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Selecting Antifreeze

Antifreeze 1

The chemical in antifreeze that resists freezing is either ethylene glycol (EG) or propylene glycol (PG). Ethylene glycol makes a good antifreeze/coolant, but it’s a poison that’s deadly to pets and wildlife and makes disposal problematic. Propylene glycol is odorless, tasteless, biodegradable, and nontoxic. While it is slightly more expensive to produce, it offers a combination of safety and effectiveness we feel makes it a superior choice for marine use.

Antifreeze applications

The main applications for antifreeze include winterization of potable water systems, sanitation systems, non-running engines and air conditioners. In addition to providing protection against freeze damage, some antifreeze can also be used as a coolant for running engines.

  • Potable (drinking) water systems require a completely non-toxic antifreeze that has no harsh chemicals. Our Pure Oceans Engine and Water System Antifreezes are ideal.
  • Boats laid up for the winter need freeze and corrosion protection for their engine’s cooling system, but they don’t need the stabilizing chemicals that protect against the breakdown of anti-corrosion additives at high operating temperatures. For winter lay-up, our Pure Oceans Engine and Water System Antifreezes are formulated to provide safe, effective protection for non-running (stored) engines.
  • Due to its slightly lower cost, some boaters prefer to use our Seafit brand -50°F Marine and RV Water System Antifreeze, which is a propylene glycol/alcohol blend. This is fine for potable water and sanitation systems, but is not recommended for winter engine storage, since some of the alcohol can evaporate, resulting in reduced protection towards the end of winter layup.
  • Pure Oceans Engine Antifreeze and Coolant contains an additive package that prevents corrosion and antifreeze breakdown at high operating temperatures. This enables it to be used for freeze protection during the winter, and for use as a coolant in the summer. Due to the additives it contains, it should not be used for winterization of potable water systems. You can find it in the Engine section of this site.

Freeze vs Burst Protection

The temperature ratings (-50°F, -60°F, -100°F, etc.) that we associate with our antifreeze products denote the “burst point”, which is the temperature at which a sealed copper pipe filled with the undiluted product will burst. Burst points are a standard created by the plumbing industry in the 1930s to indicate the relative strength of antifreeze.

The freeze point is the temperature at which ice crystals (slush) will begin to form in the undiluted product. Slushing is not injurious to engine or potable water systems.

For an expanded discussion of freeze points vs burst points, and more about West Marine Antifreeze, click on the link at the bottom of this article.


No matter what type of antifreeze you choose, be sure to follow the manufacturer’s dilution ratio instructions carefully. When buying antifreeze, you are mostly paying for ethylene or propylene glycol, the proportion of which can vary from 25 to 95 percent. That’s why lower temperature rated products are more expensive. Products that advertise protection to -50°F may require that you use the product full strength, while other products can be diluted with water at a 3:1 ratio and provide the same -50°F protection. Shop carefully and remember that cheapest isn’t always the best value!

Selection Chart for Seafit, Pure Oceans and Star brite Antifreeze

  PG Percent Burst Rating Freeze Rating
Seafit -50° Antifreeze 28% -50°F +14° to +18°F
Pure Oceans -50 30% -50°F +12° to +16°F
Pure Oceans -60 35% -60°F +7° to +10°F
Pure Oceans -100 60% -100°F -58° to -63°F
Star brite -200 96% -200°F -98° to -103°F