In article , Peggie Hall
wrote:


hard PVC is
likely to crack, especially in cold weather.

Not. (incompetently piped?)

Incompetently piped is only one cause. Schedule 40 gets very brittle
below 40F. Even a sharp rap against a dock can crack it. The kinds of
stresses a boat can be subjected to in a storm at sea can crack sch 40
even in warm temps. PVC glue can crack, resulting in a leak.

I really wonder about this. First, not all yachts have hulls that are
all that flexible - steel boats in particular. Second, PVC *is* used on
commercial fishing boats all the time, and it works well. Third, there
are low temp plastics available. Fourth, PVC glue *can* crack, but
rarely does if the joint is done properly - cleaned, primed then glued.
It's a solvent cement rather than a glue.

There are also other plastic products. I'm thinking of pressure rated
black plastic pipe widely used on farms worldwide. The fittings are a
bit on the bulky side but work very well. Some pipework on my country
place has been exposed to UV and heat/cold cycles for 20 years now,
under 10 metres of water head pressure. No leaks.

Have you never worked with PVC fittings & layout? It is very fast &
easy.

You've obviously never had to pay for labor. 5 extra minutes per
union on your own boat is one thing...multiply that by several hundred
or even several thousand boats, and the cost to builders becomes
prohibitive.

Yeah, but that's a different argument. We all know builders of anything
need to look at costings, esp when any potential problem is going to
happen far enough in the future to be not theirs.

In the case of blackwater wouldn't it last far
longer & be more vapor-impermeable?

More impermeable, yes. But the stresses on the glue in unions results in
more maintenance (or repairs at sea).

I really doubt this is true, if the hull is rigid enough and the
pipework is well supported. Personally I think the whole business of
running flex hose everywhere is plain crazy and a throwback to a time
where the choices were flex hose or metal pipework.

Put it this way - I spend some months of every year on an icebreaker
bashing about in sea ice. It's COLD, the hull is cold, there's a hell
of a lot of vibration, shuddering and shock loads that the average boat
NEVER sees. There's a hell of a lot of PVC piping for the toilet
systems, potable water systems etc etc on that ship. Sure, the final
runs are all welded steel pipe, but the point is, we *don't* have
massive and continual failures of the PVC pipework, despite cold and
vibration.

Haven't decided what I'll use, but flex hose will be considered only
when I can't use something better. If the flex hose is better
structurally than rigid pipework, why don't commercial vessels use it?

PDW

One more time: it's YOUR boat. Pipe it any way you want to and live the
with consequences. If you're lucky--and also more competent and
knowledgable than the "average bear," you won't have any negative
consequences...but I've learned from experience that it's hard to go
wrong tailoring advice to the lowest common denominator.

The real question is: how far removed from the lowest common denomator
are you? These days, a lot of boat owners aren't as far as they'd like
to think they are. Otoh, those who are smart enough to find their way
here are usually are smarter than "the average bear.' At least the
questions I see about sanitation systems seem to indicated that they are.

Peggie
----------
Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://www.seaworthy.com/store/custo...0&cat=6&page=1

In article , Peggie Hall
wrote:

One more time: it's YOUR boat. Pipe it any way you want to and live the
with consequences. If you're lucky--and also more competent and
knowledgable than the "average bear," you won't have any negative
consequences...but I've learned from experience that it's hard to go
wrong tailoring advice to the lowest common denominator.

The real question is: how far removed from the lowest common denomator
are you? These days, a lot of boat owners aren't as far as they'd like
to think they are. Otoh, those who are smart enough to find their way
here are usually are smarter than "the average bear.' At least the
questions I see about sanitation systems seem to indicated that they are.

When it comes to engineering stuff, I'm a long way from the LCD. My
work group builds marine gear that's pressure rated to in excess of
2000m pressure, for example. Specifics of marine sanitation systems,
pretty ignorant. Personally, I think the engineering of marine heads is
ridiculously primitive. The Lavac is about the only one I've read about
that makes sense.

My point is, there's nothing that's real rocket science about low
pressure plumbing runs. You need acess to the seacock no matter what,
and anything below WL needs to be reasonably secure against leaks.
Engineered flex hose with 2 connection points (sea cock & toilet) has a
lot less points of failure - provided it doesn't get abraded - than
anything with many joints, regardless of material. However it does have
its downsides. I just can't accept that worrying about vibration
cracking PVC pipes is a realistic issue in practice provided the pipe
is properly supported. It's just as likely that a flex hose will abrade
through and you don't seem to think that's a concern.

PDW