Metal Marine Tanks (mainly welded aluminum)

Steel and aluminum boatbuilding. See: "Boatbuilding Methods", in left-hand column of the Home page, for information about alloys.

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regal
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Re: Metal Marine Tanks (mainly welded aluminum)

Postby regal » Tue Jul 03, 2018 6:09 pm

Hello Kevin - as promised I wanted to update on the status of my leaking fuel fittings. I removed the brass anti siphon valve (your "king nipple") and closely investigated the threads on the valve and on the street el. All threads looked perfect. I got a replacement AS valve (aluminum), and used some fuel-resistant sealant (Permatex 80019 Aviation Form-A-Gasket No. 3 Sealant) to attach it to the street el. I have seen no leakage.

I wanted to thank you for helping cut through my corrosion concerns by stating what really is the obvious - the first thing I needed to do was remove the leaking components, inspect, and go from there. I got so worked up thinking that I had corrosion that would require extensive repairs (like replacing the tank) that I couldn't see the trees for the forest.

So all is good - thank you for being my sounding board!

Bill

Kevin Morin
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Re: Metal Marine Tanks (mainly welded aluminum)

Postby Kevin Morin » Wed Jul 04, 2018 11:40 pm

Bill, glad that you found a solution to your fittings leaks and have cleared up the confusion. That's why the Glen-L Forum is here! For all of us to find solutions to questions about our boats.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

Kevin Morin
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Re: Metal Marine Tanks (mainly welded aluminum)

Postby Kevin Morin » Sat Jul 07, 2018 11:14 pm

During our discussion of making welded aluminum tanks for your own boat; I've referred several times to making use of a press brake to form bends in the sheet material- potentially eliminating a weld and if you're hiring the welding (?) the bend could be less expensive than welding?

The cautions I'd offered were to make sure you understood the shop or service who'd do the work: What die would be used to bend your aluminum? I'd cautioned (above) that many shops use knife edge dies to do this work. This is not adequate, and in fact can damage the parent metal (which you'd own if you followed this thread) and therefore you'd be at a loss for the material!!

If you recall, I'd suggested that all marine aluminum alloys should be pressed/bent/braked to a radius compatible with the thickness and the alloy. For those considering bending services you can look up the radius for a given alloy and thickness online- and then discuss what the shop or forming service is planning to do to your tank? After all, it is YOUR tank.

I've mentioned in the previous posts about tank building the importance of the dies used to bend marine aluminum but I didn't have any images to help my poor text along. Since then I've had a few examples appear before my phone's camera (not the highest rez) so I can reinforce this point about strain hardening to the point of stress cracking.

strain crack_3.jpg


this first image shows a piece of remnant 1/8" that has been pressed using a knife edge. The point is to look closely at the surface of the outside of the bend? Notice the mottled and rough surface? Let's zoom in a bit.

strain crack_4.jpg

This closer view helps you to see the metal has been cracked into a 'gazillion' pieces- all still holding together but not nearly like the two adjacent flats. This is an example of a stress cracking region from using a "too small" radius die to press the alloy. I suspect this was 5086 and the die was a steel knife edge- but I don't know as I just found this piece as an example.

Moving to another pieces of aluminum that is thicker- and from a completely different source- this bracket is part of a anchoring jam cleat and fair lead for a river rowing dory (McKinsey River drift boat anchor system) but the purpose of showing the bracket is the outside of the bent edge!

strain crack_1.jpg


Inside the green rectangle you can see the tears or breaks in the metal. It was pressed to a radius that was too tight for the metal to bend smoothly and the outer edge "tore" into long-wise rips or ridges in the metal.

Looks like I've exceeded the attachment limit because the last image file won't upload so next post just follows up this one.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

Kevin Morin
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Re: Metal Marine Tanks (mainly welded aluminum)

Postby Kevin Morin » Sat Jul 07, 2018 11:21 pm

Just above I posted some images of the strain or stress tearing of sheet aluminum when the bending dies are not correctly sized for alloy and thickness. This is another image of the same reminder;

strain crack_2.jpg


here the material failure can be seen very dramatically! Imagine your tank being bent using this size die?

The reason to follow up on the tank thread about bending tank parts is to remind you that there is serious time and work savings by planning to bend tanks' bottom and sides or baffles and other parts. However... it is critically important to be sure you know who's doing your bending work; what they plan to use for tooling die radii (inside "nose bar size") and to make sure if they tear or over stress your material- they're agreed to buy you new metal !

