Water Tank Design, Construction, and Installation
The tank design needed to take maximum advantage of the space in the bilge. In other words, the tanks needed to conform as much as possible to the shape of the space in the bilge--narrow at the bottom and wider at the top. The current plan for water tankage is three tanks on the centerline with a capacity of about 70-75 gallons. We will keep the 30 gallon quarter berth tank. We will built a new tank under the V berth at about 30 gallons which will get us to about 130 gallons. I researched various tank materials. SS was high on my list. SS tanks are strong and there are no lack of facilities that can repair them. But, the more I read about them and the more sailors I talked to that had experience with SS tanks the less confidence I had in my ability to get tanks that would be reliable and at a cost I could afford. Crevice corrosion plagues SS water tanks. I heard of tanks lasting 20 years. But I also heard about many tanks that had a far shorter lifespan . . . some less than three years due to massive pinhole leaks along the welds. If the design allowed them to be square or rectangle then the cost would not be out of range but as soon as I got into custom complex design the cost went up very quickly. I drew up the design and faxed it to a number of SS tank builders. The number one metal for water tanks is monel. The estimate for monel tanks were $4500! Way to high. SS 316L was next on the list but the cost ran to about $2500-$3000. 304L from $1800-$2500. This was more than I was prepared to spend--plus there remained the issue about getting tanks that would last.
I tried finding prebuilt roto-molded tanks. These were not my preference because they don't have full baffles and they can't really be repaired. But they are the least expensive and can be pretty reliable. I thought they would be a good choice if I could find ones that would closely conform to the bilge area. I contacted Kracor who at one time made bilge tanks to fit the Cape Dory 36. But, they destroyed the mold in 2003 and wanted $3000 to build a new mold. I contacted RONCO tanks in SOCAL but they did not have tanks that would fit my bilge. I took a hard look at building epoxy tanks. This seemed like a good option. It was not too expensive and I had the skills. I found an epoxy approved by the FDA for potable water tanks . . . but it was a big undertaking and I already had a full plate. Then I heard about custom welded plastic tanks. They are FDA approved. They can be built to fit any space and can have full baffles built in. Unlike roto-molded tanks these can be built very thick and can be repaired, though not in as many places as SS. I faxed the design out to three companies and got references and called them. I decided to go with Dura-Weld tanks. They have a great reputation and were easy to work with.
After revising the drawings I faxed them out for quotes. After selecting Dura-Weld to build the tanks I called them and spent some time discussing the tanks with Garreth. We talked though the design and made sure we were both on the same sheet of music. It only took them about two weeks to build the tanks.
I'll build shallow fiberglass trays, molded to fit each tank bottom. I'll fastened the trays to fiberglass runners that I'll glass into the bilge. The trays will keep the bottom of the tanks from moving around. I'll use blocks on the top end of the tanks and SS straps to hold them in place.
The middle picture depicts the trays I built for each tank. They were simple to build. I turned the tanks upside down. I covered them with plastic sheeting and laid four layers of 17oz biaxial cloth wetted out with West System Epoxy over the tank bottom. The next day I removed the cured trays from the tanks and trimmed them down with a grinder.
The bottom right picture shows how it will look in the boat. The forward tank is sitting in its custom molded tray. The tray will be secured to the runners with mechanical fasteners so I can remove the trays for cleaning, etc. The runners will be glassed into the bilge. All three tanks will be lined up and spaced to allow the floor timbers to fit between them. I will drill 3/8" holes in the lower aft corners of each tray to allow any water that gets into the trays to drip out into the bilge.
The bottom two pictures to the right are photos I took this winter and they show how the whole thing fits together. The forward tank is the closest to the front. The tanks get deeper as they go aft because the bilge slopes down and back. The aft two tanks were divided otherwise they would not have fit through the companionway hatch. Each tanks has baffles and an inspection/clean-out port for every baffled compartment.
Today, I also filled the tanks with water to see how much each tank would hold. The forward tank holds 35 gallons, the middle tanks holds 18 gallons, and the aft tank holds 20 gallons. With the 30 gallon quarter berth tank that gives me just over 100 gallons. I'll add another 25 -30 gallon tank at some point.
Today, for example, I installed the water tanks. All went well. After I sat them in the trays I installed yesterday, I checked that there were level both athwartship and fore and aft. I removed them from the trays then I used #8 SS self-tapping screws to secure the trays to the stringers--they are temporary screws as they were the shortest ones I had on hand. After I reinstalled the tanks I sat back to admire them. I was thinking how much more proper they look in a boat than on a garage shelf. Then I noticed they were not level with the forward floor timber. Hmmmm. So I checked the tanks with the level. Dead on. Then I check the floor timber . . . high on the right and not just a little. I check it against the other floor timbers. They were also high on the starboard ends. I check the cockpit-- dead level. I check the cleats the supported the settees "back in the day." Level. Very interesting. Either the deck is on crooked or the boat was not level when the builder installed the floors. I will investigate this more tomorrow.
For more on floor beams go to Rebuilding the Interior.