Winterizing Your Boat: Cover, Engine & Systems

In a previous article, we detailed the importance of winterizing your boat as well as your storage options. This is only the tip of the iceberg! If you want to prevent fiberglass damage, engine block cracks, and other nightmares, you’ll need to take great care in winterizing your engine and finding the right boat cover.

A Boat Cover by Any Other Name

There is a natural desire to protect your investment, and covering the boat is one way to do so. A cover is a cover is a cover…right? Wrong! Covering a boat involves much more than simply throwing and tying a tarp or installing a fitted cover. A cover should be a good fit, well supported and installed so as to not cause damage to the boat due to rubbing, shredding, tearing and collapsing.

Boat covers are placed on boats stored onshore, on the water and in a storage facility. As is true for most things today, you have options as to how you cover your boat: canvas, synthetic material or shrink wrap. There are good reasons to cover a boat.

  • Keeps debris out of scuppers (outlet on the side of the boat that carries water off the deck and drains it overboard)
  • Keeps debris off the boat deck where it can freeze and/or rot, staining the fiberglass (you don’t want to look at a brown outline of a leaf on your white deck)
  • Prevents snow from accumulating in the cockpit, the weight of which can cause a boat to sink lower
  • Prevents water from pooling on the deck where it can freeze and cause damage
  • Protects the gelcoat from the ravages of winter weather
  • Stops dust from collecting inside the boat when stored in a storage facility
  • May or may not discourage vermin (moment of honesty here)

Homemade boat covers, like plastic tarps sown together, are not a good idea because they are never going to fit right. If you get stubborn and still choose to make a homemade cover, it will need to be placed on a frame to allow air circulation but also be installed tight enough to prevent water pooling. If a cover is not installed correctly, your boat may experience weather damage, but it can also experience wear-and-tear damage from things like tie-down lines rubbing against the gelcoat.

One year a boat owner (no names!) took a homemade tarp off the boat and discovered months of nylon line rubbing against the boat had worn through the gelcoat, leading to spiderweb cracks that had to be repaired. We won’t even talk about the leaf outlines where tree debris had made its way through a shredded tarp. Ah well…live and learn.

Boat cover styles and options

Keep in mind while reading through the following sections that there are different boat cover styles. The most common is the cover that covers the whole boat. There is also a cockpit cover made of one or two pieces. It attaches to the window and snaps into place. One piece may cover the cockpit and another piece the seating area, or a single piece extends from the windshield to the transom.

Your boat design is taken into consideration when selecting a boat cover. A pontoon boat cover is different from a sailboat cover is different from a power boat with a transom or other features.

Here are your options for boat cover materials:

  • Plastic tarps
  • Generic or custom made synthetic or canvas cover
  • Shrink wrapping

Plastic tarps

So many people use plastic tarps to cover their boats. It often ends up being a cover that does more harm than good because throwing a tarp over the boat and tying it to the trailer is not the right procedure. They almost always end up shredded from regular flapping due to wind. Tarps are very light, tend to wear anywhere they touch the boat and have grommets that are likely to pull out after a short while, meaning the lines can now shred the tarp even more.

  • If you do use a plastic tarp, don’t let it sit on the boat. The tarp is ideally placed on a frame and pulled taut so it sheds water and snow, rather than collecting it in loose areas. The frame forces the tarp to look like a tent of sorts with pitched sides.
  • If you don’t use a frame, make sure there are vent spots for air circulation; place the vents at the base of the tarp to keep water out
  • Don’t let the tarp connect with anything that has a sharp edge, like windshield corners; if there is no way to avoid it, place a towel or other cushioning material between the tarp and the point where it touches
  • Secure the tarp with tie downs or bungee cords, making good use of the grommets if available (see the next piece of advice) or connect the securing lines to something heavy on the sides of the boat
  • Don’t secure the tarp to support blocks or jack stands because one strong wind could cause the blocks or jack stands to topple (not good for your boat!)
  • Buy tarps with reinforced grommets
  • Buy a heavy duty, high-quality tarp

Air circulation is important, unless you want to spend a month trying to get rid of mold and mildew in the spring.

