYC certified and owned operated by naval architect, Jamie Halk. There will be no compromises on the safety and quality of any tank that we sell.

Purchase and Design Process

The fuel tank product page is designed to let you enter your requirements, get a quote, and checkout with your tank deposit to get in the build queue as fast as possible. However, there are still a few details to iron out before we can start building your tank.

You will notice that there are no options to identify exactly where all your fittings will go or the fine details on your tank shape. In general, these details do not affect the price unless you have an unusual requirement. We want you to get your price, secure the deal, then we will develop the details with you soon after.

  1. After we receive your order, our team will review your specifications and get in touch with you through phone or email to start the initial design.
  2. There is an option in the fuel tank product page to upload your photos, files, or drawings. You can upload pictures of your old tank, drawings from the manufacturer, or your own dimensional sketches. All of that is helpful to speed up the design process.
  3. We design your tank fully in CAD and develop a 3D model and 2D drawings showing all the important details. You’ll get the first revision of this drawing and be able to approve it or work with us on tweaking the fine details and ask questions.
  4. If you’re a local customer or have some way of shipping us your old tank or providing the OEM drawings for the old tank, we can simply design the tank to be an exact copy.
  5. It is critical that you review the drawing and verify that the dimensions are exactly what you need to install in your boat. The tank will be built exactly as drawn and we cannot accept returns or refunds on custom tanks.

Welding and Fabrication Process

At Halk Marine, we take pride in our welding and fabrication skills and attention to detail. Here’s the process you can expect to be followed on your tank.

  • The tank plates will be CNC machined on our routers to within 0.005″ tolerances. We also machine all the cutouts for fittings and penetrations ahead of time so there will be no drill or saw shavings left in your tank.
  • The plates are bent on our CNC press break. We design the tanks in a way to maximize the length of formed seams and minimize welding. This improves the strength and performance of the product and makes the process more efficient.
  • The plates are sent as a kit with all the required fittings to the welding station and cleaned and prepped for welding.
  • After tacking the tank together, the interior seams are fully TIG welded. The baffles are fillet welded on all sides inside the tank. In some tank forms due to the depth and inaccessibility, we may MIG weld some of the baffle joints.
  • The tank top will be installed. This is the only joint that doesn’t have interior welds but we take care to get proper fully penetration corner joint welds. The baffles are welded to the tank top through slots CNC machined in the tank top at the exact center of the baffle top flange.
  • The fittings are located and clocked in the correct orientation, and tig welded into place.
  • The mounting flanges are installed on the outside of the tank.
  • When welding is complete, the tank will be visually inspected and pressure tested to 3 psi minimum.
  • After complete inspection and testing, we install the manufacturer’s tag with all the required information.
  • The tank openings are sealed with tape or plugs and prepped for shipping.

Fuel Types

We can make aluminum fuel tanks for both gasoline and diesel boat engines. A diesel tank typically requires a few extra fittings and valves, but otherwise they are constructed the same. If you need return fittings, just add those to the field “pickup fittings” in the tank product page.

If your tank is for a fluid other than fuel, such as water or a holding tank, please make a note in your order comments.

Material grade

Most fuel tanks are built from 5052 aluminum. This is a marine grade, strong, corrosion resistant material used extensively in most small production boat hulls and non-structural components on larger aluminum boats.

5086 alloy is an upgrade option with a much higher tensile strength material than 5052. It is used on the hull and critical structural areas of larger aluminum boats due to its greater strength to weight ratio. It is similarly corrosion resistant compared to 5052.

When choosing between the grades, it is our opinion that it is more beneficial to choose a thicker plate material due to the added corrosion resistance against pitting versus a lighter, stronger tank. However, the 5086 alloy is about 2x the tensile strength of the 5052 alloy so if weight is critical and you want to double down on strength, you can choose the higher-grade alloy and increase the thickness.

If you are installing a tank below deck in way that limits access for inspection or servicing, we STRONGLY recommend upgrading to 3/16″ or 1/4″ thick tanks.

Thickness

Aluminum tanks are typically offered standard in 1/8″ thickness which is generally sufficient for tanks up to 100 gallons depending on their shape. At higher capacities, 3/16″ or 1/4″ would be required for structural reasons. Additional thickness also gives you additional insurance against pitting corrosion. 1/4″ thickness is required for most commercially classed or passenger vessels.

In general, on newer boats you should be safe using the same thickness as the OEM tank. Older boats often had unacceptably thin aluminum tanks (0.060 to 0.090″).

Corrosion Protection

A properly constructed and installed aluminum fuel tank should last the lifetime of the boat. However, many don’t and that’s the reason for the content below and probably why you’re reading this page.

Aluminum oxidation is self-protecting. Aluminum instantly forms an oxide coating when exposed to oxygen and this eventually causes the dull gray color. This coating, unlike rust on steel, is self-protecting from further corrosion after it forms. Even underwater, aluminum exposed to oxygenated water will still exhibit exceptional corrosion resistance.

The critical exception to this material feature is when the aluminum is deprived of oxygen, especially in stagnant saltwater. Because the oxide coating cannot form, the aluminum instead corrodes in the form of aluminum hydroxide, aka “poultice corrosion”.