The images are to show exactly what I'm talking about in regard improper die sizes and the disastrous results that can occur! What if the corner of a your tank looked like these examples?

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

Kevin Morin
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Re: Metal Marine Tanks (mainly welded aluminum)

Postby Kevin Morin » Fri Jul 20, 2018 12:43 pm

The last few posts show some images of bending failures- pictures of aluminum that was pressed to a "too tight" radius inside - and the resulting tearing/cracking/crazing of the outside of the material because the alloy's mechanical properties COMBINED with the tooling shapes and geometry induced too much strain.

In the previous images the tooling and alloy, for that thickness combined to over stress the metal and tears were formed. I've tried to emphasize the need to communicate effectively with a fab shop that will perform forming services for your tanks. Here is another set of pictures I took of a part that shows even more starkly the results of ignoring bend radius standards.

Image

this picture shows the inside of a 90 degree bend- trimmed to a bracket after bending. I'd call your attention to the narrow knife edge print of the bending die in the center of the two 'legs' of the bent strip. This material (don't know alloy) in this thickness (looks 0.187"- 3/16" material?) could be bent using this steel die tooling; but it was not the best practice.

Image

The outside of the bracket's bent surface shows the tearing and internal cracks that resulted. If this were a tank edge- it wouldn't be safe to include this part in your tanks' construction.

not far 'back' from the bend center- there are 'tracks' on the suface of the still flat material- this is where the bottom die or V block into which the metal was pushed to form the bend- galled or "hung up" and marred the surface of the softer metal- dies are hardened steel. This area is mostly cosmetically abused, but is a secondary location of overstressing marine aluminum alloys.

Not many shops will include a plastic or paper strip to ease the marring but its worth asking your metal forming service if they'd try?

Image

Just a close up of the outside of this bent area shown in the two photos above - it looks worse close up!!

Welded aluminum tanks can be partly formed by pressing/braking the materials to save welds - and properly done- add mechanical stiffness to the tank's final form. Aluminum marine alloys actually gain "strength" when cold worked- the become "harder-to-break" in the areas of the bends. However, just as proper bending helps 5000 series sheet materials' mechanical properties- improper bending tooling for any given alloy and thickness- will reduce the mechanical properties and create a susceptibility to rapid corrosion and tank failure.

Hoping that these images and a strong word of caution will help you to value the discussions you should have with shop who may do any forming work. I hope you'll consider being very exact about the tooling used on your metal and cautious about 'steel knife edge tools' used on your aluminum tanks' parts?

It would be better by far to cut all the tanks' panels as flat rectangles and edge join them by welding than to allow your material to be pressed into overstressed, too tight radii and torn or cracked. The press brake in fully educated hands can be a good cost savings in tank welding and fabrication- but in other, less informed hands, is a waste or money and metal.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

Kevin Morin
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Re: Metal Marine Tanks (mainly welded aluminum)

Postby Kevin Morin » Tue Jul 24, 2018 8:30 pm

As part of explaining tank building we’ve looked at some pretty refined details regarding welding and welding preparations. The primary reason to be very detailed in the explanations are that a Forum reader may choose to build a tank- but not to do the welding of the tank? Aluminum welding is an expensive process- the powers supplies and torches are fairly high priced and the time to learn is expensive in that it takes lots of electricity, inert gas – argon; and consumables like wire and SS brushes.

However, if you’re preparing the tank for welding- OR if possible you’re actually helping the welder tackup and prepare to weld the tanks’ seams- then knowing exactly what to do or what you need to provide for the welder will make your ‘help’ not only worthwhile but actually a real assistance to the welder.

In previous posts we’ve looked at tacks (small temporary welds used to hold parts before the seams are fully welded ) on seams and there is a series of sketches showing a tack that has been cut down in preparation for welding. Regardless if the tack will we welded over with MIG or TIG the prep is similar- the bulk of the tack is cut to the two planes of the adjoining sheets/plates and the main center of the tack is hollowed out.

dressed tack_1.jpg


Here is a tack that has been 'dressed' prior to welding. The tool used was the grinder/saw in the Tool Review thread which works well to cut tacks in this profile.

dressed tack_2.jpg


another view of a dressed tack. When the weld passes over this reshaped tack weld- it will melt the entire volume of the tack and fuse the tack weld material into the full length weld allowing the tack to do its temporary job and not compromise the final weld as might be the case when a tack is too large and acts to "chill" the weld puddle and perhaps provide a couple of unfused edges in the parent metal?

