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* Fibreglass reinforced plastic
* Metal
Either linear (not cross-linked) polyethylene or fibreglass is a good choice. Aluminium or stainless steel tanks are easy to fabricate in place, and not subject to odour permeation. But urine will corrode any kind of weld within a few years. Therefore metal tanks should be coated inside with a ceramic-filled epoxy. Use 3/16-in steel.
Ceramic-filled epoxy is a two-component, reinforced polymeric com-pound that can be brushed on.
Polyethylene, in 40-gal sizes, should be a minimum of 3/8-inch thick, for structural rigidity and resistance to odour permeation. Keep it out of direct sunlight.
All connections should be through the top of the tank (input, deck discharge, overboard discharge, vents, level indicator, inspection port).
== Capacity ==
The size of the tank depends on the boat's intended usage (day trips, weekend or extended cruises, etc.). Assume three to five gal (11-19 L) per person day. The size of tank required is thus:
<pre>Size = k(P*D*E)
Where:
K is 1.4 (the inverse of 75% rounded up)
P is the number of people on board
D is the number of days in the cruise
E is gallons or litres per person per day</pre>
Therefore, for four people on a 10-day cruise, you would require a holding tank of 168 gal (636 L). For 10 people on a two-day cruise, you would require an 80-gal tank (300 L). Most commercial tanks are no larger than 40 gal (150 L), so larger tanks must be custom constructed.