Hoses "Do's" and "Don't's"

[Pub911]

Can you use pvc pipe and fittings on a boat's salt-water plumbing?

Some background...
I'm fixing some issues with the cockpit drains and bildge pump plumbing. The prior owner did not route the bilge output to a high point, before plumbing it into the existing cockpit-scupper-to-thru-hull lines, causing the boat to actually back-feed water into the bildge via the bilge pump hose. This only occured when the boat was sitting low in the water at the stern, like when too many of us were jammed in the **** pit having beers at the dock. It wasn't an issue under normal load and most points of sail.

Nonetheless, I'm fixing that problem and was wondering if the use of hard PVC pipe, elbows, step fittings, etc. are ok for marine application? None would be used where a flexible route is required - in these cases I intend to use the white, thick, sanitation-type hoses.

FYI, the boat is a 38' NorthStar S&S sloop rig build in '72.

[camaraderie]

Pub...I think PVC is a bad idea on a boat...not that it isn't done...but you have vibration and pounding and multiple joins rather than a continuous piece of reinforced hose.
You only need to use sanitation hose for sanitation lines. Better and less expensive choices are available for other uses.

[sailaway21]

I'd recommend against it. The problem with PVC pipe is not that the pipe is not strong or that it isn't flexible; it's actually both of those. The problem occurs when you glue on fittings such as elbos and male adapters (slip x thread fittings) and the like. Each of those fittings creates a hard spot, or a stress point, in the pvc piping system. Any stress or bending on the pipe is transmitted down the pipe to those fitting and that's where the system will fail.

If you use Schedule 80 pvc it is less likely to do so but only if you use threaded fittings. Note that you cannot thread the more commonly found Schedule 40 pvc, there is not enough wall thickness to do so. Even with Schedule 80 I'd be very leary of using it. it's just not very good where flexing and vibration are concerned.

If you used "tiger flex" which is similar to the sanitary hose you mentioned, and available usually at plumbing supply houses or a swimming pool dealer, you can then use hose barb fittings with it and, if you absolutely have to, can glue pvc fittings to it to adapt to something else. I'd far prefer an avoidance of all critical joints being glued.

Of course you could just re-route the bilge pump discharge hose so it has a high point above the lowest aft waterline. Or you could install a line check valve-I recommend one size over-sized and bushed down to your hose size. Some here do not like line check valves due to potential clogging while other's have had no problems with them. I personally don't like water being able to flow back in through the discharge either.

[sailingdog]

There's a good reason it isn't commonly found on sailboats, either for through-hulls or for sanitation purposes. Unlike a house, boats move... and the parts on a boat will flex and move with it. Often, plumbing on a boat is located inside lockers and such, and may accidentally get stepped on as well. Flexible hose generally bounces back from such mistreatment...PVC pipe doesn't.

[Pub911]

I am interested in alternatives to white sanitation hoses suitable for this below-the-water-line application (e.g. "tiger flex") and alternatives to the nylon hose-barb type connectors which are not as versitile as PVC (step ups, downs, T's, elbows, etc) - I'm find that when I peice together the nylon barb-type to accomplish the need, they require short hose-runs between each, enlarging the assembly and increasing the complexity and number of failure points.

Here's a schematic of what I'm trying to build...

[catamount]

Here's an argument from some experienced cruisers in favor of using PVC pipe for selected applications on boats: http://www.morganscloud.com/stuff/stplumbing.htm

Some uses of PVC pipe for head plumbing are illustrated here: http://www.plasticclassicforum.com/viewtopic.php?t=2804
and here:
http://www.plasticclassicforum.com/viewtopic.php?t=1710

Personally, I plan to use PVC pipe for most of the discharge piping from one of my manual emergency bilge pumps, and will likely use some PVC pipe in my head plumbing. One key is to isolate rigid sections from each other with sections of flexible hose.

Regarding the cockpit drain arrangement shown in Pub911's schematic -- I wouldn't do that. Rather, I would plumb the deck and seat drains either separately, or so that they discharge into the cockpit footwell (rather than into the cockpit drain hose). The bilge pumps should have their own discharge outlets and should not connect to the cockpit drains.

Regards,

Tim

[zaliasvejas]

Pub,
Can you simplify this set up? Just think of each clamp and each wall of the t-connector as the thickness of your hull, your safety...
Personally, this looks scary.... I would not be able to sleep at night with this...

[sailingdog]

Pub-

I see so many different points of failure in that setup that it is rather worrying. I can understand that you want to simplify the setup and use a single through-hull, but a single through-hull also increases your chances of something clogging the drain and blocking all three drains on a given side.

Originally Posted by Pub911

I am interested in alternatives to white sanitation hoses suitable for this below-the-water-line application (e.g. "tiger flex") and alternatives to the nylon hose-barb type connectors which are not as versitile as PVC (step ups, downs, T's, elbows, etc) - I'm find that when I peice together the nylon barb-type to accomplish the need, they require short hose-runs between each, enlarging the assembly and increasing the complexity and number of failure points.

Here's a schematic of what I'm trying to build...

[sailaway21]

Catamount's suggestion to drain the deck scupper and seat scupper to the cockpit and then the cockpit to the thru-hull is exactly what I would do in your circumstance. As drawn, and a nice drawing it is, you'll find that you'll not get good drainage due to the number of tees you'll be using off the cockpit drain. The tee's themselves will intoduce turbulence in the flow and it will only be aggravated by actual flow from the deck and seat.

If you are going to plumb it as illustrated though you will want to hard pipe the tees below the cockpit drains with brass nipples in between bronze tees. and then flex hose down to your thru-hull. Before everyone starts screaming about not using brass let them first offer a source for bronze nipples. I am against using plastic nipples for this as they are virtually impossible to work with once installed. You have to heat the female fitting that they're going into to soften them to back them out or even tighten. You trash the nipple in the process. Thru-hulls should be either bronze or marelon. I am of the opinion that the brass nipples used in the manner dsicussed will not be a problem but, like anything, should be monitored. Brass and bronze are pretty close on the table of dissimilar metals too. You'll find brass and possibly bronze bushings and reducing couplings that will make the job almost as easy as doing it in pvc. I strongly recommend Catamount's suggestion of draining to the cockpit and then everything drains from there to the thru-hull. It simplifies things and avoids a host of potential troubles.

If someone wants to use PVC for their head or other non-critical plumbing, as suggested in Catamount's link, that's fine but my reasons for avoiding it still are valid, in my opinion. One thing also you might consider in regards to pvc is that, when a joint manages to work itself loose, and it does happen, it's almost impossible to reprime and reglue the existing joint successfully. I say almost impossible because I've never been able to do it in any circumstance but am willing to allow for miracles that may have benefited others. You'll have to cut out a section and re-pipe with new couplings and so forth.

[craigtoo]

Originally Posted by Pub911

I am interested in alternatives to white sanitation hoses suitable for this below-the-water-line application (e.g. "tiger flex") and alternatives to the nylon hose-barb type connectors which are not as versitile as PVC (step ups, downs, T's, elbows, etc) - I'm find that when I peice together the nylon barb-type to accomplish the need, they require short hose-runs between each, enlarging the assembly and increasing the complexity and number of failure points.

Here's a schematic of what I'm trying to build...

Awesome post. I vote this for the funniest of the night!

I can just see you drawing the thruhull and going... "Wait a minute... I know what that looks like...!!"

Now... back to the useful posts above and below...