The post provide pictures of real live tacks cut down as shown only in sketch/illustration before.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

Kevin Morin
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Re: Metal Marine Tanks (mainly welded aluminum)

Postby Kevin Morin » Tue Jul 24, 2018 9:06 pm

In previous posts we also discussed "key hole" welds where the weld hold two pieces together but they are stacked; on piece on the other. To weld this orientation of sheet/plate a hole is cut in the upper sheet and the weld fuses to the lower metal surface and to the sides of the upper sheet along the cutout.

This cut out, or opening for the weld, is often called the keyhole providing the name for this particular weld. Tank tops and decks commonly use this weld method in order to 'seal up' the volume. ON larger boats where there's room below the deck for a welder and his tools; the decks can be secured to the framing by welding from below- but for tanks and small welded boats welding through a 'keyhole' to secure both sheets is most often the case.

Depending on the depth of the top plate and the welding technique (TIG in this case) the keyhole has to be wide enough to allow the arc to 'wet' or melt the supporting material's flat surface before completely fusing/melting/wetting the sides of the opening. If the cut out/keyhole is too narrow; the arc will not melt the bottom material sufficiently- and the weld will not hold the top sheet down during a tank's pressure test.

keyhole tack_2.jpg


This picture shows a typical keyhole weld - tacked using TIG- where one entire end of the hole (drilled to 3/8" and sawn along the sides) is welded to the bottom supporting surface. This tack has been 'tapered off' by reducing amperage and filler. The entire tank top's keyholes should be tacked first- before they are filled completely with welds. Otherwise the concentration of heat in small areas would warp/distort/lift parts of the tank top before all the welds could be tacked down evenly.

keyhole tacks.jpg


Here are two more keyhole tacks- one from the end and another long the side- what is important is the width versus depth of the opening must allow the arc to wet the lower surface quickly to get a tack- before the material warps up due to uneven heat.

If you were preparing to key hole a tank top for a welder's access to the support framing inside the tank? You'd want to know exactly what type, shape, size and layout pattern the welder expects to see; before laying out and cutting these critical weld access openings.

These tacks seldom need to be trimmed/dressed if they will be welded over with TIG. IF they're going to be MIG welded then I'd want to dress them down. The reason to dress tacks, as mentioned, is to reduce their mass so they minimally 'chill' the weld which can reduce the full fusion of these critically important one-sided welds. I'd suggest a carbide burr on a die grinder, non-ferrous metal fluke type burr, used to carefully gouge the tacks down so they have just enough weld left to hold- but not any more!

Again, just pictures of a real live tank build so the previous illustrations have a better sight picture for your tank build.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

Kevin Morin
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Re: Metal Marine Tanks (mainly welded aluminum)

Postby Kevin Morin » Sat Jul 28, 2018 6:15 pm

I've bumped this thread for several reasons. One, I'm building again and that has involved a tank; two I'd prefer to use photos (regardless of the poor quality of my "shades of gray" metal images) over my sketches and illustrations as ways to show a workable method of fabrication; and three, many of the points shown above may not be part of tanks you've seen or owned? So my remarks and previous sketches may not be familiar to all potential tank builders or designers? If possible I'd like to show what these principles look like in practice.

In order to give a more complete explanation of this tank (shown below) I'll list some of the details for context, but won't repeat them in each image's notes. The tank is 1/4" for the body and ends but is 0.160"/5/32" thick on top with 1/8" baffles and center divider plate. The tank is 12' long X 35" wide X 20" center depth but has an angle bottom so it is not a true rectangular form. The tank is about 380 gallons of volume and will suspend the fuel off the sides (like an engine mount) but is only 60-65 lb/ft^2 on the bottom which 1/4" 5086 will support!

The tank was formed using a 1-1/2" nose bar and the neutral radius of the bend is 4 X thickness so there is no strain cracking but there is a very significant strain hardening/cold work tensile increase in these bends. There are baffles full length in the center so the side to side wave action is limited to 16-17" and transverse baffles are every 16" so each 'cell' of the honeycomb is about a 20" tall 18"x16" cubic volume. The baffles, including the centerline lengthwise full sized baffle will help support the bottom load of the fuel mass.

