How To Winterize A Boat

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How To Winterize A Boat

Ian Fortey Updated on April 23, 2021. In nauticalknowhow


Before freezing temperatures roll in, you’ll want to winterize your boat. This is true if you want to store your boat or keep using it. When the temperature gets too low, water in your boat is going to freeze. This can happen in the bilge, in the boat’s internal plumbing, and in the engine.

Every year insurance companies deal with freezing boat claims. This happens very often due to engine damage. As ice expands it can push with a remarkable force, enough to crack an engine block. When that happens, the engine is essentially destroyed.

A boat owner in the northern part of the country is usually prepared for winter. Winterizing boats is commonplace there. But those in southern states need to prepare their boats as well. Most freezing claims actually come from states that are traditionally considered warm. Texas leads the nation with freezing engine claims.
Winterizing your boat can prevent costly repairs. It will also ensure you get to enjoy your boat when and how you want to. Let’s take a look at the process of winterizing your boat, step by step. You need to prepare you engine components. Also the cooling system, drain plugs, bilge pump and more.

There are a number of steps that can be simple and won’t cost a lot. If you learn to change the oil, clean the freshwater system, and prevent moisture you’ll improve performance. This will prepare it for boat storage and a safe winter.

Winterizing a Boat: The Basics

The process of winterizing boats depends heavily on the kind of boat you have. Different boa engines require different methods of winterization.

Inboard Engine Winterizing

Start the process of winterizing your inboard engine by allowing it to run long enough to warm up. Change out the engine oil and transmission fluid with new fluids. Engine oil filters needs to be changed also. A freshwater cooled engine should have two different systems.

The engine has a raw water side. This circulates water to help cool the engine. This side uses water and antifreeze. You need to make sure you have proper antifreeze in your engine to keep it cool. Flush the system with clean water. Then follow the directions on the antifreeze to make sure it works properly. Typically this will require a 50/50 mix of antifreeze and water. Just make sure you run the engine long enough to allow antifreeze to fully circulate.

You can remove the spark plugs at this point as well. Look for any corrosion or scoring. There may be carbon buildup as well. If possible, you can clean the carbon build up off. Use a metal brush to clean out the cylinders as well. Spray them with some fogging oil. You can clean up the engine with a microfiber towel and then spray the whole thing with some fogging oil as well.

Outboard Motor Winterizing

The best method for winterizing an outboard motor involves the owner’s manual. There are too many different kinds and different designs to give a 100% accurate answer. In general, there are a few steps you can follow. This ensure clean fuel systems.

  • Start by cleaning your engine down. Soap and water will get it cleaned up well.
  • Flush the inside with fresh water.
  • Disconnect the fuel hose and let it run until it’s drained.
  • Fog the engine with fogging oil, including cylinders and the exterior as well.
  • Change the gear oil
  • Grease the propeller shaft
  • The owner’s manual is important. It will tell you what you need in order to continue winterizing your outboard engine.

Sterndrive Engine Winterizing

Like outboards, there are several kinds of sterndrive engines. You will need to check your owner’s manual to make sure you know precisely what to do. Depending on the type of inboard/outboard motor you have it may take in water from a thru-hull. Other models require different flushing techniques. This is why it’s important to keep the manual and follow it closely. You may need just a garden hose. Or something more in depth.

Make sure that you clean the engine thoroughly. Also make sure you allow the antifreeze to fully run through the engine. This will allow it to circulate completely. This will ensure that your engine is able to stand up to any weather that pops up over the winter.

Choosing the Right Antifreeze

Winterizing antifreeze is not the same as engine antifreeze. Antifreeze that runs through your engine is made with ethylene glycol. It’s potentially lethal and for that reason can’t be used in your boat’s freshwater systems.

Winterizing antifreeze has to be non-toxic. It is a propylene glycol antifreeze. It’s used to winterize your plumbing system. That means you may end up drinking trace elements of it. Likewise, in your bilge and other systems it can end up circulating back into the environment. For that reason it needs to be safe to use. Read the labels carefully to make sure you’ve chosen the right product.

Some formulations of antifreeze are only good down to certain temperatures. You need to be aware of what temperatures to expect where you are. Also, you don’t want to cut it close. If you have an antifreeze that is only good to -32 degrees watch out. If your average cold temperature is -30 you could be in trouble.

The other important thing to remember is what an antifreeze label means. If the antifreeze says it’s good to -50F you need to know what that means. At -50F the antifreeze will be frozen. However, it will start to freeze much sooner. At around -10F your pipes may actually begin to crack.

A winterizing antifreeze is not used the same way as engine antifreeze. You don’t want to dilute this 50/50 with water. It should be used at full strength. If it is diluted it can’t work the way it’s supposed to. It may end up causing damage as a result.

Anything that holds water that you may end up drinking cannot use this. You should be able to find winterizing antifreeze at boating supply stores. You can also order it online. Or buy it at a marina. Always make sure you check the specifications for your engine before using it.

Winterizing Boat Plumbing and Water Systems

Next to the engine, this is one of the most important parts of the boat to winterize. Your sanitation systems have fresh water running through them all the time. In the winter, any number of pipes and hoses can burst. This can damage the head, sinks, and more.

Draining your sanitation systems will not winterize them. There will still be enough water in the lines to cause serious freeze damage. You need to run antifreeze through the system to get it properly winterized.