Installation mistakes that will cause corrosion: 

  1. Foaming the tank into the cavity and allowing stagnant water film to lay on the tank sides and bottom.
  2. Painting or epoxy coating the tank. Any breaks in the coating will create targeted corrosion conditions under the coating.
  3. Poor ventilation in the tank cavity and lack of drainage. Any condensate or green water should have a pathway to drain into the bilge.
  4. Dripping water from the decks pooling on the top of the tank. A light slope designed into the tank top is a great design feature to mitigate this.

Dissimilar metal corrosion is also a culprit in aluminum tank failures at component installations. Never use any bronze or stainless-steel fittings directly in the aluminum tank fittings. Skip Home Depot and find the correct aluminum fittings and hardware. Where stainless fasteners are unavoidable (e.g., sending unit flanges, inspections hatches, etc.), use a copious amount of Tef-Gel anti-seize and nylon washers to minimize surface area contact. It’s also important to minimize any exposure to electrolyte (aka saltwater) on the fittings.

Read here for more information on corrosion prevention for aluminum in marine environments.

Tank Bonding

Because fuel filling can build up a static electricity charge, the tank and stainless fill port should be bonded to the grounding circuit of the boat. We include a grounding tab for securing the bonding wire to the tank.

Tank Coatings

For the reasons above, we strongly believe properly installed bare aluminum tanks will have the longest service life. Many well-known manufacturers figured they can avoid the chemical reality described above by coating the tanks and foaming them in, but there will be inevitable imperfections in the coating and sealing, and this will create the perfect conditions for targeted corrosion.

We do not offer any coatings on the tank and recommend that they are avoided. However, we recognize some retrofit installations don’t easily allow you to meet the corrosion prevention guidance above. If you must, many tanks are coated in a product called “coal tar epoxy” and other epoxy coating products. Follow the manufacturer’s guidelines on the coating. Prep and timing are critical for coating aluminum correctly.

Dimensions and Volume

We measure our tanks by the outside dimensions. It’s simple to calculate the volume of the tank and convert it to volume in gallons. 1 gallon equals 231 cubic inches, so multiply your length x width x depth / 231 to get your maximum total volume. If your tank has a shape other than rectangular on the ends, multiply the area of the end x length / 231 to get the volume.

It’s important to note that this volume is not quite your usable volume in the tank. There will be 0.5″ to 1″ gap between the pickup and the bottom of the tank and a very minor deduction for the volume of the tank wall thickness and baffling and internal structure. A good estimate of usable volume is 90% of absolute volume. This will be higher on taller tanks and lower on short, flat tanks.

NOTE: It is a mistake to specify your tank too tall in order to maximize volume. You need to allow at least 3″ between the top of the tank and bottom of your deck structure in order to allow clearance for the fuel fill neck and hose. Squeezing this too tight can cause difficulty installing and servicing the hoses and increase your chance of kinking a fill or vent hose.

Similarly, you need to allow 1/2″ between the bottom and sides of the tank for proper installation. The tank needs to “breathe”. It shouldn’t be sandwiched tight between the hull bottom and bulkheads or you’re asking for corrosion problems.  

Mounting Tabs

Mounting tabs and brackets are included in the price of the tank. You can choose your mounting tab locations during the design process.

Tank Shapes

Most tanks are rectangular, but others follow the hull shape and can be quite unique. We have options to identify your tank shape on the fuel tank product page, and here are the definitions which should cover most tank shapes.

  • Rectangular – pretty simple. The tank has the same width and depth through its length.
  • Belly Tank – tank has a vee shaped bottom with 5 or 6 sides.
  • Saddle tank – one side is deeper than the other, but the tank has 4 sides.
  • Other – tank has additional shape features not covered by the descriptions above.

When measuring your tank and describing its geometry, it’s important to take careful measurements of all dimensions. Many tanks are asymmetrical or tapered.

Note: we calculate the tank price based off the maximum of length/width/depth. While a V-shaped tank will technically require less material and hold less volume, it also requires additional design, bending, and welding lengths which offsets the small material reduction. It’s simpler to calculate material and labor requirement based on the largest rectangular prism containing the tank shape, and most of a tank’s cost is fabrication labor anyway. 

Particularly odd-shaped tanks may require case by case quoting, but you will get pretty close by using the fuel tank calculator with the maximum dimensions. 

Internal Baffles

Fuel tank baffles are required every 30″.

Tank baffles are internal plates welded inside the tank with small openings to allow fuel to pass through. Internal baffles in the tank serve two purposes: 1. provide structural framing inside the tank against the static pressure of the fuel level; 2. protect the tank and mounting structure from the dynamic loads of sloshing fuel.

ABYC requires baffles to be installed in any tank dimension exceeding 30″. If you have a 30″+ tank, you will need 1 lengthwise baffle; a 60″+ tank will need 2, etc. Tanks wider than 30″ will also need a widthwise baffles.

Baffles are fillet welded to the bottom and sides of the tank before the top is installed, then plug welded through CNC machined slots for a secure weld on all 4 sides.

