This article applies to replacement of diesel fuel tanks aboard a boat fit with a diesel propulsion engine and a diesel generator. This article DOES NOT apply to gasoline fuel systems, which carry different risks, and different handling and construction considerations.
There are several choices for dealing with diesel fuel tank leaks. Most if not all Taiwan built boats have tanks made of “mild steel.” Also called “black iron,” these tanks are well known to develop leaks at welds and often, on the tops of the tanks. A common cause of tank top failure is rainwater which leaks through deck fill openings.
Some tank leaks can be plugged with sealants and/or adhesives, and while that may save up-front costs, it undoubtedly delays the inevitable and impairs the resale value of the boat. Sanctuary developed a leak that could not be accessed for simple, external remediation. After careful review of my options, and in consideration of the age of the boat, I chose to replace my OEM tanks. I did this replacement as two completely independent projects, the first being to replace the STBD tank (2017) that was leaking and could not be used. The second project was to replace the PORT tank (2018) as “predictive maintenance.” This article documents my approach to the tank replacement project.
The major steps of the project plan for replacement of a diesel fuel tank include:
- Assess the extent of personal involvement to be invested in this project, based on personal preference, personal skills and boat configuration.
- If professional help will be hired, define the scope of the work to be contracted.
- Settle on design of the replacement tank solution.
- Contract/hire professional assistance.
- Empty the tank to be replaced.
- Gain physical access to the tank to be replaced.
- Perform demolition and removal of OEM/old tank.
- Qualify and hire fabricator for new tank.
- Wait patiently for the fabricator to complete the build.
- Receive and place new tanks.
- Restore disrupted plumbing and other vessel infrastructure.
- Fill and calibrate new tank.
- Celebrate completion!
Because I have the necessary skills and tools, I decided to handle many parts of the project work myself. However, I also decided I would hire a mechanic to cut out the OEM tanks and install the replacement tanks. My tasks would include gaining access to the tanks so the mechanic could come in and begin to cut. The mechanic would manage removal of the old tank, transport the replacement tanks from the fabricator to the boat, prepare the install location, move the replacement tanks into place, mechanically secure the tanks in place, and re-plumbing the tanks. I would then take over to button-up the work once the new tanks were secured in place. This approach worked well for me, and saved many thousands of dollars of professional hourly-billing labor time.
Aboard Sanctuary, the OEM configuration consists of two, one-piece tank units of 160 gallon capacity, each, located athwartships in the hull, in a “saddle tank” configuration. The OEM tanks were placed into the hull before the deck was installed, so physical clearance limitations made it impossible to install a single replacement tank of the OEM dimensions. The OEM tank is 48” long, with a baffle in the lengthwise center. It would have been possible to reduce the height of the OEM tank by 3”, but physical placement of a 48”, one-piece tank would have required removal of the engine to gain the needed clearance. Since we live aboard, removal of the engine was a significant impediment. However, two 24” tanks could be fit without engine removal, so two side-by-side 24” tanks became the design point I adopted. This approach also provided equivalence with the baffle of the OEM tank.
Using Lotus FreeLance drawing software, I created an engineering drawing for my replacement design, as shown in Figure 1 for my STBD side project.
The complete drawing set for the OEM tank, STBD and PORT replacement units and fabrication notes is here: 20180506_Monk_Fuel_Tank.
Between the mechanic and myself, it was agreed that I would do the site preparation work to gain physical access to the tank. On the STBD side, that involved total removal of the DC electrical system and batteries, relocation of AC distribution wiring to the aft half of the boat, and removal of a non-structural bulkhead covered with soundproofing tiles. Gaining access to the PORT tank involved removal of the main fuel supply rail and primary filter plumbing and removal of the control unit and hydraulic pump for our hydraulic thruster system. On the STBD side, the house batteries needed to be removed from the boat, so I used the genset start battery to power the house water pump and the waste macerators for the duration of that project. Because the OEM STBD tank had leaked fuel, it was already empty. On the PORT side, I pumped fuel from the OEM PORT tank to the newly replaced STBD tank to empty the PORT tank.
