Winterizing Your Boat’s Water System

The right anti-freeze ratio in winter helps prevent water tank contamination in the spring.


Amazingly enough there are still forum posts and online “experts” advocating vodka as anti-freeze solution. Practical Sailor has thoroughly debunked this myth in extensive testing of anti-freeze solutions. Like the advice to lubricate your head pump with vegetable oil, this practice could cause more harm than good. (As we pointed out in our report in on head maintenance and reducing toilet odor, vegetable oil can actually accelerate the degradation of the waterproof grease used to lubricate your head.)

There is little doubt in our minds that microbial infection is what causes most fouling of water systems. In Practical Sailor‘s report on winterizing with alcohols versus glycols, it became clear from our testing that under-treatment was a leading cause of water tank contamination. The combination of weak solution and mild temperatures in fall and spring proved especially hospitable to growth.

Much of your winterizing success will depend on the initial condition of your existing water system. If it is nearly sterile (effectively chlorinated water), there may be too few bacteria and fungus present for infection to start. However, if you use less than 20 percent alcohol or glycol (after dilution with water left in the system) you are at risk of biological growth; the lower the concentration the greater the risk. This is the reason why vodka and other alcohol-based winterizing fluids should be avoided. In our testing these solutions acted like an invitation to a microbial feast.

The alkalinity (pH) seems to correlate with infection resistance, although this depends on the microorganism types. Although high pH has long been used as an antimicrobial defense in industrial systems, we are not suggesting you add anything to change the pH. This might affect the formulas ability to fight corrosion.

However, pH is certainly a factor that can be monitored using simple dip sleeves used for pool or aquarium maintenance. If you find the pH goes below 7, there’s likely a problem. At that level, glycol-treated potable water systems might be infected, and glycol-treated cooling systems may be experiencing exhaust leaks or certain electrolysis problems. For testing, we like the Tetra EasyStrips (about 69 cents per test), which simultaneously test for nitrate, nitrite, hardness, chlorine, alkalinity, and pH. Other factors are described in great detail in the report, “Keeping Water Clean and Fresh.”

Time and temperature make a difference. Our warm incubator temperatures encouraged growth. Boats winterized only during the coldest months of winter will have fewer problems with growth.

Based on our testing, there is evidence that winterization products could be improved with additives. We are not suggesting regular use of DIY formulations for potable water systems, but perhaps the vendors can be encouraged to investigate biocides that are non-toxic to humans. We/ll see.

For our full report and test results of winterizing solutions, see “Will Your Tanks Be Clean Next Spring?” For the cruising sailor who wants to ensure clean safe water where ever they go, our four volume e-book series Onboard Water Treatment, Storage, and Production: Complete Series covers everything you’ll need to know about collecting, making, storing, and filtering water for onboard drinking.

Drew Frye, Practical Sailor’s technical editor, has used his background in chemistry and engineering to help guide Practical Sailor toward some of the most important topics covered during the past 10 years. His in-depth reporting on everything from anchors to safety tethers to fuel additives have netted multiple awards from Boating Writers International. With more than three decades of experience as a refinery engineer and a sailor, he has a knack for discovering money-saving “home-brew” products or “hacks” that make boating affordable for almost anyone. He has conducted dozens of tests for Practical Sailor and published over 200 articles on sailing equipment. His rigorous testing has prompted the improvement and introduction of several marine products that might not exist without his input. His book “Rigging Modern Anchors” has won wide praise for introducing the use of modern materials and novel techniques to solve an array of anchoring challenges. 


  1. Very interesting and informative, but useless in 2021 for most sailors who haul their boats for the winter. Publishing this on Dec 2nd after 98% of boats in the colder climates have been hauled and winterized already shows very poor planning or concern on your part. This article should have been published in late September or early October. I’ll save it for next year.

    Ron Canizares


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