How to Drain Anti-freeze from a Boat Engine

Last Updated on October 1, 2022

Although it doesn’t happen often and it does not need special technical know-how, you should know how to drain antifreeze from a boat engine if you want your boat engine to continuously perform at its optimal ability and save yourself some cost on servicing your boat.

Antifreeze is a combination of propylene glycol and water and non-toxic materials. Its only purpose in a boat engine is to prevent the engine and its components from freezing during winter. That means, it is mostly used during the winterization of a boat and is removed from the boat engine when it is to be de-winterized. To drain antifreeze from a boat engine, you can use one of two ways, although one is better than the other; let the antifreeze leave the engine when you start to run it and go on to the waters, or run the engine and collect the antifreeze in a bucket.

We will explain both ways and give you a detailed explanation of how antifreeze works in the cooling system and engine of a boat.

What Does Antifreeze Do in a Boat’s Engine?

Your boat engine uses two fluids, antifreeze and coolant, to keep the engine from freezing (in cold weather) or overheating (in warm weather), respectively. While these two can be confused, they are not the same.

The cooling system of a boat either employs the raw water cooling system or the fresh water and heat exchanger cooling system. Older boats employ the raw water cooling system; however, due to its adverse effect on boat engines – reduced engine life – most new boat engines don’t use it. You will majorly find boat engines with heat exchangers now.

The boat engine pulls water from the water body and runs it through the engine block. Since boat engines have no radiators that force heat out of the engine by blowing air, they use heat exchangers that transfer heat from the engine to the cool water introduced to the block and passing them out. The freshwater mixes with coolant to further drop the temperature, so it can absorb more heat from the engine, before returning to the water body.

Antifreeze, as opposed to a coolant, does not interfere with keeping the boat’s engine cool. It majorly ensures that the temperature in the engine does not drop so far that it causes the components to freeze, causing expansion and damage to them. This usually happens when the engine is not in use and the prevalent weather condition is cold.

Antifreeze, already containing water, is passed through the engine via the sealed tubes and into every part of the engine, to prevent them from drying out, keeping them in contact with water, and the propylene glycol ensures that the water remains in its natural state.

How to Drain Antifreeze from a Boat Engine

To drain antifreeze from a boat engine, you need to decide if you want to use the boat immediately or later. This is because there are two ways to drain antifreeze from a boat’s engine, and this is due to the relative safety of antifreeze, unlike coolants that are toxic.

The two ways you can drain antifreeze from a boat engine are:

Recapture Before launch

If you’ll be using your boat later, say in a couple of minutes, you can drain the antifreeze you have left in the engine during winterization. In essence, as part of the de-winterization process, you can remove all the antifreeze you have introduced into the engine block before you get your boat on the water.

You’ll need a bucket of water, an empty five-gallon bucket, an intake hose, and fuel for the engine.

  • Insert the intake hose into the inlet of the boat engine, making sure the other end is immersed in the bucket full of water.
  • Have someone place the five-gallon bucket at the exhaust end of the boat engine.
  • Run the engine and watch the water from the bucket go into the engine block and come out of the engine into the five-gallon bucket.

Usually, the liquid that comes out of the engine is pink.

Caution: Continuously fill the inlet bucket with water as your engine may get damaged if it runs dry.

Recapture In transit

This approach is for when you do not have time to take the long process of de-winterizing your boat. While it is not advisable that you use this approach, it may come in handy sometimes. All you have to do is to get your boat on the water and run your engine. You may recapture the antifreeze in your boat engine by placing a bucket at the exhaust end of the engine.

This may be difficult if it is difficult for you to reach the exhaust from the deck or if the exhaust is close to the water.

Note: You may be unable to catch all the pink fluid from the exhaust with this method and some will leak into the local water.

Antifreeze Disposal.

Antifreeze is a relatively safe material that is sometimes part of a drinking water cooling system, in small quantities. However, their safety in large quantities cannot be ascertained, making them potentially dangerous for lives in the water. This means it will be irresponsible to dump the bucket of antifreeze you have recaptured in the water.

You can contact your local waste disposal authority on the right way to dispose of antifreeze in order not to have a run-in with the law. If you have a marina around you, they may have a disposal area that allows waste oil and coolant disposal. Ask for information in the marina office.


Antifreeze is a liquid composed of water and propylene glycol. It prevents water from freezing in your boat engine, protecting the engine components from expanding and damaging the engine. It is relatively non-toxic and can easily be drained from your boat engine during de-winterization.

The two ways you can drain antifreeze from a boat engine are recapturing antifreeze during transit, and recapturing antifreeze before launch. However, ensure that you dispose of the collected antifreeze properly.