A normal 50:50 mix of propylene glycol provides a protection level that will allow an engine to operate at -26°F (-32°C). That means that no ice crystals will form in the coolant till the temperature drops below -26°F (-32°C). This is referred to as the freezing point of the anti-freeze.
At any ratio of 35:65 (35%) or higher, propylene glycol provides additional protection to lower than -50°F (-46°C). This protection level is referred to as the burst point. The burst point is the point where the crystals in the coolant are able to expand and cause damage to the cooling system. In other words, it is OK for the coolant in the cooling system of an engine to turn to slush as long as the engine is just being stored and will not be expected to operate till the weather gets warmer.
Since it is pretty unrealistic to think that a customer will actually be using their boat at -26°F, the bursting point becomes the more important value to pay attention to. Following is a chart showing the anti-freeze ratio and the corresponding freezing point and burst point.
A 40/60 (40%) ratio provides adequate burst protection for most of us. The 50/50 (50%) mix historically has been the recommended concentration because of the convenience being able to determine how much anti-freeze and water you need to use for a given system.
Remember to use a refractometer to get an accurate measurement of the anti-freeze concentration.
Adequate protection from freeze damage is the responsibility of the boat owner. This service publication is for informational purposes only. Indmar does not assume any responsibility for damages caused by freezing of the engine's cooling system.