Yes the tank is overbuilt by a factor of several but.... the owner/builder of the boat is very familiar with aluminum and wants to be totally confident of the tank performance at planned traveling speeds.

The tank was bent as a U shape then, ends, baffles and all fittings prepared, installed and fully welded inside using MIG. Then the open top tank was fully etched inside and out- and rinsed in preparation for the top to be installed and welded out. TIG was used for the top install and to prepare the fill downcomer, level stilling well and the four draws from the tank bottom sump to the tank top.

The sump is two depths- first is a shallow 1" x 6" pipe with a 3/8" floor/bottom and second- recessed into the first sump is the water bottoms sump with a separate down comer to draw out the water bottoms from this lower and smaller diameter sched. 80 pipe with a 1/2" plate bottom plate. The sump, all down comers and stilling well were all fabricated off the tank using TIG and then added to the tank in the locations required for the planned installation. The tank will occupy a volume low in the boat, just above the keel and be centered, fore and aft, above the center of buoyancy.

Image

From the side, top inverted in the foreground, the tank with baffles and center divider. Keyhole openings cut and the three circular openings will accommodate the fill downcomer, level sensor stilling well and the flange mounting the four draw pipes.

Image

End view from the top, each baffle and the divider have angles welded to their upper edges to back up the keyholes - shown in a previous post. In this case; the tank will suspend the fuel off the sides so the baffles and center divider will help to structurally suspend the fuel's wt.

Image

Inverted to show the bent shape in cross section. The tank is not fully rectangular due to fitting close to the bottom V in the hull. The surface of the plate (1/4") has been buffed with a Scotchbrite (tm) "like" material AND etched with acid to further insure there is no mill scale left to promote corrosion.

At this point the tank has been welded fully inside, turned over and TIG used to fill the outside seams, all seams were back gouged and cleaned to a fully open 90 fillet. The butt seams were router cut with a 3/8" ball to a depth of about 3/16" providing a very nice weld zone for TIG in 1/4" plate.

Image

This image of the end of the tank-upright and after it was etched with acid to clean any mill scale remaining from buffing; shows the outside fillet TIG bead on the end plates.

Image

Here, the top seam is welded - all from the outside, no back weld. Using TIG for this process allows a little more control of the weld quality than a MIG weld- however- if an experienced MIG welder did this seam (?) there would be no reason to fault that process or the welder. The reason I use TIG for so many seams is that I use a motorized wire feed, TIG torch that allows seams like this with very low effort and fast travel speed by comparison to hand TIG.

What I've shown is the the correctly bent tank corner seams- in this case the chart calls for 4T radii of the inside bend; then the baffles and interior framing has been MIG welded to the tanks' shape helping to hold its form and structurally reducing the load of any given panel in the tank- and finally a view of what an outside corner TIG tank weld should resemble.

Next we'll look at some details of the tank fittings and plumbing.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

Kevin Morin
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Re: Metal Marine Tanks (mainly welded aluminum)

Postby Kevin Morin » Sun Jul 29, 2018 11:01 am

After posting the last set of images, I realized that some were 'worse for the wear' of "photo handling" on my part. The result of my questionable photography skills, coupled with the camera in an iphone6 and edited down from the huge file size to a cropped 800pixels max on one side image; has lead to some pretty out-of-focus and blurred images of the welds (at least).

I'd hoped to show that the welds could be uniform in reasonably long runs once the tacks were dressed properly and left to minimum size required to hold the two pieces being welded together; but no so large as to chill the weld and show a raised or over sized puddle along the weld reducing the uniformity. ( It is good to recall that I'm using a cold wire feed TIG gun to weld- NOT doing two handed TIG which would be much more commonly used to do these welds on a smaller tank that you cut for your own project and had welded in a local shop.)

I'll look for some weld images that aren't so blurred and move on to the tank sump, draws, fill and stilling well for the level. The tank mounts will have to wait for several months as the hull progresses, since the tank won't be mounted until much (much) more hull work is done on project.

Sorry for the blurry images, I'll pay more attention in future posts.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

Kevin Morin
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Re: Metal Marine Tanks (mainly welded aluminum)

Postby Kevin Morin » Sun Jul 29, 2018 2:07 pm

Water in a marine tank is a fact of life; water in fuel from the gas dock may not be as common as it was, but a tank vent allows the gases atop the fuel to expand out the vent ports but.... more importantly when the tank cools with the setting sun; outside atmosphere is allowed into the tank via the vent hoses and fittings mounted to the top of the tank.