Synthetic and canvas covers

Naturally, a custom made boat cover is better than an off-the-shelf cover. Both are extensively used. Many of the same rules apply to synthetic and canvas boat covers that apply to tarps. You do want air circulating, but you don’t want water pooling anywhere. You do want vents placed to encourage the air circulation, but you don’t want to secure the cover to anything that can be yanked out, like jack stands.

Though boats stored inside and outside are covered, the requirements differ. Storage covers for boats stored on the water or on the trailer are loose fitting so they allow air circulation but must be tied down to prevent flapping. Trailer covers for boats that are pulled down the highway will fit snugly to prevent flapping.

  • Polyester fabric – Most boat covers are made with polyester fabric. It is breathable, water repellent, durable and lightweight. Polyester fabric is cheaper than acrylic fabric which is one reason it is used so much. It is also lighter weight than acrylic fabric, so it’s easy to handle the cover. However, lightweight is not always the best choice for winterizing a boat. It depends on your weather and how you will store the boat. Some polyester fabrics are treated with an acrylic, urethane or vinyl coating to increase their water repellency and UV resistance, but a coating reduces breathability.

  • Acrylic fabric - Acrylic boat covers are superior to polyester boat covers. Acrylic is a synthetic, man-made fabric. It’s heavier than polyester fabric, i.e. 9.25 oz compared to 7.5 oz. Sunbrella material is some of the highest quality acrylic solution-dyed fabrics with the longest warranty. It is fade proof due to UV resistance, breathable, water repellent and mold and mildew resistant. It’s also abrasion resistant. As discussed below, the combination of water repellent and breathable is one of the best qualities in a boat cover. This is not an attempt to sell Sunbrella products but only to demonstrate that high quality fabrics have many features.

    Pigment-coated fabric means the color dye was applied after the fabric was woven. Solution dyed means the fabric fibers were dyed before the fabric was woven.

  • Cotton-poly blends and nylon fabrics – These are very lightweight fabrics and not recommended. They have low UV resistance and tear easily. The best thing we can say about these covers is: Don’t buy them unless you only need a temporary cover for a very short period of time while you wait on your custom made cover.

  • Vinyl – Vinyl is another synthetic material used in boat covers. It usually has a polyester fabric base and is then coated or laminated with vinyl layers. Vinyl covers are made with a different type of vinyl than seating vinyl which is designed to stretch and bounce back. You don’t want your boat cover stretching because that’s when indentations form and collect water, and the cover will sag. To make laminated vinyl covers, sheets of vinyl are bonded to the top and bottom of the polyester fabric. A coated vinyl top is more durable because the vinyl is integrated into the fibers. The heavier the vinyl, the higher the abrasion resistance. You will find there are different types of vinyl covers on the market, and some contain more vinyl than others.

  • Shrink wrap - Shrink wrapping boats is becoming commonplace. Shrink wrap is a polymer plastic (polyethylene) material that shrinks when heat is applied. When it shrinks, the material molds tightly on the boat, encasing the boat to some point down the hull sides. A high quality shrink wrap has a fabric liner and layers of shrink wrap material. It is good for indoor and outdoor storage. There are custom made fitted shrink wrap covers and universal shrink wrap covers. Technically, you don’t have to shrink the material, but getting a tight fit is usually the purpose of choosing this type of material. Shrink wrap is manufactured in a variety of thicknesses, generally ranging from 6 mil to 10 mil. The 8.5 mil is a good thickness for boats that might experience a high snow load. For boat owners living in a milder climate, 6-7 mil will likely be adequate.

    There are some points to keep in mind about shrink wrap. First, it’s waterproof and non-breathable, so it’s good at trapping moisture. Always use vents to promote breathability. This type of cover is only used once too. It’s not a cover you take on and off. However, it’s very good at keeping debris and dust out of your boat which is one reason you see new boats transported on a trailer while shrink wrapped. If you plan on storing your boat outdoors, use shrink wrap with UV inhibitors.

    Some boaters try to shrink wrap their own boats. This is definitely not advised. A propane-fired heat gun is used to shrink the material. One wrong move and you have a serious problem. Either your boat will get damaged or something will catch on fire and your boat will be destroyed. There is a whole process for properly shrink wrapping a boat, including taping the fuel vent and protecting sharp corners with padding.

