Belly Gas tank and stringer design

I would not have anything for your type of application as the tanks that we installed normally spanned several stringers, and if not, we welded in additional support pieces , glued on neoprene, normally built an inverted "U" strap with tabs and bolted to the stringers.

Under our straps, on the tank was another strip of glued on neoprene.

 

Yes, Barry. This was the common method for many of these off shore boats at time of manufacture. Donzi, Magnum, Cigarette, Formula, Martini, Nova to name a few. Many of the restorations I see replicate this method with the caveat of a properly coated tank. The tank is blocked in place to prevent the expanding foam from pushing it out of position. Then the liquid foam is poured in. I was told the foam under the tank supports the tank structure when loaded (and g-force applied to the contents). Some installations leave a tab or bracket of sorts, attached to the stringers. Maybe this is considered in addition to plastic foam to satisfy the "other than plastic foam" requirement.

Perhaps "encased" in the regulation refers to all six sides covered. Typically these tanks are not foamed on top, thereby not being totally "encased".

There is a drain tube that runs directly along the keel under the tank to pass any moisture.

Picture is my 1965 Ski Sporter before removal (this tank was steel) of the tank. Certainly not the neatest of jobs... I am replacing this tank due to age. It was difficult to remove and did not leak. Not bad after 50 years.

 

The ABYC standards are for boats manufactured from July 31st onward.

Steel tanks installed in boats now must be hot dipped galvanized inside and outside.

50 years is pretty good service for a tank, luckily it did not leak during this period.

 

Foam will absorb fuel and oil in case of a spill. It is virtually impossible to clean it, and the fumes may be a problem. Also, encased in plastic includes a tank fiberglassed in. The resin is a plastic. I like plastic tanks because they will never corrode and can are somewhat easier to install; cheaper too.

 

I would like a plastic one too, but I am limited on space so I need to have an aluminum one custom made. I plan on using speedytanks.com and they coat the whole tank in epoxy.

Seems like I have a good idea of how to mount the tank and set it up...

I looked at some Doug Fir tight grain at the wood store today and I plan to put two stringers in to replace the thin fiberglass ones. I have to figure out how I can cut a sharp angle on the bottom of the stringers so I can make them stand up straight. I cut a lot of wood in my workshop so I can figure it out on my table saw

How thick do you think the stringers should be?

 

The metal tanks in foam lasted 25-30 years before corrosion caused a leak.

 

Yea that's a pretty long time and probably ok but I have a personal issue with the foam. I just spent HOURS ripping old soggy foam out of the boat's sponsons, and is the main reason the floor is ripped off and being replaced. At the same time I am doing some other improvement too.

I'd rather not include foam, what happens if I hit a rock and get a hole in the bottom of the boat? Then I have water pouring it before I can patch it on land and probably working it self into the foam to get trapped. This boat will be free flowing on all three sponsons for excellent drainage and ventilation.

 

I would use 3/4". That's more than enough for the job.

 

Nice that will make the thickness easy to work with (cut) and I plan to fiberglass it in attaching to knees connected to the transom

 

I think I got the tank design all set but still trying to figure out how to install the tank under the floor so I can remove it. I don't think I can do it without it looking ghetto