how to keep a boat motor from freezing

How To Keep a Boat Motor From Freezing and Cracking

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After the pleasures of an adventurous cruising season, suddenly, winter comes. The routine of winterizing a boat mated by a headache about how to keep a boat motor from freezing – follow.

Be ready to spend some time and money, because you want your baby fresh when unveiling her in spring after a few months sleeping.

For this we never underestimate winterizing a boat as the most necessary precaution to pass winter safely for the boat and its equipment, especially the motor. In particular – the motor.

This article is to help summarize how to keep your boat motor from freezing to “death”, $$$! We do not want a freezing temperature to win the battle. Don’t panic and nothing will crack in your boat without a reason.

Let’s just find out the reasons along with the means of a necessary winter care for a marine engine.



How Cold Does It Have To Get To Crack A Boat Engine Block?

Start at the beginning – at what temperature will water freeze in an engine? The potential danger of freezing and of the consequent crack of the boat engine block appears at the water freezing point equal to +32F and below, as for freshwater.

For saltwater the freezing point is at about +27F providing that your engine is cooled by the outboard water.

If you cruise in saltwater, cooling the engine with outboard water, the latter is naturally in the boat engine system. For other cases it is freshwater in the system.

General provisions


The boat motor freezing temperature should last a while before it causes any severe damage, because it takes a while to freeze the interiors of the boat and the insides of the motor. Nothing too bad should happen from a short exposure to a freezing temperature.

Making the engine environment warmer is a good idea if a cold period strikes suddenly and/or is short enough through the year. However, taking extra precautions will set your mind free.


Depending on the region, you should choose in advance how many and which of the winterizing precautions you want to apply, and NOT necessarily all of them. Understanding how freezing works, be sure to choose some of them.

Keep in mind when a frost strikes the land suddenly it shall be too late (and cold for you) to run in panic carrying marine antifreeze and an electric blanket to the boat parking. Panic is your enemy but you prevent it wisely.

Thinking ahead is the safest way at sea, thinking ahead is what we do when winterizing a boat motor to keep it far from any possible freezing damage.


There is a serious difference between winterizing an inboard boat motor compared to winterizing an outboard boat motor. Below on this page we suggest directions and considerations for both cases.


ANTIFREEZE is not a panacea but a strong solution. It is a thoughtful idea to have it if winter provides some periods of cold below the motor freezing point. There is snow – you need antifreeze.

Apparently warming the insides of the boat or of a motor compartment may create a warm cap as soon as your heating solution can align with the outside temp of the region.


Exposure of the Motor to Low Temperatures

The very constitution of the problem about how cold does it have to get to crack a boat engine block as a result of exposure to freezing temperature depends on water inside the engine whether inboard or outboard.

Water does not freeze quickly, but you are a boater and you think wider than the actual problem. Not only the engine is in danger but its pipes and pumps and all the way till seacocks and seals.

Below +32F is the point where the nightmare starts for freshwater, and about +27F for saltwater.

The constitution of the problem is in the laws of nature again – when water freezes it adds to its volume and doing so it is unstoppable.

Reads: when water has no space to expand, it can crack metal, regardless of the metal’s strength; water is stronger. Aye-aye, we can feel its rage when it throws a boat as a toy at sea.

Winterizing a boat motor is draining all water out of the system, lines and pumps included. No water – or antifreeze! This or the cold may get to crack a boat engine block making it unrecoverable or even sink a boat. Ye been warned.

boat engine freeze

How To Keep An Outboard Motor From Freezing

If you are still reading, then winter temperatures in your area get below 32F. For the inboard motor winterizing recommendations scroll the page down and now we speak about how to keep an outboard motor from freezing.

The outboard motor must be properly drained under the exposure to the temperatures below 32F (freshwater freezing point).

Here are the best practices and directions for safety and durability to keep an outboard motor from freezing during winter months.