Winterizing antifreeze can be run through the system to clear out any water present. You’ll want to check in on your boat periodically over the winter to ensure that it is in good condition. Sometimes water can be where you least expect it and a line may freeze despite your best efforts. Check the bilge to ensure it’s still dry. A quick inspection of lines, hoses and the engine are recommended.

Fuel Stabilizer and Other Winterizing Steps

There are some additional steps you need to take to finish winterizing your boat.

  • Wash down the outside hull of your boat. This can be done with a simple garden hose. Clean the prop, rudders, shaft and so on. This ensures no residue or build up can cause issues with corrosion. Mold and mildew growth during the winter storage may also be an issue.
  • Add a fuel stabilizer to your fuel system. You don’t want to leave your engine empty over the winter. It’s better to top it up, in fact. This can prevent moisture forming. Condensation can build up in an empty tank and then end up freezing. However, a stabilizer can keep it in good condition. Let your engine run for a few minutes to circulate the stabilizer.
  • Change the fuel filters.
  • Change the water separator as well.
  • Clean the bilge. Use soap and water for a thorough clean. Mop everything up afterwards. Dry the area completely when you’re done. Add some antifreeze to ensure any water you may have missed will not freeze. You should also spray the bilge with a water displacing lubricant. Something like WD-40 will do.
  • Make sure there’s no water in any additional systems. Clean out the bilge, the raw water washdowns, the livewell and wherever else water may be found.
  • Remove any drain plug from the boat.
  • Remove any valuables from your boat. This includes any electronics that may be on board.
  • Remove any safety equipment. Things you have had stowed like first aid kits. Also personal flotation devices and life jackets. Make sure you take the fire extinguishers as well.
  • Cover your boat. This can be done with a fitted tarp or boat cover. Professionals may also shrink wrap the boat for maximum protection. If you choose to cover your boat yourself, make sure you allow for some air circulation. If a boat is sealed without air circulation, moisture can build up. This can lead to corrosion on metal parts. As well, it can also end up leading to a lot of mold and mildew growth. Professional shrink wrap can avoid this.

Using a Boat Heater

Some boaters recommend using a heater to get through the winter. Small space heaters or heat lamps plugged it and directed at the engine block and other systems are common. This may get you through the winter once or twice, but it’s a very bad idea. Many boaters have discovered this after the winter. Their boat has then suffered the consequences. Heaters can short out or break. That can result in freeze damage that cracks pipes and engine blocks. In addition, heat lamps or heaters can overheat and cause fires.

Power outages are common during things like winter storms. A heater can end up losing power at the exact moment when you’d need it most. That can mean the boat will freeze right after the heater fails and your boat engine could be ruined.

Winterizing in a Marina

Storing your boat at a marina for the winter is never a bad idea. These can keep your boat safe and secure. But you need to make sure you have a contract for what is expected. Many marinas are just going to store your boat. You may be surprised to learn how many boaters expect winterizing as a benefit of winter storage.

Most marinas will not touch your boat in preparation for the winter. You have to specifically pay for someone to winterize your boat. Never expect it as a part of your winter storage. If this was not a service the marina offered and you accepted, it won’t happen.

Hiring Someone to Winterize Your Boat

The process of winterizing a boat can be stressful and time consuming. If you’re not used to it or don’t have the time, you may want to hire someone. If you choose this as a course of action, make sure you are being careful and thorough.

  • Write out a clear contract for what you are going to have done. You want this to spell out every step the person or company is going to take. Just saying “winterizing” is not specific enough. Itemize your list. Include things like the engine, the manifold, the bilge, the plumbing, and so on.
  • Make sure you know what needs to be winterized. Don’t forget things like the head. Anything you forget that is not in the contract could end up breaking if it’s not properly winterized. If the company was not contracted to winterize it, the responsibility falls back on you.
  • Use a credit card to pay once you have finalized a contract. If necessary you can work with your credit card company to dispute charges later. This may be necessary if the boat is not properly winterized. But you have a signed contract showing what was supposed to have been done.

Categories: nauticalknowhow


  • Alice Carroll on August 20, 2020

    Thanks for the remind that changing the fluids in my boat is crucial when preparing for the winter. I recently bought my boat about three months ago so I’m not that familiar yet with the necessary maintenance it needs. I hope that I can find a service that give a boat motor engine flush so that I can see for myself how the replacing of the fluids happen.

  • Robert on December 3, 2020

    Great article.

    Winterization will cost you less than having the burden of repairs after winter. How much does it cost to winterize a boat is not the right question to ask, but how much can you save by winterizing your boat. You don’t only save yourself from the costly repairs; you also keep your boat’s extended life for a longer time. Having a DIY winterization process can keep your financial worries at a minimum.

    Use a high-quality antifreeze for boats. Change the oil of your engine before winter kicks in. A warm engine helps flush out the old oil. Disconnect the batteries of your boat. Removing and storing them in a warmer location can maintain its condition for a good time. Get a durable boat cover. It may be expensive, but this is a significant investment you can bank on.

  • Robert Hogward on September 15, 2021

    Great article. Include the suggestions from the websites supplied and consider the manufacturer’s instructions for winterizing my boat via the owner’s handbook as well. Now, I am in excellent condition for the winter.

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