PS Advisor: 11/01
Aluminum Fuel Tank Mounting and Corrosion
I need to replace a 27-year-old steel diesel tank in my wood Dickerson 41. New custom tanks seem to be available in aluminum, aluminum, or aluminum. It is apparently the material du jour.
The old tank was framed around its perimeter in two bands, at 1/3 and 2/3 the length of the tank, on the hull frame centers. The original painted steel is in very good condition in these areas. I am reluctant to mount an aluminum tank in the same way, without isolating it from the corrosion potential of wet wood.
One recommendation that I have read is to glue 1/4" thick strips of fiber reinforced plastic to the tank with 3M 5200, to seal the aluminum from water where it contacts the plastic, while allowing it to be exposed to air to dry elsewhere.
Please comment on mounting and plumbing practices, to avoid corrosion problems with aluminum tanks. Is powder coating the exterior a viable extra step to assure long life?
We intend to live aboard and cruise this boat, so will be in saltwater at some point in time. It is, by the way, a great boat which the market under values at present...
The mounting and isolation system you describe is good. Here are the ABYC recommendations: "For [aluminum fuel tanks]... all non-integral tank supports, chocks, or hangers shall be separated from the tank surface by a non-metallic, non-moisture-absorbent, non-abrasive material suitable for the purpose, e.g. neoprene, Teflon, and high-density plastics, that shall adhere to the tank."
Once you have the tank mounted, don't be tempted to cushion it with foam, soft padding, or anything that will absorb moisture. The whole idea is to make a secure installation that provides air circulation and prevents crevice corrosion.
As for the pros and cons of powder-coating, while some builders do use a baked-on epoxy-enamel to seal tanks against moisture, our guess would be that powder-coating would not be needed; in fact if the coating's adhesion were less than perfect, or if it were scratched or abraded down to the bare aluminum, it might give rise to the sort of crevice corrosion you're trying to avoid in the first place.
Just keep the water out of the mounts, and air circulation everywhere else.
In the November 1, 1997 issue you indicated that navigation software demands a lot of power and will eat up battery life fast on a laptop. What is a reasonable estimate for power consumption for a laptop? Why did you recommend using an inverter? Would a laptop "air-auto cigarette lighter" adapter suffice?
The initial power consumption on laptops varies a lot, depending on how old they are, how big their screens are, and the type of processor they use. Since 1997 most laptops have become more power-efficient, but some have actually become greedier. Let's say a typical late-model laptop runs between 25 and 50 watts when doing "normal" chores – that's total power consumption for processor, monitor, fan, and hard disk spinning. Graphics-intensive programs like navigation software do demand more power, but again, the amount depends on how bright you keep your screen and how often you change charts and zoom in and out. You could probably estimate use at between 50 to 75 watts, and maybe up to 100 watts with the CD drive spinning. That begins to look like real power consumption.
J.P. Vellotti, a formal technical editor at PC Magazine, reminds us that computers that display the Energy Star logo are most efficient. He also suggests loading all the CD data onto the computer's hard disk if there's room—that'll save a lot of time and disk spinning. Vellotti says that the typical computer battery should hold up through about 1000 charge/discharge cycles, although it may begin to show signs of weakness before then.
If your computer has a cigarette lighter adapter, use that instead of an inverter. It will surely be more efficient.