Stainless water tank under swim platform

Hello, I have been given to task of constructing a stainless water tank and placing it on the outside of the hull under the swim platform.

Now that I have your eyebrow raised, I need to explain a little background of this project.

It is a retired Aluminum research vessel at 61.6' in length and has a shallow draft for it's size at 5.8'. The boat has been converted to a live aboard and used for pleasure. The shallow nature and the lack of machinery on board provides for a sickening roll when out in light seas. The pilot house and cabin are located quite far fwd and it has a very large aft deck. This vessel has been recently fitted with a Seakeeper Gyro and in the process of a re-power. Weight has been relocated wherever possible to the aft. The fresh water tank has been removed from below the cabin and will be relocated to the exterior of the transom. That should bring you slightly up to speed on our mission to push the aft of the boat down as it was designed for when in working use and also reduce pitch and roll.

I am looking for your professional opinions about this project. The tank will be constructed of 1/4" stainless plate and bolted to the transom via a 1" aluminum flange with 1" bolts. Possibly even running supports along the bottom. I am in the design phase of the tank and just thought I would throw it out there to get more insight.

Finally, the reason for the stainless tank is water quality at the tap. Water was horrible tasting previously and upon removal of the tank we found a white chalky residue up to a 1/4" thick in the bottom of the original tank. There is no room inside for a plastic tank.

 

Is this tank going to be submerged ?

 

Yes, It will be tucked underneath the swim platform which will be right at the waterline.

 

If the tank is submerged you negate the weight of the water inside it and then as soon as you start emptying the tank it will want to float and raise the stern higher than it was to begin with.

 

Thank you. The boat nets 45 ton and gross 66 ton. It had a welded on trim tab that was at a fixed angle forcing the bow down. We assume this was added later to counter the weight of the large tanks it used to haul. I have since removed the tab. The bottom of the water tank will follow the natural transition of the bottom. Along the process allowing the bow to stay up. A future installation of hydraulic trim tabs will be determined after sea trials. I will continue to try and tell the whole story of what has led us to this point.

There are a lot of changes that have been made that all add up. Ex: House battery's moved from engine room to Transom. 5000 lbs of lead ballast has been purchased that will be placed as low and aft as possible and the list goes on.

Also worth noting, the boat will spend most of it's time at the dock. Occasionally It will make a trip out for a leisure cruise and 2-3 times a summer will make a voyage up North for a week or so. The tank will always be full when leaving the dock and I would say maximum water usage will be while at the dock.

 

Yes..the tank is buoyant...your stern will rise.

As for stainless. Ive never had much luck with stainless plumbing or tanks.. corrosion. Crevice corrosion internally, particularly in the area of welds. .


Stainless and aluminum are not friends..

Well painted steel tanks last for many many years. Steel and Aluminium are reasonably good friends.

 

It is difficult to comment with such little details available.

As for the tank..why not use Aluminium??..we have used ally tanks for years, you just coat the interior with epoxy, cures all. All stiffening to be on the outside, to prevent any electrolytic action.

As for the motions, the VCG plays a part, but also without actually knowing/seeing anything about the hull, have you considered bilge keels, to reduce roll?

 

To keep water in tanks " Sweet" the tanks must be cleaned every year or so. This requires a manhole size access hatch in the tank top.

As Ad Hoc states. Alluminium makes great tanks. Ask your paint supplier for the correct paint for the interior of the tank.

 

Sorry Higgins, the whole project doesn't make sense.
With the tank full of water, just the weight of the empty tank contributes to lowering the transom, when empty there is a force equal to the tank's capacity pushing the transom upwards. A guy called Archimedes figured that out long ago.

The choice of material is another matter. Unless you figure out a way to electrically isolate the tank from the hull, you create a giant short circuited galvanic cell that will start eating the aluminum flange as soon as the water surrounds it.

 

Excellent advice from you guys as I expected. I am the fabricator and really need this outside looking in advice. We can address the internal corrosion problems that will arise as most tanks are either SS or Alloy alum. This will be an issue no matter where it is. The dissimilar metals submerged in water directly attached to each other is what may force us to find another solution. The hull is painted and the tank will be attached with stainless fasteners. The flanges can be isolated so there is no contact at all with the SS and boat hull.

CDK has pin pointed what my major concern is. Self destruction.

I apologize for the lack of details, just figured I would throw it out there and fill in the blanks as it went.

We totally understand the effects the tank will exert upon the hull and will be able to deal with that. To focus on a point, will It be inadvisable to use stainless as our tank material? The next options will be aluminum welded in place or a stainless tank installed internally.

Thanks again for your time.

 

Epoxy eglass makes a good tank. When properly painted you will get no unpleasant taste.

 

Thanks Michael

 

Do you mean a complete tank or coating the inside?

 

You should also consider what your holding tank capacity should be. Grey and Black water. The stern of a boat makes a nice location for waste tanks...easy pump out , easy vent

 

When stainless is used for fuel tanks, whoever makes the rules limits them to 20 gallons and they must be cylindrical. From what I understood, welding stainless makes hard spots that when flexed have a tendency to crack.

If the white chalky stuff was caused by the tank materials that's one thing, but if it's caused by the water supply, the tank material wouldn't seem to make a difference.

I found the reference for the tank regulations. Although they only apply to fuel tanks, they are there for a reason. In reality, it might be perfectly fine making big water tanks from stainless, although I haven't seen many on boats.

ABYC H-24
GASOLINE FUEL SYSTEMS
24.19.2.2 Stainless steel tanks
shall be cylindrical, with domed heads, and
a capacity of less than 20 gallons (75 liters)