New SS water tank

[pcmm]

We're currently in the process of doing a major refit of our 1978 Morgan OI 41 and currently have the aft cabin apart ( repairs to stern tube, hydraulic steering rebuild, etc.) While the aft cabin is "un-pretty" we're thinking now is the time to replace the main water tank which is on the port side. Its a massive 135 gallon tank that is under the port side deck. To get it out I have to cut it up as its too large to fit out the companionway. I have plans for a replacement tank that WILL fit though the companion way with only a 10 gal loss in capacity. The plans call for the tank to be built out of 18 gauge SS. Seems really thin for such a large tank. I was thinking that 14 gauge would be more suitable, thoughts?

I am also considering having the tank made of 3/8" polyethelyne <sp?>

[gonesail]

seems like a polyethylene plastic tank would be better for water. good luck cutting up the 40 year old SS tank.

[pcmm]

Originally Posted by gonesail

seems like a polyethylene plastic tank would be better for water. good luck cutting up the 40 year old SS tank.

The original tank is aluminum. which is easier to remove but not a good material for potable water!

[coopec43]

When I was building water tanks Jim Skoog's Book Cruising in comfort stipulated certain requirements.

They had to be capable of being cleaned ie you have to be able to reach every corner of the tank

Four smaller tanks are better than one big tank.

• They are easier to build
• They are easier to secure
• Big tanks need heavier construction.
• If you get water from a dubious source and it is contaminated you still have potable water in other tanks.
• A series of water tanks allow you to manage your use

[jhulmer]

I agree. 18ga is too thin. It will take a while, but the flexing of the metal from sloshing water, near the baffle welds will eventually fatigue and crack. Depending on quality of construction 16ga can work well but 14ga is much better and much easier to weld. We made water tanks out of 12ga for a friends steel boat and that was over kill and heavy.
Use only 316L material.
I made these fuel tanks (45gals each) out of 14ga and they are solid!

Jim

[chouliha]

Jim really nice job on the tanks. I especially like the rod that runs lengthwise as it adds considerable strength to the tank.

Chuck

[jhulmer]

Originally Posted by chouliha

Jim really nice job on the tanks. I especially like the rod that runs lengthwise as it adds considerable strength to the tank.

Chuck

I thought so too ... I can't take credit, it was Cabo Ricos tank fabricator that came up with that idea. I copied it from the old rusty Steel tanks I removed.

It makes a lot of sense, braces the baffle and the ends with the least amount of weight.
Jim

[StoneCrab]

Flat sides are your enemy. Shape = strength.

I agree with Jim's concerns with flexture but that can be dealt with by the application of shape to the design.

18 gauge should be plenty in my opinion, but that depends on a few variables.
The greatest variable being the height and width along with the shape.

A bead roller can add contours to flat sections for added strength from the shape and the baffles can have the flanges rolled for a similar stiffening effect to reduce flexing.

If you want to engineer the tank instead of just making it thicker, the calculation for determining the load requirement for the wall plate is:

Ms= (0.00292 * NHW * 1.6) / (D/A)

Ms is the minimum material strength for the wall in pounds per lineal inch
N is the design load in "G's" - (military aircraft use 35G to withstand a crash) (civilian aircraft would use 9G max)
H is the "head" of liquid in feet
W is the density of the liquid - 62.4 lbs/cuft for water
D/A = ((height of the tank in inches squared) + (width of the tank in inches squared)) divided by the (height) * (width)

This formula describes a round bottom tank with a rectangular top. This is presented for relative comparison purposes... sort of an order of magnitude TLAR type of estimate.

For a 12 inch wide tank 48 inch long holding water:
minimum material strength =
(0.00292)(9g's)(1ft head)(62.4 lbs/ft3)(1.6) / (148) = 0.018 lbs/inch

the 148 above was calculated as12 inch) squared + (48 inch) squared divided by 12 * 48 = 148

The max load of 16 gauge 6061 T4 aluminum is 1360 lbs per inch so you can see that 0.018 lbs per inch calculated is many thousands of times stronger than required.

Now to compare aluminum to stainless steel, the 6061 T4 has a yield strength of 16,000 psi and the stainless steel yields at 42,000 psi.