    So, this is the bottom line: find a marine professional to shrink wrap your boat. The do-it-yourself videos are readily available online, but watching a video and applying heat to your boat in the real world are two different things.

One of the special considerations for sailboats include whether you want to cover the boat with mast up or mast down. Some covers are designed to use the mast up as the ridge to create a pitched cover.

So Many Points to Consider

Breathability is an important consideration for covers because of the potential mold and mildew problem. Add a terrible odor that can permeate cushions and other items too. Moisture trapped under a cover can ruin your electronics, wiring, seat cushions, items you have stored under the seats, and power plant.

The issue with breathability to keep in mind, when choosing fabric for a boat cover, is that the more breathability it has, the less water-resistant it will be without taking additional steps. The extra steps are buying covers with built-in vents or installing the vents. Vents are important in all types of boat covers.

Polyester and acrylic fabric covers are made with different weights, typically ranging from 3.0-ounce to 9.25 oz. The weight is a good clue as to the quality of the boat cover. A heavier fabric will, of course, be a better choice for boats stored outdoors or in the water. Lighter weight covers are mostly for keeping debris and dust out.

The length of the warranty is another clue to the quality level. If a boat cover has a one-year warranty, it’s probably not much better than a plastic tarp! A boat cover with a 10-year warranty has high grade fabric.

The terms “water resistant” and “waterproof'' do not mean the same thing. Water resistant fabrics are fabrics treated with a coating that isn’t supposed to let water leak in. Unfortunately, pooled water allowed to sit on the fabric’s surface and a coating that wears away will lead to a leaking boat cover. Waterproof fabrics are usually vinyl or fabric treated with an acrylic, urethane or vinyl coating. It’s important to have vents on all covers, but especially when the fabric is waterproofed because it isn’t breathable. Without vents, you’re likely to grow a nice garden of mold and mildew on your boat. Ugh! That’s just the start of potential damage. Some new technology has created fabrics (i.e. Top Gun® 1S and Odyssey®) that are supposed to be almost waterproof, so there is some breathability.

Boat covers made out of marine-grade pigment-coated polyester fabric purchased off-the-shelf usually have a hem with a shock cord that tightens via elastic around the boat and/or there are straps you hook to the trailer. You get what you pay for. Draw cords should be made of rope because it’s stronger and doesn’t lose elasticity.

A custom cover fits the best, as expected, and can accommodate different boat designs and brands, like a boat with a swim platform. When you get a custom-fitted cover, it’s easier to put on, provides better coverage and is of higher quality. Have a quality manufacturer make your cover, and it will have the essentials like vents, loops for tie downs, extra padding for areas known to experience problems and double-stitched seams and hems. A cover like this could last up to 10 years. You pay more upfront but less over the cover’s lifecycle. Loops and rings are best for tying down the cover because metal grommets can cause damage to the boat if they happen to flap in the wind.

If you want a quality boat cover, look for things like:

  • Sewn in reinforcements at wear points
  • Fuzzy backed reinforcement that prevents windshield scratching
  • Draw rope instead of stretch cord
  • Strapping that has strong webbing
  • Over-lock and lock stitching
  • Vents
  • Material type
  • Material weight
  • UV protection, breathability, water resistance or water proofing
  • Fit
  • Ease of putting on or taking off

Who would have ever believed boat covers would get so complicated?

Winterizing 100%

Winterizing your boat is not just about covers. In fact, putting the cover on is one of the last things on a list of steps you should do before you put your boat keys away. You have to ensure that your engine is prepared for winter weather, antifreeze is added where it should be added and water cannot get in where it shouldn’t enter. Engines are particularly at risk of freezing, leading to a cracked engine block or a cracked oil cooler, to name just a couple of problems.


