Marine Fuel Tanks - Plastic Vs Aluminum
This isn't a thought that crosses every boaters mind, and unless you hold on to your boat for a long period of time, are rebuilding a boat, or got a boat with a bad tank, you probably won't ever read this article. For those that do indeed find themselves in that smaller crowd, I hope to shed some light on this situation.
There are 4 main materials that are used for marine fuels tanks; polyethylene, aluminum, fiberglass, and stainless steel, of which the latter two are not as common so I am directing this article to polyethylene marine fuel tanks and aluminum marine fuel tanks. In addition, this article is intended for standard gasoline permanent mounted boat fuel tanks and not for diesel fuel.
Since I am not an expert in metal or types of plastic, I am basing this article more on the known pros and cons of marine plastic tanks and marine aluminum tanks. Again, I am no expert on this, and much of this information comes from talking to many "experts" in the industry, as well as some research. From my research, the following are my beliefs on the subject. If anyone has any "cited" information to contradict my opinions, please chime in via the comments below this article. When I say plastic I am using the term loosely, as most tanks are made with some sort of cross-linked high density Polyethylene resin, or better known as HDPE.
Pros and cons of Aluminum and Plastic Marine Fuel Tanks.
Plastic 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. I believe the tanks are made from HDPE (High Density Polyethylene).
2) Plastic tanks, if properly made will outlast aluminum marine tanks.
3) Plastic marine fuel tanks cost less than 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) Newer polyethylene marine fuel tanks are not affected by ethanol.
7) Polyethylene fuel tanks have stronger seams than aluminum. 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 would be weaker.
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 undue stress on the tank walls.
2) Plastic tanks release vapors and after a period of time your boat begins to smell of fuel. Personally, I often smell a light odor when the boat is being stored for long periods, so when I do smell fuel on a boat I suspect it's coming from another source. All boats smell of fuel regardless of the tank, so even though I have this listed under cons, I don't believe it is a deciding factor con.
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 with a wide selection for the popular boats, but if you have a not so popular boat and need a plastic tank, well it may be a tough choice.
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 25-30 years, but I think they may go even longer as I've seen them past that mark and still in good shape. I've also seen some in the 15 year range that looked to be 50 years old due to crevice corrosion as noted in the con below.
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 marine aluminum fuel tank can take advantage of extra space and allow for more fuel capacity.
4) Aluminum fuel tanks are stronger and resist punctures better than plastic. Aluminum tends to dent and withstands more punishment. On a rebuttal side, I really don't see how a properly secured tank would be at risk, but during install this could be of concern.
CONS
1) One of the biggest issues in aluminum marine fuel tanks are from builders foaming in the tanks. Aluminum tanks do not like moisture over prolonged periods, and the foam against the tank enables major corrosion to occur. The constant jarring of the boat eventually creates a seem between the tank and foam, which allows any moisture, whether from deck water running in or from simple condensation to do its damage. A properly mounted aluminum will have good airflow around all surface areas, thus allowing it to dry. Studies show that aluminum marine tanks are structurally sound for 10 years in most cases, and beyond that they begin to weaken and should be replaced.
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 stainless on many of today's boats will rust out in a year or less. This is simply due to the quality of the alloys. Aluminum comes in different mixtures, and a lower grade alloy will not give you the 30 plus years. The only real difference I see in the aluminum marine tanks on the market today are the thickness being offered. 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 by now attracts water. As mentioned earlier, constant water in contact with aluminum will corrode the alloy, and is doing so with aluminum tanks. Despite what you may have heard regarding aluminum marine fuel tanks having a protective coating inside is not true. The protective coating is the actual oxide itself, and this gets destroyed as soon as the massive amount of heat takes hold from the welding of the seems.
4) Constant pressure and jarring of the boat puts a lot of stress on the aluminum welded seams, which are more apt to fractures. I've never had a plastic seam on a decent product fracture unless it was dried out from sun damage, which won't be the case in the below deck fuel tanks . On the flip side, my push pull platform has fractured at 2 of the 4 deck braces around the welds.
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 take well to alloys that are not of the same. A simple loose bolt, screw, nut or any other metal object that were to fall on the tank and continue contact would eventually corrode that area. Loose washers or other items seem to always find their way to the tank area. In fact, as you may know we are doing a 225 Aquasport Osprey boat rebuild, and upon pulling the deck off we found several washers and screws that had been laying on the tank for many years, as evident by the fossil imprints they left behind. I have decided to go with Moeller, as they have a great reputation and also had the design that I needed. Check the link above and follow the build.
Conclusion of Aluminum Vs Plastic Marine Fuel Tanks:
Basing the conclusion on the winner between the pros and cons above really isn't fair, as there are many factors in determining the clear cut winner, such as the install being done correctly, airflow in the hull design, dry docked or wet slip, boat usage, maintenance, and many more.
I have a 55 gallon aluminum marine tank in my 1993 21' Hewes Light Tackle flats boat and after inspection it still looks pretty good! I have no idea what the inside looks like other than a very narrow look through the sending hole, which looked absolutely fine. My tank is not foamed in so that gives it a fighting chance. I know Hewes used all the best material available, as nothing on that boat has ever rusted that came originally on it. The tank is not sitting on a wood bottom, and the support rails are angled to allow full drainage. The entire 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, have put a meter on it and observed no current flow, so I am comfortable in saying that this tank should last the life of the boat.
The example above is a good case scenario, however, with the introduction of Ethanol, there's no telling how long my aluminum tank will hold up from here, as I'm sure it'll be greatly reduced. I have always been a fan of aluminum, as it's stronger, and as far as metal goes, I feel it is the best alloy for marine use hands down. But, since the introduction of ethanol, my favoritism is beginning to shift to plastic for marine fuel tanks. Plastic seems to be the winner by way technicality, as ethanol may be aluminum's Kryptonite.
I chose to go with Moeller for my replacement fuel tank on my Aquasport rebuild project. The had a tank that matched the original shape and size nearly perfect. It was several inches shorter, and several inches taller, but still fit with little issue. The only issue was a larger gap at both ends of the tank, which took a few more support blanks to close the gap.
I installed this 102 gallon Moeller fuel tank in 2011 as part of a full rebuild on this boat, which can be seen here Aquasport Boat Rebuild by Florida Inshore Angler.com It is now April 2018 and the tank is doing as designed. I've ran this boat through some extremely rough conditions, spending too many trips for my liking is solid 4-5 footers offshore. The bilge remains free from fuel, so I know it is holding up just fine. One of the big concerns I often hear is the non baffle aspect to plastic tanks. It logic, it would appear that not having baffles would be a very noticeable issue, leading to boat surge as the fuel sloshes around. I have not noticed any surge or recoil effects regardless of the tank fill level. Perhaps it's more noticeable on smaller boats, or lighter boats, but this 24'6" hull weighing in at 3600 dry isn't phased, so anything larger I can only assume would not be either. The one draw back that I noticed after a few months of holding fuel was permeability inside my center console. The smell is pretty strong, and the effects are certainly corrosive. The wire chase coming from below deck simply will not keep the fumes out, so adding a quality vapor barrier should be considered. I have added this to my to do list, and will post my remedy and outcome when I get around to fixing it. In closing, the Moeller fuel tank is a quality product that I would certainly buy again, and highly recommend. Be sure to properly install the tank to specs, and take the steps needed to combat unwanted vapor.
Allen Applegarth of FloridaInshoreAngler.com