The tank debate:
Poly Marine Fuel Tank:
PROS
1) A major advantage, and one that persuades me right from the get-go is that tanks made from this material aren’t susceptible to corrosion. The tanks are made from cross-linked (very stable, dense and strong) polyethylene.
2) Plastic tanks, if properly made will outlast aluminum marine tanks.
3) Plastic marine fuel tanks generally cost less than quality aluminum marine tanks.
4) According to many experts, plastic tanks are “incredibly durable” and much more reliable than aluminum tanks for gasoline. Diesel fuel may be another story.
5) Being a non metallic composite, plastic fuel tanks won't introduce galvanic action to other components on your boat.
6) Polyethylene marine fuel tanks are not affected by ethanol.
7) Polyethylene fuel tanks have stronger seams than aluminum. In fact, roto-molded tanks are essentially seamless and contoured at the bends - no sharp, stressed corners. I'm not say that plastic is stronger than aluminum, but for the tensile strength of each when compared to it's own counterpart, the aluminum seam will often be the weakest link.
CONS
1) To the best of my knowledge, most plastic tanks don't have baffles inside the tank to limit the swashing of fuel and stress on the tank walls. When mounted low, near the stern, the sloshing may be limited anyway.
2) I've heard that plastic tanks can release vapors. Personally, I've never encountered this with a plastic tank, and when I do smell fuel on a boat I suspect it's coming from another source. According to experts, it's doubtful that the tank itself, made of cross-linked polyethelene, releases anything. This may be a myth.
3) Limited selection due to most plastic tanks being roto spun in a mold, which makes custom plastic tanks expensive. There are a few companies like Moeller and Inca, with a wide selection for the popular boats. If you have a less popular boat you may require aluminum for the budget.
Aluminum Marine Fuel Tank:
PROS
1) A correctly mounted aluminum marine fuel tank of high quality material, and under ideal conditions should outlast the boat. Some say they last 20-30 years, but I think they may go even longer as I've seen them at that mark and still in good shape. My 1992 Malibu Tyee had an aluminum tank, well installed and it looked great 2 years ago; except for corrosion at the electrical ground connection and at the tabs where bolted to the stringers.
2) Aluminum marine fuel tanks often have baffles in them to stabilize the fuel from swashing around excessively.
3) Aluminum tanks are custom made, so getting one to match your boats design is a snap. A custom-built marine aluminum fuel tank can take advantage of extra space and allow for more fuel capacity.
4) Aluminum fuel tanks may resist punctures better than plastic. Aluminum tends to dent and may withstand more punishment. However, a properly installed and secured tank would not be at risk.
CONS
1) One of the biggest issues in aluminum marine fuel tanks are from builders foaming in the tanks. Don't do it! Aluminum tanks do not respond well to trapped water, especiallly salty water. The foam against the tank enables major corrosion to occur. The constant jarring of the boat eventually creates a seam between the tank and foam, which allows any moisture, whether from deck water running in or from condensation to cause corrosion damage. A properly mounted aluminum tank will have good airflow around all surface areas, like in the Malibu, allowing it to dry.
2) Buying an aluminum marine fuel tank of quality is easier said then done. I'm sure you've seen stainless steel on a 40 year old boat that still looks new with no upkeep, yet the poor quality stainless on many production boats will rust in a few years. This is simply due to the quality of the alloys. Aluminum comes in a variety of alloys and a lower grade alloy will not give you the 30 plus years. The significant difference in the aluminum marine tanks on the market is the thickness. The main protection in aluminum is the natural oxide on the surface of the aluminum. A simple scratch removes this coating an opens the door to corrosion.
3) The new ethanol fuel, as we all know is hydroscopic - it readily absorbs water. As mentioned, constant water in contact with aluminum will accelerate corrosion. If the interior had an epoxy (plastic coating) that was durable this could address the problem. The naturally protective coating is the aluminum oxide itself and this protection gets destroyed with cutting and grinding , scratching or damage.
4) Constant jarring of the boat puts stress on the aluminum welded seams so they become susceptible to fractures.
5) Due to Ethanol's action of attracting water, aluminum marine fuel tanks can and most often do create galvanic reactions throughout your boat. Zinc can be placed on the tank for exterior protection, but this does no good for the interior, thus leading to problems extending beyond just your tank, as well as corrosion attacking the welded seams from the inside out.
6) Aluminum does not respond well to dissimilar metals. It ranks low on the galvanic scale. Connections or a simple loose bolt, screw, nut or any other metal object that were to fall on the tank and continue in contact would eventually corrode that area.
Conclusion of Aluminum Vs Plastic Marine Fuel Tanks:
There are many complex factors in play, such as the installation being done correctly, airflow in the hull design, fuel composition, dry docked or wet slip, boat usage, maintenance, and others.
I have no idea what the inside of my 1992 Malibu tank looks like, other than a very narrow look through the sending hole which looked absolutely fine. That tank is not foamed in so that gives it a fighting chance. I know Malibu used quality material. The tank is not sitting on a wood bottom, and the supports are designed to allow full drainage. The entire aluminum tank has plenty of breathing room and there are open air pathways that keep the air moving. I have seen no foreign metal in contact with the tank so I am comfortable in saying that this tank should last the life of the boat. It better be good - my grandkids fish from it with my son-in-law!
The example above is a good case scenario, however, with the introduction of Ethanol, there's no telling how long that aluminum tank will hold up. I have always been a fan of quality aluminum and as far as metal goes, I feel it is the best for routine marine use. But, since the introduction of ethanol, my preference is beginning to shift to plastic for marine fuel tanks. My 2007 Seafox 256 WA has a poly tank. Plastic seems to be the winner by way of technicality only because ethanol may be an aluminum tank's Kryptonite.
Credit to the Florida Angler and others.