  1. Make sure you have a fuel tank almost full, just leaving enough space for adding a fuel stabilizer into it. Because the full fuel tank shall leave less space for air in it, while air is the source of water, water in its turn sneaks along with fuel into an engine and freezes there.
  2. Get the outboard motor out of water. Either you haul-out the watercraft or just the engine, but we herein speak about saving the motor from freezing. Place your boat (or your engine) on the level ground to emulate its working position in relation to gravity.
  3. Stabilize the fuel. Mix the fuel stabilizer into the fuel tank (following the directions about the amount) then run the engine for about 10-15 minutes at low reps.
    TIP We recommend STA-BIL Fuel Stabilizer, which is one of the most used fuel additives in America since the 1980s, and it works. It resolves the quasi-impossible task of removing fuel completely from the system (including the engine and pipes) but it stabilizes fuel instead changing its chemical nature. It is essential for a long storage.
    TIP Do not forget to provide a water supply into the cooling system through the intake seacock once running the engine out of water on the dry stand, if you have an outboard cooling system. Keep an eye if exhausted water is thrown away after it cycles. Stop immediately if you see no water coming out, check both the source and the intake then continue.
  4. Drain out all the water. Trim the motor to the downward position, similar to what you do when cruising. You can also emulate the positioning for the case the motor is detached from your watercraft, if she is a small watercraft or something.
  5. In the downward position – let the motor rest for a sufficient while then get it up again. Repeat a few times. Don’t be greedy about the time spent to get every drop of water out of your engine. Patience is your friend, give it time.
  6. You can also add a dry run, some boaters advise doing so. For this, cut the water intake and run the engine for a short time to throw the remains of water away. You can repeat it a couple of times as soon as it cools again. Let us leave this step upon your sole discretion. Many marine engines are suggested to carry well a quick dry run.
    TIP Keep low reps just above neutral at all steps when running the motor.
  7. To add antifreeze into the system mix the proper amount of marine antifreeze into the intake-water. You can also add the antifreeze depending on how low shall be the winter temp as from your expectation. Small amounts of water in antifreeze shall only set the freezing point a little higher than the antifreeze suggests as a pure. It’s not a big big deal until you are in the Arctic. With an antifreeze bucket connected to the intake, run the engine and wait until antifreeze starts splashing out from the cooling exhaust, allow few splashes, then stop.
    TIP Keep in mind how low are the winter temperatures in your location. The lower they are the more paranoid you are to be about winterizing your outboard engine. However, if you are not going to put antifreeze into the system (find details about marine antifreeze below on this page) then you need to drain water until it is desert dry. When there is no water in the engine nor in the lines, no damage may occur.
  8. Remove the cover, and spray the fogging oil upon the carburetor and spark plugs. The engine better be warm enough when you are putting fogging oil. Cover every sensitive part with the fogging oil honestly. It prevents water penetration into the insides and rust.
  9. Disconnect everything. Disconnect the battery, use fogging oil upon open plugs and connections where reasonable. Disconnect all lines and sparks. Close all holes with cloth, add fogging oil generously upon this cloth. You don’t want any accidental water, including from condensation.
  10. Changing oil in the engine before winter is a good move yet less necessary. Once it’s already time to change then you choose to change before winter rather than after. For sailing boats where the engine is mostly an auxiliary equipment this step is totally a matter of free will.

Do You Need To Put Antifreeze Into an Outboard Motor?

Not really.

The important thing is that the real necessity to put antifreeze into an outboard motor system depends only on how dry and how well sealed that engine is. For sake of comparison, for the inboard motor the dryness of the whole system is hardly reachable.

Also mind how long the peaks of below 32F really last in winter. Once the engine is honestly dry and properly sealed, nothing will happen even if an ice-age is coming.

The short summary on using or not using a marine antifreeze upon a boat motor is:

  • A totally dry, desert dry, honestly dry engine, well drained, well fogged, well sealed, shall not suffer.
  • A day or two of a super-frost is not an issue at all once all decisions are wise and done well.
  • Wrap your outboard motor tightly regardless of the severity of winter, but periodically check what is under the wrap. Even if it is not a specially-designed cover. Wrap it to minimize the condensation as much as possible.
  • Use the external heater (like an electric blanket or a bilge heater) in the place of storage if such is reasonably possible. The inside of the boat is already a place which keeps warmth effectively. In general: the warmer, the better.
  • Once you are on the antifreeze mission, use marine antifreeze only. Marine antifreeze is the propylene glycol product while those for cars are based on ethyl. The latter is very corrosive and especially for the rubber seals and other rubber stuff, car-antifreeze can cause severe damage to the whole system. Use marine antifreeze only!!
  • Apparently propylene glycol antifreeze (“marine antifreeze”) shall coat all the insides of your outboard engine where you cannot reach with the fogging spray. Use marine antifreeze only!
  • Marine antifreeze works in super low temperatures, like -50F. Therefore you can mix it with water to save on price, also to enjoy an easier draining.

How To Keep an Inboard Motor From Freezing

Taking a pro-winter care of an inboard engine is slightly a different trick as it is a strongly different system. For example, using antifreeze is more likely a must while for the outboard one you can really skip it.

Here are some principal differences between winterizing an inboard and outboard motor:

  • An inboard engine sits inside of a boat where you can regulate temperature by other means creating a warmer environment. Think of it as if your engine, compared to an outboard one, is already in “garage” and you can really warm it up at a moderate cost.
  • You shall need antifreeze once there is any chance of temp drop below 32F (freshwater freezing point). This is because the lines and pumps of the inboard engine are less accessible, also less dis-connectable, and water stays there, and especially when you keep your boat in your pond for the whole winter and no haul-out.
  • Even if you warm the bilge and the motor compartment you’d better still have antifreeze in the system in case the power supply fails. Or double it with a battery. However a short cutoff is not the end of the world yet, as it takes time to cool down the whole system to the motor freezing point.
  • You really can warm your boat equipped with an inboard engine and consequently add comfort for the latter and create conditions and a longer life for the whole system.
  • Making it warm inside your boat for your inboard engine, say, using the shore power supply, you can host a boat party even at the coldest night adding heating from the invited guests but promoting it as a romantic time-spending. Yay! You must not suffer from the winterizing precautions, for yachts are pleasure crafts.
sail boat motor winter
A sailboat during the winter time.