  • Change the oil and oil filter – run the engine, shut it off, change the oil and replace filters
  • Fill fuel tanks to 95% full
  • Add fuel stabilizer to the fuel - ethanol absorbs moisture and that can lead to phase separation in which a caustic mixture forms in the fuel tank’s bottom
  • Change the fuel filters – ensures there are no leaks
  • Ensure the coolant is at the right level in the freshwater-cooling system
  • Fill water strainers with antifreeze
  • Run antifreeze through a raw water cooling system
  • Ensure the battery electrolyte level is topped off
  • Put batteries on a marine charger
  • Fog the cylinders in gasoline engines
  • Look for signs of corrosion and address

Outboard Motors

  • Fill fuel tanks
  • Add fuel stabilizer to the fuel
  • Turn off the fuel supply while the engine is running
  • Flush the motor with fresh water while using muffs (flushing device that fits over the engine’s water intakes on both sides of the gear case)
  • Drain the gear case and add fresh lubricant
  • Inspect and replace anodes
  • Look for signs of corrosion and address
  • Place the outboard motor in its lowest position
  • Empty fuel in portable motors, remove the motors and take home

Cleaning and closing down systems

  • Drain the water heater and bypass it
  • Pump out the holding tank
  • Put antifreeze in the head
  • Drain the freshwater system and flush antifreeze through it
  • Drain the shower sump
  • Run antifreeze through the live well or other units your boat contains that use water, like a wash down pump
  • Winterize the air conditioner
  • Check the bilge pump and switch are working
  • Clean the bilge and ensure it is dry
  • Close propane valves and remove any portable canisters
  • Close the seacocks except for the one that drains the cockpit
  • Plug exhaust ports
  • If storing onshore or in a building, remove the drain plug (don’t forget to put it back in next time you go boating!)

Final steps

  • Remove food (take my word for it that it gets gross when you forget to take food out of the freezer)
  • Open all the cabinet doors and storage areas
  • Place cushions on their sides or take home
  • Remove all bed linens, towels and clothes (unless you like the musty smell)
  • Remove all portable electronics
  • Remove any loose items or appliances on counters and in the galley and take them home or place them on the floor (in case your boat gets tossed around)
  • Secure all ports and hatches
  • Tie off the steering wheel or tiller
  • Check dock lines for condition and placement, and make sure they are securely connected to the boat and the dock
  • Add chafe guards to dock lines as necessary
  • Add fenders as necessary to prevent chafing
  • Cover or shrink wrap the boat
  • Say goodbye to your boat!

Always leave a key with a marina manager or a storage manager in case there is a problem with the boat. Also, if you leave your boat on shore or in the water, be sure to visit it periodically and check for signs of developing problems, like clogged drains, accumulated snow, chafed lines and so on. Always check the boat after a snowfall, heavy rain or high winds hit. If possible, check under the cover to make sure everything looks good.

Let Marine Professionals Do Professional Work

The recommendation is that you let a certified marine mechanic winterize your boat, especially the steps involving the engine or motors and any systems. Adhering to manufacturer instructions is critical, and steps like draining the engine and flushing antifreeze is not always easy. Boat U.S. Marine Insurance claims indicate that improperly draining the engine or improperly adding antifreeze accounts for the vast majority of boat freeze claims. You also need special tools and supplies, like muffs and 3-way valves.

Don’t leave a lightbulb on either, believing it will create heat. It can explode or start a fire if it comes in contact with something (like the seat cushions you left standing on edge that the storm tosses around).

Also, it’s difficult to remove ALL water from systems unless you know how. Leaving pools of water anywhere in pumps, the marine head, plumbing pies, valves and elsewhere is inviting serious trouble should the water freeze. Invariably, something gets missed too. You forget to put antifreeze in the head, or you forget to make sure the bilge is dry. The DIY-er always forgets something. It’s just a fact of life.

Never ever leave a heater in your boat. The power can go out, the heater can fall over and catch something on fire.

Winterizing your boat is serious business. Freezing water in a line, snow lowering the boat in the water, a cover blown off and other events can seriously damage your boat. Winterizing 100% is one of those things that should not ever be put off, and each task must be done correctly. There is a lot more information that could be added, but this should get you going in the right direction.

Mariner Exchange makes it easy to find the right boat service provider, from making custom covers to winterizing the boat to storage businesses. Take advantage of the knowledge and expertise of certified marine mechanics and professionals. They are your key to enjoying your boat when it’s time to cruise the water again.