pre-fab fuel tanks

[Ken Liden]

Looking for a method of tank construction that would allow pre-fabrication ashore and final assembly aboard the boat. I don't want onboard welding (wood boat) if it can be avoided. The tanks would be about 6 foot cube so drop in is out of the question. Any thoughts?

[PugetSound]

That must be some boat you're working on!!! You sure you want tanks (plural?) that are 6 foot on a side? That comes to: 216 cubic feet = 1620 gallons of fuel = 11,340 lbs (diesel oil). One thing for sure is that if you really want to have such large tanks, you'll want baffles built in to control the free-surface movement.

[Barnacle Bob]

Ken,

I face the same problem in my 56' Bertram cruiser I'm in the process of restoring. I have two main alluminum alloy tanks (400 gallons each)for diesel fuel that have been leaking slowing and I have to pull them out and put new ones in. This means tearing out the guest cabin wall and then the floor to get at them. I have been studying different ideas and since this Bertram was built in 1973, it has a very thick solid Fiberglass hull, so I am thinking of putting in an integral tank using the bottom of the hull in this section (between the twin shafts in the stern - the most stable part of the hull) as the bottom of the tank and then building in the sides and a lid. Of course it will have baffles like the original tanks that I will look at when I remove them. I plan to use a urethane foam board that is 9mms thick and I will laminate three sheets together to make it thicker so it would be 9mm x 3 for the walls and the lid. after grinding the surface section of the hull and cleaning it with acetone I will bond in the walls with epoxy putty and then tape the inside corners and joints with fiberglass cloth and then cover the inside with cloth and coat with epoxy. It's easy enough then to just bond in an inner lip (using cleats of the same material) around the inside near the top down the thickness of the lid which will then be bonded to that. There can be access covers with seals located in accessible places on the lid so the tank can be cleaned once in a while, etc. (something the alluminum tanks don't have. So, by building the tank in place, it saves tearing up even more of the boat that would be necessary to get the new tanks in. Also, the urethane will never corrode nor rot, is plenty strong, (used in aircraft and boat construction and for municiple sewage wastewater treatment tanks here in Japan). The bilge will stay cleaner as nothing will accumulate under the tanks like with the alluminum tanks. And there will be a double bottom where the integral tanks are located providing added saftey at sea in the event the hull is breached in this area. The U.S. Coast Guard Website had a report on integral tanks and advantages and disadvantages of the different materials used. They said Fiberglass tanks were the best, but cost more to construct. I have a small fiberglass day tank in the engine room that was installed along side the larger original day tank that was the first to leak. The original owner had a fiberglass tank made and it has never leaked. If the main tanks workout the way I want, I will also install a urethane/epoxy tank inside the original day tank in the engine room,(cutting the lid off the old tank and building the urethane tank inside it)to enjoy near the original capacity and then remove the smaller fiberglass tank which is crowding the space down there. There is a supplier in the U.S. of a polyurethane foam board they call "Last-A-Foam" for aircraft, industrial, construction, marine, nuclear, shipping and modeling industries that I have been looking at and have samples from them and I may use that if I can import it at a reasonable cost for Japan. Their website is: www.generalplastics.com (http://www.generalplastics.com) I would be interested in hearing how you progress and what you do for your final solution. If I can be of any futher help, please contact me.

[Barnacle Bob]

Just some added comments. The Urethane board is easy to cut and handle, is light, but strong. There is no welding to worry about, this board cuts easier than plywood, bonds wonderfully with epoxy.

[Ken Liden]

Thanks for the feedback gentlemen. PugetSound. I have based by calculations for fuel weight on the standards for # 2 diesel ie 7.19 - 7.42 lb/gal. The actual tanks will somewhat less than 6 foot cube. Current tanks are mounted at deck level in the engine room. I hope to take advantage of the 22 inch bilge depth on the inboard side for added tankage. Total weight is not a concern. The original engines weighed 11,120 lb. each and have been replaced with Cats weighting 5,620 lb. ea.. The original full width gas tankage of 2000 gal. had been previously replaced with a pair of 1,100 gal.diesel tanks located 4 feet further aft. The current tanks actually use part of the space available after the orignal 12 ft. long engines had been removed.

Barnacle Bob. Thanks for the tip on urethane board. I had considered building glass tanks in place but discarded the idea because I don,t have much experience with glass. I will give the urethane board some strong consideration.

[Barnacle Bob]

Ken, The sawdust from the urethane board makes a strong putty. The board I can get in Japan has a glass fiber grain running through it and this results in a urethane powder mixed with microthin glass fibers, mixed with epoxy to a peanut butter consistency makes a very strong, but light putty when cured. Great for bonding in the urethane board walls. I alternate the grain 90 degrees when I laminate the three sheets together and this makes it even more dimensionally stronger like plywood. The only drawback with cutting this is the fibers ich if you get them on your skin. The Last-A-Foam does not have any glass fibers and the sample of the various densities I got from them are stronger than the urethane board I get in Japan. So, I may use the Last-A-Foam instead if I can import it at a reasonable price. Keep me informed of your progress.