The Captain's Blog

When is it Safe to Dewinterize Your Boat?

April 5, 2023 by Michael Kiel

When the temperatures rise and the sun starts shining, it is tempting to dewinterize your boat right away. But is it time yet? The key is knowing when your area will see its last spring freeze. Dewinterize early and you risk exposing your boat engine to serious damage. Dewinterize late and you waste precious days that could be spent on the water.

dewinterizing boat
Here is the lowdown on how to know when the time is right, what to do if you dewinterize too early and more advice on getting your boat back in the water.

When is My Last Frost/Freeze?

You can find the latest information about frost/freeze in your area by visiting the National Center for Environmental Information. They provide a map of when to expect your last spring freeze. The map is derived from a 30-year average of climatological variables like temperatures. It shows the time of year, on average, that areas across the United States can expect to see their temperature dip to 32 degrees or below for the last time. While this map shows the average time frame of the last spring freeze across the United States, this year's actual conditions may vary widely based on weather patterns.

map of areas last spring freeze

What Exactly Do the Numbers Mean?

As soon as the weather gets below 32 degrees, water begins to freeze, with water with impurities or saltwater in it starting to freeze at 28 degrees. In most cases, the water will not freeze solid (leading to cracked engine blocks) until the temperatures stay below 32 for several days and nights.

Note: This is an average and does not guarantee against frost/freeze. Do not play fast and loose with something as expensive as a boat engine. You also have to keep an eye out for sharp temperature drops into the teens or below zero, which can do damage much quicker, sometimes as fast as overnight.

Frozen Water is No Joke

One of water’s most remarkable properties is that it expands when it freezes to a volume that is 10 percent greater than in the liquid state. In other words, 10 cups of water put into the freezer is going to turn into 11 cups of ice when it freezes. Water expands when it freezes because its molecules are closer together in the liquid state than they are in ice.
dewinterizing boat
The expansion of water when it freezes takes place with tremendous force, as anyone who has left a full can of soda in the freezer knows. The force is enough to burst the strongest pipes and engine blocks if the water in them freezes. It is not possible to make a usable pipe or engine block strong enough to withstand this force.

“Looks Good to Me!”

Let’s say that you are at the point where a hard freeze (multiple days below freezing) is probably extremely low to non-existent. You may want to opt to dewinterize and then keep an eye on the weather in the coming weeks. If a freak storm is on the way, reverse the process and re-winterize. Or, you can dewinterize and run an engine-compartment heater if it gets cold again. This option extends the boating season, which is definitely what we all want.

Homespun Logic

Statistics and forecasts are good but never take into account all the particulars of a boat’s unique location. Some boat owners use homespun frost/freeze logic and put a small cup of water in the cockpit or on the boat’s trailer during questionable temperatures. The idea is that, if the water in the glass has not frozen, then the boat’s engine block is good. On the other hand, if the water in the glass starts to freeze, they drain the block with the idea that there is a time margin, as water in a glass will freeze way before anything in the motor.

Interim Dewinterizing

While you may not want to drain the systems and crank up the engine until you are sure a late freeze is out of the question, the warm/cool days of early spring are the perfect time to get a jump on the rest of the spring commissioning processes.

So, instead of wasting your days on shore:

  • Air out the cabin.
  • Put boat linens and other things you stored off-site back on board.
  • Check the boat exterior and make any necessary repairs.
  • Check the safety equipment aboard.
  • Inspect the boat trailer lights, frame, springs, and suspension.

You get the idea. It may seem like winter will never end, but this is a great time to print out our spring commissioning checklist and make sure the rest of the process is done before you wake up the engine and water systems aboard.

Summer is on the horizon, and you want to get your boat back where it belongs — on the water. The above guide will help you get there as soon as possible.

What are your guidelines for knowing when the time is right to dewinterize your boat? How do you deal with surprise freezes once your boat is ready to hit the water? The tips you provide could save someone a lot of time and money on repairs.

Michael Kiel