Rigid pipe is problematical
Because it doesn't bend, it should only be used for long straight runs. Every bend means breaking it to insert a radius fitting, and every joint becomes a bump in the line to traps waste. Rigid pipe should NEVER be directly connected to anything else rigid--head, tank, macerator through hull. It must be "soft coupled" with enough hose to act as a shock absorber (more connections/bumps). Otherwise, when the boat flexes, or takes a pounding in heavy seas--or even from a bad wake...bumps the dock hard, etc, the weakest link in the hard-hard connection can crack. That's either gonna be the pipe or a tank fitting. Hard pipe only makes sense on boats that have straight runs of 5' or more. No metal is suitable for waste holding. Urine is so corrosive that it typically eats through a weld at a seam or a fitting within 2-5 years. It takes a bit longer to eat through stainless sheet metal than it takes to eat through aluminum, but I've yet to see a metal tank--stainless OR aluminum--last longer than 10 years (any time the tank has been older than that, it's because it wasn't used). Otoh, a good quality, thick-walled plastic tank will last for decades (the one on my last boat was 16 years old when I bought the boat, and the only part of the sanitation system I didn't replace. It was 21 years old when I sold the boat and still in fine shape). And the good news is, a really good quality from Ronco costs about half what any metal tank does, especially stainless. Bite the bullet and spend the money for SeaLand "OdorSafe" hose. It's just about as "bulletproof" as hard pipe. Yes, at $8/ft, it's expensive...but which is really more expensive--$8/ft once...or $3.50/ft every 2-5 years? Not to mention what a ROTTEN job it is to replace sanitation hoses! Do it right, and you'll have to do it once.
