When plumbing a boat’s pressure water system, flexible tubing is inevitably part of the equation. It is simple to install, and the hose, connecting hardware (hose clamps) and fittings are readily available. Before beginning any plumbing project, the do-it-yourself should be careful to use the right hose for the job. The hoses and coupling methods should follow guidelines set by the EPA, ABYC, and other regulatory agencies, including the U.S. Coast Guard.
But which tubing best withstands the bending needed to lead the water lines through the torturous routes they often must take? The risk is a kink, which severely restricts the flow of water, burdens the water pump and hastens the day when the plastic becomes brittle and cracks.
Kink resistance can be impacted by three factors: (1) the size of the tubing (including wall thickness) (2) the temperature of the water inside (2) the tubing brand. We have tested all three. In a comparison of five varieties of tubing in various sizes, we bent 1-foot sections over a measured grid until each hose first flattened discernibly and then kinked, usually suddenly. We carried out the test at room temperature, and then after the hoses had been immersed in hot water. Our finding should help you decide how to best route your new hoses as you make any plumbing changes.
Size of Tubing
In general, small diameter PVC, either clear or reinforced, will withstand sharper bends than larger sizes. In our testing:
– ½-inch clear PVC can be bent at 65°F to about a 2-inch radius before it kinks.
– 5/8-inch clear PVC can be bent over a 3-inch radius.
– ¾-inch clear PVC can be bent over a 4-inch radius.
– Equivalent sized reinforced PVC can be bent in a radius that is about 25 percent tighter.
– Heavy-duty reinforced PVC can be bent to fit even tighter turns.
– PEX tubing can be bent to a 3-inch radius, but it kinks more suddenly, sharply and severely than the softer PVC.
The Cost of Kinks
To avoid kinks, small diameter tubing (like ½-inch) can be used, but this introduces friction into the system that can affect pump efficiency. The other option is to add a pre-formed loop, “swoop,” or elbow.
While pump efficiency is not as critical in a potable water system as it is in our bilge pump system, we still want to maximize the system’s efficiency by minimizing the loss of hydraulic head pressure (also called head pressure).
In some applications, high pressure is essential (as we saw in our washdown pump test). However, even in lower-pressure potable water systems, better efficiency will reduce the pump’s running times. Less run time can translate to longer pump life. Although all pumps eventually wear out, as we discussed in PS Advisor, a diaphragm pump is generally more susceptible to the rigors of time.
Elbow fittings are often the cheapest solution to eliminating bends, but consider how these will affect head pressure. Several resources can help you get an accurate picture of head pressure prior to installation. For example, as I discussed in my blog post on bilge pump installation, a 90-degree bend in your bilge pump is like adding approximately 3-feet of vertical hose to your line. In a paper presented to the American Council for an Energy Efficient Economy, Gary Klein has an informative Powerpoint presentation on plumbing efficiency that tabulates the impact of elbows, bends, and swoops in plumbing lines. Several online plumbing suppliers offer similar tables online.
You can use any these tables and formulas to calculate flow rates at the outlet, but the bottom line is to avoid sharp bends and elbows whenever possible.
Hot vs. Cold
We still see boat hot-water systems with flexible tubing that is not rated for this purpose. To check what heat does to flexible tubing, we made identical 1-inch loops of more than a dozen varieties and sizes of tubing. We bent them until a flattened section just began to develop at the top of the loop at room temperature.
One of each pair then was dropped in hot (160°F) tap water. In less than one minute, the clear PVC samples, the reinforced PVC samples and PEX tubing all kinked at the slightly-flattened location. Only the heavy duty reinforced PVC emerged unchanged from the hot water.
Use only plumbing materials rated for that use and avoid kinks. Even an materials designed to carry hot water can kink if it has a sharp bend. When installing hot water lines (or when leading any line through an area that will subject it to heat), a bend that may look acceptable when installed can kink when heated.
For withstanding sharp bends, by far the best of the flexible PVC tubing, in any size, is the heavy-duty reinforced variety.
Our reinforced samples included PVC tubing made by Shields and Trident. At the time of our test, we found no measurable difference that between the equivalent types tubing offered by the two companies, including clear PVC, reinforced PVC or heavy duty reinforced PVC. It has been a while since our last test, and we look forward to seeing if there have been any significant improvements in this area.
Our kink report just scratches the surface of our past reporting on boat hoses. If fighting odor is your main goal (as it is for many), check out our report on odor-proof hoses and vent filters. If it’s a new pump you need, we’ve tested the full spectrum of pumps for every job.
For those who are contemplating installing a new system, adding to their existing system, or ensuring high quality drinking water onboard wherever you roam, our recently released eBook four-volume series, “Onboard Water Treatment, Storage, and Production,” covers all aspects of such a project.