Welded Aluminum Tanks

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This post is about a pair of aluminum alloy plate tanks I put in a GRP 30' boat a year or so ago.  The reason to post here is they are welded aluminum and I did get a fairly sequential set of photos as we worked.  Also this type of work can be done by anyone who’s interested in aluminum fabrication and since all tanks are air pressure tested any chance of leaving a tank with a leak is eliminated.The tanks are made of 0.160" and 0.187" 5052 with various parts added in 5086 plate scraps and 6061 extrusions and pipe.   The tanks are fully suspended between wooden structural members glassed into the hull and would not touch the bottom or rest on any bedding except for engine style side mounts. The side mounts were designed to hold the wt and allow the tanks to be removed and installed in a fairly tight space and still have access to the bolts and lags that held those mounts in place.Because the bottom would be suspended, all the wt of the 140 gallons of fuel would be carried by the bottom of the tanks so there are more baffles/egg crate framing than would have been needed if the tanks were simply bedded.  Bedding tanks of this style might be a good choice, but I preferred to rely on the hull's longitudinals rather than the glass fiber surface of the tank void for support.More aluminum tanks fail from corrosion coming from the installation than from any other factor.  Mill scale is a shiny, oily, slick water retaining film that is on sheet aluminum when it is sold- looks nice for a while. There is a galvanic potential between this film and the underlying aluminum so removing it is basic good metal handling.If the mill scale gets wetted and allowed to keep wetting and drying the galvanic potential to the underlying parent metal can form a spot of corrosion; inside or outside an aluminum tank.  Failing to remove the mill scale contributes to more tank corrosion than just about anything else I’ve seen in metal boat building in the last forty years.   The sheet on the left is a dull, buffed, or stripped sheet of the same material as on the right.  The sheet at right is factory mill scale coated.Here’s a closer look at the surface of the two sheets/plates; one cleaned of mill scale the right side not cleaned.Close up of the cleaning process results.  We use a 50" wide drum sanding machine with a power feed belt and 3M Scotchbrite-like abrasive strip wound around the drum to 'sand' the mill scale off.  This could be done with a circular disc on 4" or 7" grinder motors, and it could be done with hydrofluoric & phosphoric acid mixtures [Zep-a-Lume (tm);  Aluma-Brite (tm) or equivalent products] at any stage of the construction including when the entire tank was finished.  So while its not necessary to get the scale of at this stage it is handy for us and helps reduce weld zone clean up while building.The main baffles are almost full height, one shown here at the back of the bottom panel, and the bottom supporting cross baffles are shown as notched/mouse holes/limber holes cut.Back of the previous view just to locate the two main longs.In this photo the bottom and baffle system is being clamped in preparation for tack up.By cleaning all the mill scale off of all surfaces before tacking up, the clean up for a MIG weld tack or bead is just a few strokes with the SS 'toothbrush' that is used by all aluminum MIG welders just before welding.  By cleaning inside plates the need to rinse inside with acid is eliminated and the welding smut can be washed out with acetone which will evaporate and not require lots of extra cleaning effort.Cheers, Kevin Morin
Reply:Tacking up the bottom with the sides off is a nice luxury as you won't have to reach in to get tacks, if you were working with a pistol grip style gun, reaching is can be a bit of a challenge in some spaces.  The entire baffle system is clamped to a strong back below the bottom plate in order to give a flat result and help make the tank square.The sides are on, the baffle system tacked to the bottom and then to the sides as they intersect.Most of the tack up was done on a wooden pallet to keep the tank off the floor and help to keep the weld joints free of contamination. The visegrip is the welding ground lead's copper lug attachment to the tank.As the welding progresses all the baffles and plats are all tacked and the weld areas are re-cleaned with a SS wire wheel on a 4" grinder.  [The wire in these power wheels is 0.01" diameter max.]  