NOTE: Our fuel tank product calculator assumes the required baffles in length and width. Under no circumstances will we not install the minimum number of baffles. However, installing 1 additional baffle if you’re on the borderline of the 30″ limit is a good idea, especially for offshore boats. 

Inspection and Cleanout Hatch

Inspection/cleanout port for fuel tanks.

Its practice is to install a cleanout hatch between each baffle in case you need to open the tank for water/debris cleanout. Unfortunately, many boats do not provide accessibility to reach the entire length of the tank for cleanouts and inspection. At a minimum, we recommend at least 1 cleanout hatch in the area where you have a deck hatch and where your main fuel fittings and sending unit is located.

We offer machined inspection hatches that are welded directly in the tank top. The inspection hatches are made from machined billet aluminum and offer blind threaded holes and Viton gasket for long service life.

Fuel Pickups

Fuel Pickup Tube

Standard fuel pickups use a 3/8″ NPT threaded female fitting welded into the tank. We install a fuel pickup which has an aluminum elbow that threads into this fitting (prevents dissimilar metal corrosion). One end has a plastic tube which is ethanol fuel compatible and is trimmed to extend just 3/8″-1/2″ above the tank bottom. The other end of the elbow is a 1/4″ NPT female thread which can either take an aluminum hose barb or an anti-siphon valve.

As an optional upgrade, we can use a welded aluminum pickup tube instead of plastic.

Fuel feed hose is usually 3/8″ type A1 for most outboard engines. Smaller engines may use a smaller hose.

Anti-Siphon Valve

Anti-siphon valve

This is an optional fuel pickup accessory. If your fuel tank pickup is above the height of any point of your fuel supply line towards the engine, this valve prevents a siphon from occurring if you have a leak at any point in the supply line. It’s a simple ball check valve that cuts off excessive fuel flow beyond engine consumption and prevents you from draining your entire tank if a break occurs in a fuel line below the tank level.

Fuel Shutoff Valves

We sell aluminum fuel shutoff valves for all your fuel pickups or returns in the tank. Diesel tanks require shutoffs at both the return and fill. Gasoline tanks do not; however, it’s still very beneficial to have one provided you have good access to it from the deck.

Avoid mixing brass/stainless shutoff valves installed directly in the aluminum tank fittings. If you must, use a length of hose to separate the two to prevent galvanic dissimilar metal corrosion.

Vent Fitting

The tank needs at least one fitting for a vent hose to be plumbed over the side. The vent fitting is typically a 90 degree hose barb for 5/8″ hose connected to your overboard vent screen.

Boats manufactured after 2013 also have additional venting hardware installed in the tank top and fill/vent deck hardware to meet new EPA regulations. We can assist you in configuring your new tank to meet these requirements if you desire.

Fuel Fill

Typical fuel fill hose is 1.5″ ID type A1 or A2. The fuel fill fitting is typically a 90-degree hose barb for 1-1/2″ hose. Depending on your install situation, sometimes the hose barb can be oriented vertically or at an angle to best reach your fuel fill port.

Sending Unit

All tanks include either a 5-bolt SAE sending unit flange (standard) or a 1.5″ FNPT threaded fitting (optional) for installing the sending unit in the tank.

We offer several varieties of sending units that will be installed and pressure tested with your tank if you purchase them with the tank.

Hoses

Marine-grade fuel hose is much heavier than the same inside-diameter hose as used in automotive applications. This is done as part of its fire or flame testing, a requirement under the regulations. All fuel hose used in engine room spaces on board gasoline-fueled boats must meet a 2.5-minute fire test rating.

We stock all common fuel hose sizes as add-on purchases with your fuel tank. If you’re going through a tank replacement process, you should also replace all the hoses and hose clamps with new ones, especially those which are inaccessible or difficult to service.

Shipping Tanks

We can ship tanks all over the country. They will be packaged up with substantial protection the protect all the fittings and corners form damage. Smaller tanks can be shipped through UPS Ground. Medium tanks may be able to ship via UPS with a large package surcharge. Large tanks will need to ship with a freight carrier on a pallet.

General estimates on shipping:

  • Tanks less than 48″ long and with a length + girth (2x width + 2x depth) less than 130″ can ship UPS ground for around $50-$100 depending on your location.
  • Tanks longer than 48″ and with a length + girth less than 160″ can ship UPS Ground for around $100-$200.
  • Larger will required freight shipping. Freight shipping to commercial locations typically ranges from $250-$350 up to $1000 for very large tanks to the edges of the country.
  • Residential freight deliveries are significantly more expensive than commercial addresses so try to identify a commercial business with a forklift where you can receive your tank at.

Freight is calculated automatically with your tank pricing based on the tank size and historical freight rates to commercial location. It does not consider your location, so your actual cost may be +/- 20%. We always reconcile shipping after you place your order by either by giving a refund or store credit actual shipping cost is less, or advising you of an increase before starting tank construction.

Sales Taxes

If you pick up the tank at our facility and live in Louisiana, sales tax will be applied to your tank order. If you live out of state and we ship it to you, or you have a tax exemption certificate for your business, then we don’t have to charge sales tax. The sales tax rate in our locality is 8.7%.