Originally Posted by svcattales
I agree, James. I used 1.5" sched 40 PVC pipe in my sanitation plumbing to my holding tank
and a portion is below the water line...YIKES!
Anyone who has followed my posts will know that I am very relaxed about using PVC (on our boat even the engine exhaust
is Sched 80 PVC) and other plastics but for unplasticised PVC eg rigid Sched 40 & 80, it is subject to embrittlement as the temperature drops and that should be kept in mind for below waterline use.
At -10C (14F) its impact strength is typically around 1/4 of that at 20C (68F) but the drop in impact strength from that at 20C and that at the freezing point of water is steep with most occuring between 20C and 10C (50F) - at 10C impact strength is typically nearly half that at 20C but even that reduction is not a problem in use as these are typical working temperatures.
As you say though unplasticised PVC Sched 80 & 40 is very resistant to impact so is still so at the lower temperatures but is probably of concern enough to be wary about its use below the waterline. Of course if the boat is always in warm climates it may not be a problem but that poses the possibility (likely remote
?) of a wet adventure for future owners or users of the boat if taken to colder climates.
In my own case I plan to use PVC for blackwater when replacement of the hose becomes necessary (it is a high quality industrial hose, not "sanitary" hose, so after 12 years is still non permeated) - however, for us everything, including all the holding tank pipework, is set up to be above the waterline except for about the last 1 foot in the discharge to the seacock - I anticipate using hose to bring the end of the PVC in the drop to the seacock above the heeled waterline, this part of the pipework being vertical and self draining so no permeation and calcification problems in it.