Corrosion, Aluminum Tanks, and The 23' Venture

[Tim & Dave Kinghorn]

Nigel Calder in the "Boatowner's Mechanical and Electrical Manual" (2005, p. 520) cautions that aluminum tanks are prone to failure (leaks) due to corrosion. He quotes a Coast Guard Boating Safety Circular (#79, Apr. 97):
"Aluminum tanks failed in many different types of boats...were reported to have experienced problems associated with aluminum fuel tanks installed as manufacturer's standard equipment." He details how the major problem with corrosion occurs, particularly where the tank is in contact with bilge water.

On our 23' Venture, the aluminum tanks are a quality product. Each tank has feet with a foot that is about 2 x 4x 1/4" on each corner. We elevated each tank on four 1/2x1&1/2x24" Starboard slats, and we shimmed under each hold down with a 1/4x2x4" piece of Starboard and under each "foot" with a 1/4x2x4" piece of Starboard. In doing so, we became aware of a potential major problem with corrosion due to how the tank was installed by the builder.

On our venture, there is a gutter that goes along each side of the aft cockpit floor and drains across the stern into the sump at the drain plug. The gutter across the stern is fairly wide; and when the tanks are pushed to the stern, the rear "feet" can slip over this edge. Thus, the rear, bottom edge of the tank can rest on the back edge of cockpit floor. Water and debris can then lodge against the tank's back edge. Even when water is drained, there can be a debris laden, wet compress against the bottom of the tank. As the tanks are bonded, you will have a potential corrosion circuit with the other bonded metals. He points out that the critical bilge water level is when bilge water makes contact with metal fuel tanks.

You can easily check the clearance under the tanks with a flash light. But be aware that the bottoms of the tanks are only supported at each corner. Therefore, a full tank may sag a bit in the middle (a bit like my stomach, I fear). Again, debris can accumulate and hold moisture as fuel tanks do "sweat".

It was removing the hold-down lag bolts, that led to our finding water under the aft cockpit deck.

Hopefully we're being overly concerned, but we thought we'd pass along our findings for the general good.

Tim & Dave Kinghorn

 

[thataway]

It sounds as if you have "solved the problem". ONe of the major issues is the foamed in tanks. Foam holds enough water to keep the tank in contact with salt water.

The best way to support tanks is to use inert strips, as you have, They should be 5200 to both the tank and the platform which the tank rests on. The platform is usually a thick plywood, and should be completely sealed with epoxy, best with a layer of cloth on the top.

Any hold downs or supports should not be rubber or other material which contains carbon. This can be conductive. These issues have been discussed in the past, but it is good to refresh our memories.

 

[Tim & Dave Kinghorn]

Hi Dr. Bob,

We were unaware of the previous discussion related to aluminum tanks and corrosion. However, Nigel Calder does give an excellent summary of the issues for those who are interested, particularly the issue of foamed in tanks.

But, we were more interested in directing the attention of other Venture owners to a possible corrosion issue due to the tank's back edge slipping into the drain gutter along the stern.

However, we would not recommend using any adhesive to hold the tanks or the slats in place. Our experience does not show any movement of the slats placed under the tanks. At sometime, you will need to pull the tanks for maintenance. If 5200 is used to hold the slats to the deck and then to the tanks, you are looking at a difficult chore to remove them. For example, one needs access to the engine mounting bolts which can be difficult with the tanks in the way. Also, the discharge hoses from the bilge pumps pass behind the tanks along with associated wiring harnesses. As I consider the option, however, it does occur to me that you could use 5200 or 4200 to hold the slats to just the bottom of the tanks only.

The aft deck of the venture is cored. On our boat it is fiberglass enclosed balsa. The issue in securing the tanks is to ensure that the bolt holes are cleaned out, undercut, and filled with epoxy putty as you have so well previously described.

Hopefully, no one else has this problem! But we feel, it is better to be cautious and point out a potential problem then not say anything--especially related to the fuel tanks.

Tim & Dave Kinghorn

 

[Captains Cat]

Bob, any idea how our TC255 tanks are mounted or if they are subject to this potential problem?

Charlie

 

[thataway]

Charlie,

As best as I can tell, the tanks are laid on a platform, on the bottom of the hull and then strapped in place. Generally, if the salt water does not stand around the tanks, there is not a problem. In my boat; lift kept, the water all drains aft. If the boat is on a trailer, the tongue should be eleevated. In the water boats could have a problem.

I didn't comment on painting. If you properly clean, dry, etch and then two part epoxy stronchium chromate and apply a good exterior enamel, that will also help to protect the tank (think the paint job on the bottom of your outboard's lower unit). At least one of the Tom Cat 24 owners had to remove his tanks because of a leaking plastic tank. The port tank in the 255 can probably be removed fairly easily (with a little cutting). The Starboard tank is a whole other bag. There the shower is in the way, and that would take a lot of re-glass work at the end.

 

[Casey]

Would it be feasible to cut some of the interlocking DriDek squares to make a riser for each tank to set-on? They would provide broad support (to prevent Dave's sagging middle...) yet provide an airspace beneath from which debris could be flushed, etc.

...just a thought.

I'm finished with the current set of "boat projects" and plan to depart for Prince Rupert tomorrow.

North to Alaska - Yahoo!

Best,
Casey

 

[Captains Cat]

Charlie,

As best as I can tell, the tanks are laid on a platform, on the bottom of the hull and then strapped in place. Generally, if the salt water does not stand around the tanks, there is not a problem. In my boat; lift kept, the water all drains aft. If the boat is on a trailer, the tongue should be eleevated. In the water boats could have a problem.

Thanks Bob. Mine lives on a lift too and I can (and do) drop the stern lower, primarily so the cockpit will drain in a blowing rain. I do have a roof over it so a lot of water isn't a problem.

Charlie