If the air is humid and the temperature of the tank is cool enough? ambient air moisture will condense in the tank- evening dew will condense on tank side walls and run down below the fuel. These are called "water bottoms" in tank terminology and as I've posted above in the past; they are a source of tank corrosion if not removed regularly and thoroughly.

To review: the water run down the tank sides and becomes oxygen starved below the fuel and that shift in areation contributes to the water become acidic. If the range of ph (measure of acids and bases) is too far toward the acidic? Then water bottoms begin to corrode the aluminum alloy tank.

The US Navy and most larger shipping concerns require all tanks to have a bottom sump, or "low spot" that can be emptied separately from the regular fuel use. This sump or low spot is built with its own draw or downcomer so the bottoms of the tank can be drawn off separately to avoid having this material in the fuel filters.

The post here is to show an arrangement in a current tank that incorporated both water bottoms and fuel bottoms sumps. The water bottoms sump is within and below the fuel draw sump. The purpose of a fuel draw sump is give the boat the 100% full use of "every drop" of (gasoline in this casea) fuel in the tank.

The purpose of the water bottoms sump is to be able to pull off any contaminates that collect in the tank bottom- specific gravity does the separation work, gravity collects the bottoms and this plumbing scheme works to make the tank management less effort for the boat's Skipper.

Image

JUst when I say I'll try to post decent quality photos to explain the text!! I have to rely on this poorly arranged image! Sorry- this is looking down into the tank shown above (bend to a U with chamfered bottom) where the two layers of sumps are installed. The four fittings are fuel for three outboards (two mains and a kicker) all ending 1/4" above the bottom of the fuel sump level and one water bottoms draw that ends inside the water bottoms sump built into the floor/bottom/lowest point of the fuel sump.

Image

More of the same poor quality imagery! This shows a close(r) view of the two sumps from the inside of the tank. In the near foreground are two pipes that are for outboard fuel (the third downcomer is hidden on the right of the photo) and the middle pipe shown is the bottoms' sump draw riser. All of these 1/2" pipes will have plastic liners that reduce the lifted column of fuel to a 3/8" or slightly less diameter tube.

due to overall design and fittings required locations; the fuel sump extends slightly out of the center bottom area (flat surface) of the tank and up the chamfered side bottom area a bit. That is why the fuel sump (shallow, larger pipe sump) edge intersects the tank bottom slightly irregularly in this photo. The deeper water bottoms sump is visible off to one side of the fuel draw sump.

All the down comers were welded to a single plate (1/8") underneath - only the initial MIG tacks are visible on the top surface of the reinforcing plate that is welded to the center divider (upper edge of photo) and the nearest baffle (right side of photo). The sump was fully welded into the tank bottoms inside and out prior to installing the sub-assembly of pipes and fittings.

1/2 couplers were welded to the pipes, then seal welded to the top plate- this plate was used to make the pressure seal with the top into one large diameter (5") circle instead of trying to seal each pipe fitting individually to the tank top. The plate shown is 1/4" thick - making the fitting to top plate easier to weld and making the plate rise just slightly above the thickness of the tank top (not shown here) of 5/32"- 0.160"

Image

In this photo of the 'double sump' the tank is inverted. This set of heavy wall pipe sumps are welded one into the other and then to the tank bottom (1/4" 5086) using TIG welding process. The two sumps were welded inside and outside on all seams; including the outer most weld that mounts this assembly to the tank bottom.

Image

Last image in this set (still not the best focus!) added to show the main sump (shallowest, widest dia.) fuel draw sump's intersection with the bent bottom panel of this tank. The tank is on its side in this image, and that was done so the welds could be reached inside from a decent position to accomplish the inside weld.

Closer to the beginning of this tank building thread I showed a water bottoms sump and we discussed the different ideas influencing how to address this design feature. In this case the owner/Skipper has decided he wanted to get very last drop of fuel he could - "just in case" and also wanted to get the water out on a regular basis- daily during the fishing season. So we built heavy wall pipe sumps with 1/2" thick sump bases to withstand any corrosion that may happen - and these sumps are services by vertical downcomers that will allow them to draw the last bit of fuel and to clean the water bottoms thoroughly.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin


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