I recommend that photographs be taken at many points as any complex project proceeds. It’s amazing how these pictures help at assembly/re-assembly time. Figure 2 is a picture of the wiring of Sanctuary’s main battery box. Figure 3 shows the DC distribution wiring before the start of the project. this distribution wiring is located on the bulkhead the covers the OEM STBD fuel tank:
After removal of the DC distribution wiring and temporary relocation of aft-running AC wiring, the bulkhead could be removed. That was a destructive process. The OEM bulkhead was 5/16” plywood – well, since Sanctuary was built in Taiwan, probably 8mm plywood – but non-structural. Figure 4 shows the OEM tank with access gained. At that point, an angle grinder was used to cut out the OEM mild steel (black iron) tank. Careful examination reveals two structural angle iron retainers holding the OEM tank in place. These angle iron retainers were re-installed after the new tanks were placed. Figure 5 shows the hull space, frames exposed, after the OEM tank was cut out:
The replacement tanks were fabricated of 1/8″ (0.125″ ) Grade 5062 Aluminum. The work was done by a local SW Florida metals shop. The fabricator pressure tested and certified the tanks. The individual tanks are light enough that they could be handled by one man (a younger man than I, however). Figure 6 shows the tanks staged on the dock, and Figure 7 shows them in their installed location with the angle iron retainers in place:
Note the length of fuel hose that interconnects the two tanks at the bottom. That hose is continuously filled with diesel fuel. Use USCG Type A1 fuel hose for that application. USCG Type A2 fuel hose is appropriate for the tank fill hose. Type A2 hose is rated for fuel, but not for applications that are continuously immersed in fuel. Note also that both tanks need to have a vent. Consider the drawing in Figure 1: fuel enters the “A” tank via the fuel fill in the deck, but then fills the “B” tank from the bottom up. The “B” tank must be able to vent captive air or that tank cannot fill. Likewise, for fuel to leave the “B” tank as it is consumed, air must be able to enter the cell above the fuel in order for the tank to empty. In our case, the two vents from the “A” and “B” tanks tee into a single vent, which is mounted overboard of the hull. Finally, the tanks, the deck fill fitting and the vent thruhull fitting should be electrically bonded to the vessel’s bonding system, if equipped, to dissipate static electricity and prevent galvanic corrosion.
Fuel plumbing also merits special mention. The fuel valves used in diesel fuel systems are naval bronze, which is galvanically active in direct contact with aluminum. To minimize galvanic corrosion at the tank fittings, use a 300-series (316L) stainless steel bushing (adapter) to isolate the anodic and cathodic metals of the bronze valve and the aluminum tank fittings. Bond the tanks to the vessel’s bonding system, if equipped.
With the tanks installed and secured in place, the bulkhead and the vessel’s wiring can be reinstalled. Figure 8 shows the replacement bulkhead in place, with an inspection port that allows access to the interconnecting fuel hose and it’s hose clamps. The temporarily relocated overhead electrical wiring is still evident in this picture. Figure 9 shows the batteries and finished DC electrical distribution system in place.
When filling the new tank for the first time, I put in 10 gallons of diesel fuel at a time, and marked the tank using the sight glass meniscus as the reference. I find calibration of the tank capacity to be extremely helpful in judging my cruising options as I travel.
The loss of 3” in height resulted in a loss of about 25 gallons of total tank capacity. Each boat is different. Each tank replacement project is different. For what I’ve described above, I spent $1750 to have the STBD tanks fabricated, pressure tested and certified. Labor and miscellaneous materials – like the A1 and A2 fuel hose, hose clamps and new fuel valves – was $1800. I invested at least 30 hours of my personal DIY labor doing demo, site prep and re-install work, so for those who choose to contract this total project, consider what that would add in cash cost if performed by a paid professional. There were efficiencies gained in doing the STBD tank. The fabrication cost of the PORT replacement tanks was only $1570, and the professional labor component was $1260.
There is no question, this is a major project. With the work done, don’t forget to celebrate.