Thanks for all the inputs so far. Since this discussion has drifted towards PVC vs. hose, discussed at length in a multitude of other threads, I'll try to steer it back to the primary issue: availability of 60 mm ID hose.
We have considered PVC but have not been able to find that size pipe here in Martinique
. Having read most of the other discussions on CF about PVC vs. hose (one of which Cotemar referenced, thank you), I conclude that there are many boats out there successfully flushing
waste through pipes in spite of the warnings about flexing. Many of those discussions recommend at least a short piece of hose to allow for movement, which brings us back to our original problem of finding hose.
Our holding tanks
are wedge shaped, hold about 8 gallons each, and are mounted to the bulkheads above the toilets for gravity drainage. The starboard tank has about 1 foot of hose connecting the tank drain fitting to PVC piping which then runs to the overboard
hull valve. Existing misalignment between the drain fitting and the piping makes PVC for that tank a very remote
possibility and would require seriously modifying existing piping.
The port tank has about a 4 foot run from the tank drain directly to the dump valve through an S shape, which would require some imaginative fitting placements. That hose desperately needs to be replaced because the outer covering is bubbling and the end connected to the tank leaks
waste onto the toilet cover, which annoys the wife
Speculating here, but I believe that one reason for such large hose is to get rapid flow out of the tank for good drainage. Therefore introducing a restriction into that line in the form of a reducer as djmarchand and David M (one chandlery
here showed us the Vetus reducer you linked to) suggested doesn't seem like a good idea to me.
Still looking for hose.