How To Winterize an Outboard Engine Of a Boat

Be sure the fuel tank is almost full, just having space to add the fuel stabilizer. The same idea works for winterizing any boat motor. The purpose to have your tank full is removing air because air in a tank leads to condensation.

Air in a tank’s top leads to condensation → condensation is water getting into fuel → water from fuel gets into the motor and its lines → provides rust and may freeze at low temperatures → BOOM!

Run the inboard motor on idle (just above neutral reps) for a little while to get it warm. As soon as the engine is warm add fuel stabilizer into the fuel tank. Continue running for about 10-15 minutes.

PRO TIPS

  • Keep low reps just above neutral at all steps when running the motor.
  • For the outboard cooling systems and if running such one on the hard stand, out of the water, be sure to connect a sufficient supply of water through the intake seacock. Then keep an eye on where water exists after the cycle around the engine. Once no water is thrown out, stop, fix the wrong and continue. Do not underestimate the importance, the dry run may ruin the engine quite quickly. Do not overestimate the speed at which such may happen as the proper engines suggest a tiny-term for a dry run.

Drain the engine. This one is not simple, we are unsure if it is even possible. However the engine manual must contain the direction how to drain. But more importantly mixing antifreeze with small amounts of water is fine; the use of marine antifreeze for winterizing an inboard engine is a must.

Running the engine (with the intake seacock closed) shall blow extra water away, but be careful with this attempt, many engines suggest dry running if quickly and no overheating.

Add antifreeze into your inboard engine system using the intake seacock. Be sure, while doing this, that the antifreeze bucket is not emptied out. You continue until the water exhaust starts throwing antifreeze out. That shall mean the system is filled with antifreeze and you stop.

PRO TIPS

  • Use marine antifreeze exclusively, because as we’ve already stated above, the “marine antifreeze” (based on propylene glycol) apart from cold protection is also designed to coat the parts of the engine system where you cannot access by other means, like pipes and pumps.
  • An ethyl-base antifreeze from a car shall damage the rubber and seals and more.
  • Even if you’ve managed to drain out an inboard engine properly and fairly there is no water in it, but water most probably stays in pipes and pumps, etc. Those may crack too from being frozen.

Stop the engine, close the seacocks to let antifreeze rest in the whole system.

Spray the fogging oil on the carburetor and spark plugs, do it right away while the engine is fairly warm, and don’t be greedy with it as this also fights rust, which may arise due to the condensation (water from the air).

Disconnecting a battery is a good idea.

Connecting a bilge heater is a brilliant idea if you have a sufficient power supply.

PRO TIPS

Making it warm around the engine is the best idea yet it does not replace the winterizing procedures but adds a lot to the lifetime of a motor.


How To Keep Boat Engine Warm In Winter? Use a Bilge Heaters

Using antifreeze is a totally safe and the most correct way-out but demanding efforts and just increasing the overall cost a little. Not only the triple cost of the “marine” material but more money for a haul-out if the system cannot drain itself while lying in the pond. Until you are not in the Arctic, consider the heater!

The beautiful thing is called a bilge heater.

For sake to specify a brand – Xtreme Heaters XHEAT Boat Engine Compartment Heater.

Once you have a shore power well established, you can connect the bilge heater for the whole cold season to double your winter-care efforts. The bilge heater shall create a warm cap in the bilge and in the engine compartment, and make you sleep well through the whole winter.

Of course everything marked as “marine” may cost an extra penny, but just this heater is the professional effective way of how to keep the boat engine warm in winter. After all we’ve chosen this fate of higher prices once stepping upon the path of a boat life.


Use an Electric Blanket To Winterize an Inboard Boat Motor

There is a cheaper but efficient way to keep a boat motor from freezing, and winterize it better by adding extra heating at low cost. Compared to the bilge heater we suggest using an electric blanket!

Yes, you read right, simple as that! A life-hack if you wish.

Choose a waterproof electric blanket or two as everything is waterproof in the boat-life to align to the general safety, and your winterizing decisions must be bulletproof.

You cover the top of the motor and its sides due to less accessibility to the bottom of it. The engine shall not produce its own heat for months and you shall need to effectively save that minimal heat delivered by the electric blanket.


Keeping a Boat Motor From Freezing – Wrap-up

While you are already wrapping up the engine for winter with the electric blanket in your thoughts, we wrap up this whole engine-saving message.

We speak about the very-very expensive part of the boat, so, don’t be a scrooge and don’t be a fool when winterizing to keep your boat motor from freezing and cracking!!

You do not want a tragedy, and yes it’s going to be not a some crack but a whole tragedy once you ignore the winter precautions. In one word — buy that marine antifreeze along with other things we discussed here in this post.

Have a great winter time, Capitan!

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Igor

Hi, I’m Igor, Skipper of S/Y "The Hooker". A decade ago, I conquered my childhood dream: be a sailing captain, have a sailing yacht, sail anywhere and everywhere on the globe, and breathe freedom.
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