Antifreeze: ethylene glycol vs. propylene glycol

Posted by Darrell Nicholson at 05:27PM - Comments: (8)

The nylon strainer exposed to propylene glycol (right) was severely crazed.

In the upcoming July issue of Practical Sailor, contributor Drew Frye plunges into the not-so-funny topic of joker valves (if you don't know what this is yet, consider yourself lucky) and emerges with some valuable tips on keeping our marine heads healthy. One of his potentially controversial discoveries is that the "eco-friendly" anti-freeze propylene glycol isn't any kinder to the marine environment than the anti-freeze it was designed to replace - ethylene glycol - and it is definitely harder on plumbing components.

Granted, neither of these antifreeze products are beneficial for the environment, and we should do our best to minimize their use and participate in recycling programs. On land, relatively small doses of ethylene glycol—the product that seems to be the least harmful to our marine heads—is lethal to mammals. But in a marine environment, neither glycol packs the same punch. I won’t dive into the reasons for this, nor elaborate on the ecological arguments in favor of ethylene glycol here. There are some links below, so you can see for yourself why we think ethylene glycol has been unfairly tarred with regards to marine use.

Here, I’ll focus only on the effect that these chemicals have on the neoprene and nylon components in our engines and plumbing—including joker valves. For those who are still wondering what a joker valve is, it is a nitrile or neoprene valve in your marine toilet that prevents flushed material from flowing back into the bowl. It is also one of the most common failure points in marine plumbing. Most head manufacturers recommend replacing it every one or two years. (See the adjacent photo of a Raritan joker valve.)

The Raritan joker valve is made of oil resistant nitrile.

From a practical standpoint, the problem is clear: Propylene glycol is harmful to neoprene, a material commonly used in valves, seals, and impellers. Ethylene glycol is not as harmful. While neoprene does not fully degrade in propylene, it does change size and distort a bit, potentially causing a joker valve leak. It's not just joker valves that are at risk. We were told by several experienced marine mechanics that the best way to ruin the engine raw water-pump impeller is to winterize with propylene glycol without removing the impeller.

Probably the clearest example of what propylene glycol can do to marine plastics is what we found in our test of nylon strainers. In addition to joker valves, Frye tested nylon strainers (Shurflo) found in many potable water systems. One pair was exposed to glycols in the laboratory, and one pair was tested in use on a test boat.

The result in the lab was troubling; the strainer exposed to propylene glycol was severely crazed and ruined, and the strainer exposed to ethylene glycol was slightly crazed. The pair exposed on the boat showed even more damage (pictured at top). We think this was because temperature changes on board were more drastic than in the lab and because of the mechanical strain on the threads. The cracks caused by ethylene glycol were not visible for several days and were nearly overlooked, as glycol in the cracks rendered them invisible; only after soaking the strainers in water and allowing them to dry for several days did the cracks appear. Thus, when inspecting strainers, wash and dry them before checking for cracks. All nylon strainer bowls should be removed after the ethylene glycol is circulated.

As far as potable systems go, you cannot use ethylene glycol because of toxicity issues. Rather than put “safe” propylene glycol in the tank, perhaps a better option is to drain the tank, and empty low spots in the plumbing—although that is not always possible.

To find out which joker valve performed best in our tests, and to learn which common head cleaning and lubricating products actually do more harm than good, check out the article in the July 2013 issue of Practical Sailor.

And for more important tips on the maintenance of your engine, and cruising in general, check out Beth Leonard's The Voyager's Handbook, 2nd edition.




Conference abstract discussing ethylene glycol vs. propylene glycol toxicity

EPA review of ethylene glycol and propylene glycol aircraft deicer.

Material Safety Data Sheet for ethylene glycol

Material Safety Data Sheet for propylene glycol

Comparison of life cycle impact of propylene glycol vs. ethylene glycol


Comments (7)

I have a fiberglass holding tank (it's built into the keel). Will propylene glycol damage fiberglass?

Posted by: skylark | November 3, 2017 7:39 AM    Report this comment

I too used to use "cheap" vodka.....looks like I should go back to using it.
When I first started to use it I went to the New Hampshire liquor store. Not knowing exactly where they kept the vodka, I asked one of the help "Where do you keep the cheap vodka?" Her response before directing me to the location was "Oh, you have a boat, huh?" True story. Still laughing about it.

Posted by: William B | July 29, 2013 1:45 PM    Report this comment


Vodka is not 100% alcohol and diluting it could be a crapshoot, so I just go with straight vodka. Figure on the same amount if you would use the pink stuff. Example: about 3 quarts into the head to make sure it gets to the holding tank, a gallon or so poured into the raw water strainer while the engine is running (close that through hull first!), etc.

Run as much water out of the fresh water system first then add the vodka. You should put in a bypass for the hot water tank if not there already or you will need another jug to make it to the hot water side. As for the hot water tank: I just drain it and leave the drain valve and pressure relief valve open so even if there is a little water in it that could freeze there is plenty of room for expansion.

Got leftovers? Party time!

Posted by: JeffM | June 13, 2013 5:27 AM    Report this comment

Joseph M - what solution of vodka do you use? It's green and delivers a buzz, so what could possibly go wrong?

Posted by: jarad a | June 12, 2013 10:49 PM    Report this comment

I've used cheap vodka in the water, head, drains, and raw water cooling systems with no apparent ill effects on the plumbing or parts. I'm not aware of any damage to the joker valve (now >5 years old, no leakage). I put in a fresh impeller every spring so can't comment on any deterioration of that item.

Advantages: Any remaining alcohol either evaporates or is so diluted when you fill the tanks and run the taps in the spring after the fillip. No nasty taste or pink residual color in the spring, the alcohol won't freeze unless you live in the arctic, and the fresh water systems are pre-sanitized. Just add a little chlorine bleach when you fill the tanks and you are ready to go.

Posted by: JeffM | June 12, 2013 10:58 AM    Report this comment

I asume this article refers to winterizing for climate that freeze and not applicable to southern California ?

Posted by: Ron J | June 12, 2013 10:25 AM    Report this comment

What about glycerol based anti-freeze? Any comments on it?

Thank you.

Posted by: PA | June 12, 2013 9:21 AM    Report this comment

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