These calculations don't take into account localized impact damage from sharp objects but since the tank will be behind or under cabinetry, I assume that isn't a concern.

I'll stick to my assessment that 18 gauge stainless is WAY more than thick enough. The calculation isn't even nominally close to the strength of 18 ga stainless.

I would flange roll all lightening holes in the baffle and add some stiffening beads to the tank walls and top with a bead roller to prevent oil canning and noise. A good TIG welder can make that tank bullet proof.

If anyone wants to challenge the calculation, PM me, I'm always open to dialog; but on an order of scale, I think the calc proves that wall thickness isn't the issue. A well "shaped" and supported tank could be made out of quite light materials.

[jhulmer]

Originally Posted by StoneCrab

Flat sides are your enemy. Shape = strength.

I agree with Jim's concerns with flexture but that can be dealt with by the application of shape to the design.

18 gauge should be plenty in my opinion, but that depends on a few variables.
The greatest variable being the height and width along with the shape.

A bead roller can add contours to flat sections for added strength from the shape and the baffles can have the flanges rolled for a similar stiffening effect to reduce flexing.

If you want to engineer the tank instead of just making it thicker, the calculation for determining the load requirement for the wall plate is:

Ms= (0.00292 * NHW * 1.6) / (D/A)

Ms is the minimum material strength for the wall in pounds per lineal inch
N is the design load in "G's" - (military aircraft use 35G to withstand a crash) (civilian aircraft would use 9G max)
H is the "head" of liquid in feet
W is the density of the liquid - 62.4 lbs/cuft for water
D/A = ((height of the tank in inches squared) + (width of the tank in inches squared)) divided by the (height) * (width)

This formula describes a round bottom tank with a rectangular top. This is presented for relative comparison purposes... sort of an order of magnitude TLAR type of estimate.

For a 12 inch wide tank 48 inch long holding water:
minimum material strength =
(0.00292)(9g's)(1ft head)(62.4 lbs/ft3)(1.6) / (148) = 0.018 lbs/inch

the 148 above was calculated as12 inch) squared + (48 inch) squared divided by 12 * 48 = 148

The max load of 16 gauge 6061 T4 aluminum is 1360 lbs per inch so you can see that 0.018 lbs per inch calculated is many thousands of times stronger than required.

Now to compare aluminum to stainless steel, the 6061 T4 has a yield strength of 16,000 psi and the stainless steel yields at 42,000 psi.

These calculations don't take into account localized impact damage from sharp objects but since the tank will be behind or under cabinetry, I assume that isn't a concern.

I'll stick to my assessment that 18 gauge stainless is WAY more than thick enough. The calculation isn't even nominally close to the strength of 18 ga stainless.

I would flange roll all lightening holes in the baffle and add some stiffening beads to the tank walls and top with a bead roller to prevent oil canning and noise. A good TIG welder can make that tank bullet proof.

If anyone wants to challenge the calculation, PM me, I'm always open to dialog; but on an order of scale, I think the calc proves that wall thickness isn't the issue. A well "shaped" and supported tank could be made out of quite light materials.

StoneCrab, I agree with everything you posted here. All excellent suggestions and spot on. Shape is key to structure and if the tank where rolled into a cylinder, 18ga would be an excellent choice or might even be able to use 20ga. This cylinder shape is very stiff with little to no oil canning.

Most quality sheet metal manufactures will have tooling for the press brake to add embossed beads or cross brakes to stiffen flat metal sides. TIG welding is the preferred method to weld thin stainless like 18ga albeit slow and expensive, it produces the least HAZ (Heat Affected Zone) and could forgo an annealing process of MIG welding. Thus the tank made of 18ga is possibly an acceptable choice. Provided it is built by an experienced company.

Weight is always a concern, and using 16ga would only add about 1/2 lb/sqft of material weight and The fabrication is easier. From my experience the major problem with thin metal tanks in a dynamic environment, is fatigue. The flexing of welded joints such as bracing and baffling will lead to fatigue cracking at those weld joints.

Given the location of the OPs tank, removal is difficult and I personally would (and have) erred on the side of caution by using a little heavier material. My pragmatic opinion. YMMV
But to your point, shape (I) is the start of any structural design and material (E) choice follows based on load (L) requirements and costs ($).