Kevin,

The tanks are Aluminum 6 Lb. each. The spray hood/box is only for storage. It holds 3 and we have one working on the stern rail for a total of 6. That lasts us for over 5 months non-stop.
As you can see, the box also holds our wash-down system, stereo speakers, anchor hardware, and acts like a dashboard in the cockpit for our chart plotter and radar display.

More photos at our 2 year gear report at: http://www.svguenevere.com/2006/gear/gear2.html

Greg