The main welds are done, and the back welds almost complete using the weld positioner to get easiest access.This is the only image of the positioner/engine stand from the side. The image had so much background clutter, from my messy shop that I blocked out the parts in white in order to keep the tank as the photo subject.  Two grounds, one for the MIG and one for the TIG, some of the outside welds are already done as well.Back view of the engine stand hub with angles bolted to the flange and legs tacked to the tank's bottom inside the corners. These stand offs allow weld access to the outside and make positioning the tank for all welds to be the most convenient.One of the most problematic aspects of welding tanks that are heavily baffled or interior supported is decent torch tip access to the weld zones.Another picture of the gains from a weld positioner for MIG work in tight quarters.Cheers,Kevin Morin
Reply:After the main welds were done including the internal and external seams it was time to add the internal plumbing and top supports.All inboard marine tanks except outboard cans less than 1’ deep should have fill risers or downcomers- depending on your terminology.  This is pipe the same size as the fill hose or plumbing that reaches to within an inch or so of the bottom of the tank to control fuel frothing and vent gas flow when pumping in fuel.  If the fill downcomers are missing in a tank of this size the fill will usually burp, and froth fuel back out the fill fitting on deck making a mess and slowing time to fill up at the fuel dock. Because the fill was two inch pipe and the tank fittings would be low to the tank top, the half coupler was added to the fills so the top of the fitting could be nearly flush to the tank.  Here are the two half couplers being added to the fill down comers.The fill is mounted, ending about 1" from the bottom of the tank so that all the vent gases pass upward and not up the fill pipe/hose.  The 2" pipe is mounted to legs along its length downward, and the central full length baffle and spacer plate was welded between the fill and the nearest full depth longitudinal internal.The top was key hole welded to the tops of the [2" x 5.5" x 1/8"- 6061 architectural standard not American Standard shape] angles shown lapping the main baffle/longitudinals. The down hand lapped fillet welds are over-sized but due to access and position they work well to add these pieces of extrusion to the internals as supports for the top plate.  A 1/2" pipe draw riser is shown in the back left corner of the photo with a fitting in the top 1/2" coupler to keep the threads clean during construction.The tanks were cleaned by brushing and wiping with acetone at this point but were final rinsed with a quart of acetone when completed.  The black weld soot/smut is extremely fine powder and will clog a fuel filter in just a mile from the harbor on a maiden voyage- I learned as a young builder on his first client's maiden voyage.Cheers,Kevin Morin
Reply:The tanks were welded and tested in our shop then transported to the boat and installed to see how close they'd fit.  The mounts were to be two patterns, one was a pair of dual sided centerline mounts that straddled a 4x6" glassed to the compartment floors aligned above the keel.  The other pattern would  be a set of four outboard (side) mounts that would look much more like an engine mount.All the six mounts were built of 6" x 6" x 1/2" 6061 and the side patterns had triangular gussets while the centerline pair had boxed ends and an added vertical plate on one side to hold the other side of the tank load.here are the angles cut to begin making the six mounts.A little closer in you can see that 1/2" angle will provide a fairly stiff mount.this end view of one of the middle mounts shows the main parts but because of the colors in the photo they might not be as clear as could be if painted to show the different fits.  The original angle is pretty clear as the bottom and the right vertical and the second vertical is on the left.  A collar tie between these two verticals is a tension member, and in this short span isn't likely to be needed, but if its worth doing - its worth doing Skookum.Then there are two 1/2" x 2" x 6" blocks or bar welded to the face of the angle and the opposite vertical on three sides; the bottom and both ends.  All these welds are fully beveled into the bar stock and angle at 45 deg and 2/3" the depth of each part to be welded. The entire part was slightly preheated and TIG welded all at one time.Loosely mounted to the two bolts are the two tapped blocks that will weld to the tank sides once the tanks are blocked in place for tacking. These plates rest on the lower fully welded 2" x 1/2" flat bar so the real load is transferred there and not as sheer to the SS x1/2" bolts.Another view of the center line mount with the tap blocks tightened into the sides of the mount.  The punch marks just mate the sets for drilling and tapping then removing and welding and returning to matched pairs when the mount plates were to tack in.Cheers,Kevin Morin
Reply:Shop mock up of the centerline mounts as they will eventually be mounted.IN this photo the after dual tank mount is in place showing the welded tap blocks which are used to keep the tanks close up to the mount, where the weight is born by the 1/2" x 6" x 2" flat bars- you can see the welds above the surface where these parts meet is to the tank and below that surface the weld is to the mount.Just visible at the bottom of the bracket is a pair of flat bars used to locate the centerline mount onto the glassed in 4x6 in the bottom of the boat.an image posted on the TIG thread, that shows the tap block at welded to the tank side.This photo shows the tanks as painted and mounted, and the lag bolt hole in the base of the mount is visible beneath the collar tie that helps to stablize the mount's tension between the two tank sides.  All mating surfaces drain, but they are also prime/painted with Americoat 235 Epoxy Primer.  Socket recess SS cap screws were bedded in PTFE White Locktite (tm) thread paste which helps to seal moisture out of the threaded joint.  This material also works as a release agent or insulator between the two metals and last it will lube the threads as the parts are torqued together.This image was hard to photograph as the view is looking aft from forward the tanks on the port side of the tank compartment.  The single side mounts are visible, and their triangular gussets bracing the 6" x 6" angle legs to help make them rigid.  They have the same 2" x 1/2" x 6" long block welded to the mount face below the tank side tapped blocks, and on the after mount the vertical plate below the mounting surface added a side to side rigidity to the entire installation.  One of the plastic water tanks forms the right side of the photo and some metal retainers were welded to the toe of each mount to help keep that tank outboard on its beds.Cheers,Kevin Morin
Reply:This image is looking back at the previous one.  The view aft both tanks outboard port looking forward shows the same two mounts from another view.  Fitting these mounts here was  bit of a reach as the water tanks went in first and the only top access is from the middle of the space occupied by the fuel tanks.Installation was done by laying the four outside mounts near their mount points. One tank was brought in the aft opening in the bulkhead and slid to the side and loosely bolted to the mounts.  Then the centerline of that tank was blocked up with wood, and the other tank installed the same.  Then both centerline mounts were driven in between the tanks and all 12 mount or retention bolts were handy tight then snugged finally tightened.  Next the mounts were drilled and lagged to the outside and centerline beams.The tap block in this picture was installed just to bolt a padeye on for lifting and handling, these tanks went into and out of the boat 1/2 dozen times and we didn't want to rig slings so we used padeyes bolted to the tanks.This bulkhead was cut out to remove the first tank, and was our access to install the new tanks.  Here they're in place during one of the test fits before painting and final bolt in.To keep the tanks from moving side to side away or together we installed a pair of tapped block to carry more 1/2" bolts through a 2" x 1/2" tie bar.Aft the cut away bulkhead (painted) tanks installed and collar tie mounted.This view is downward (plan view) through the hatch in the cabin sole to the top of the installed tanks.  The fuel level senders are at the forward inside ends of the tanks, the starboard tank fill fitting is threaded into the 2" half coupler at the top of the fill pipe to protect the threads and avoid junk in the tank.  A bag of nylon insulating washers and SS metal washers is on the port tank, toes at the bottom of the photo are 4' above the compartment floor.These tanks could be in a metal boat mounted in this way - engine mount style- instead of bedded to tank beds or mounted to support on the bottom of the tank.   This is just an example of one way removable tanks might be designed and built to allow all sides to drain and dry while all the wt of the tanks and fuel is transferred to the structural longitudinals of the hull.  Of course the scantlings could be discussed as a tad heavy, but my Skipper on this boat is a believer in 'Skookum' [Native Alaskan term = strong] so I wanted to accommodate his standards and this is what we built.Cheers,Kevin Morin