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Winterizing Your Boat Systems

Winter is on its way, and for many boat owners this means it’s time to get your precious yacht ready for the colder months. Proper care of your boat is imperative all season, but by far, winterizing is the important maintenance you will need to do, as an improperly winterized boat can cause significant and costly damage.

Cleaning the Boat and Removal of Equipment

Prepare inside and out: Remove any external debris and wash the hull of your boat. Most marinas offer a power washing service, and we always recommended to have this done when the boat comes out of the water, as it is by far the best and easiest time to clean the bottom of the boat. Once the growth dries, it becomes significantly harder to remove, even with a power washer.

Remove any equipment from inside the boat, including life jackets, cushions, pillows or personal items, and store in a dry warm location. During the winter storage months, mold is a relatively common issue, and by removing items that are most susceptible, you will save yourself the added headache of cleaning these items in the spring.


Make sure all water is drained from the boat, including the bilges, live wells, and water tanks are drained. If your boat has a garber or drain plug, be sure to remove it to allow all bilge water to drain.

If you bilge is excessively dirty, now is a good time to flush the bilge with freshwater and remove any debris.

Make sure to remove any standing water from any areas of the bilge do not properly drain. A bone sponge and a bucket are often the best tools for this job.

Note: During the winterization process, we will often add leftover antifreeze to the bilge to help prevent pumps from freezing over the winter.

Fill the Fuel Tank

Fill your fuel tanks: Fuel tanks that are not full allow for condensation to form on the tank walls during the winter months, and this condensation will introduce water into your fuel supply. We recommend the fuel tanks be kept at least 3/4 of the way full.

Fuel Stabilizer:

Once the tanks are filled, add fuel stabilizer to your gasoline or diesel fuel tank. The amount added is based on the tank size, so be sure to read the directions and add the proper amount. Also be sure to use the proper fuel stabilizer for your fuel type, as gasoline and diesel fuel stabilizers are very different in both their chemical makeup and effect.

Gasoline fuel stabilizer prevents gumming or varnishing of the fuel through the engine system, prevents rust by displacing water, and helps prevent the separation of ethanol in the gasoline. Fuel stabilizer, such as Sta-Bil, is compatible with both ethanol and non-ethanol gasoline, and stays effective for roughly 2 years.

Diesel fuel stabilizers work primarily by preventing bacterial growth in the fuel through the use of biocides, partitions both water and diesel phases further preventing bacterial growth, and finally adds additional lubrication to internal engine components. Diesel stabilizers are extremely important, as bacterial growth will cause sludge to form in the tank, while will clog your fuel filters in the spring and prevent the engine from properly running. Issues with sludge appear most often when your boat is in rough conditions, which is the last place you will want your engine to fail. The only way to remove sludge and bacterial growth, once present, is to have your fuel polished, which is not an inexpensive process.

Battery Care

If your batteries have a lead acid chemistry, we recommend fully charging your batteries before decommissioning your boat for the winter. If the batteries are flooded lead acid, we further recommend checking your electrolyte levels before doing the final charge, and if low, topping off the cells with distilled water.

Once charged, and if your batteries are unable to be removed from the boat, we recommend disconnecting the negative terminals to prevent parasitic draw. If there are multiple negative leads, we further recommend zip-tieing the leads together to prevent confusion in the spring.

If you are able to remove the batteries from the boat, we do not recommend leaving a battery charger on the entire winter, and only charging the batteries not more than once a month. We further recommend the batteries not be kept of concrete, as the temperature differential can lead to stratification in the electrolyte solution.

If you have a lithium battery system, it is recommended to leave your lithum batteries at partial charge (usually 50%) but be sure to follow manufacture recommendations.

Water System

Drain the System: Begin by draining all the water from the system, including the freshwater tank, pipes, and the hot water heater.

  • Open all faucets on the boat and let them run until no more water comes out.

  • Don’t forget to open the valve on the hot water heater to allow it to drain completely.

Bypass the Hot Water Heater: Install a bypass kit if you don’t already have one in place. This kit typically consists of a few hoses and valves, allowing you to reroute antifreeze around the heater instead of through it.

  • Close the inlet and outlet valves of the hot water heater.

  • Open the bypass valve so that antifreeze can flow around the heater.

  • Lockout the hot water heater breaker if present at the electrical panel or disconnect the heads at the hot water heater. This will prevent the heating element from burning up in the event the boat is plugged in before the water system is refilled.

Why bypass the hot water heater?

The main reason for bypassing the hot water heater during winterization is to prevent antifreeze from entering the unit. Not only is this a waste of antifreeze, but it can also be difficult to flush out come spring. By bypassing the heater, you can ensure that antifreeze goes only where it's needed, saving both time and money.

Add Antifreeze:

  • Pour marine-grade non-toxic antifreeze into the fresh water tank(s). We recommend -50F antifreeze (the pink stuff). How much you’ll need depends on your boat's size and the length of your plumbing runs, but it's always better to have extra on hand.

  • Turn on the boat’s water pump and open the faucets one by one, starting with the one furthest from the tank.

  • After draining the water tanks, there is a potential that an airlock can form in the water pump or the water filter. If an airlock occurs, disconnect the output side of the pump and run antifreeze into the unit. If the boat has a filtration system, loosen the filter housing while the pump is running to remove the airlock.

  • When you see antifreeze coming out of the faucet, you know that section of plumbing is protected. Close that faucet and move to the next one.

  • Remember, since you’ve bypassed the hot water heater, both the hot and cold taps will draw antifreeze from the tank. Be sure to switch each facet from cold to hot to ensure both sides are flushed with antifreeze.

  • If there are multiple tanks, make sure to winterize each water tank individually.

Don’t Forget Other Water Systems: If your boat has other systems like a raw water washdown, livewell, or an ice maker, shower sumps, be sure to winterize these systems as well.

Final Checks: Walk around your boat and ensure you haven’t missed any faucets or fixtures. Ensure all valves in the bypass system are in the correct positions, and the hot water heater remains isolated from the antifreeze.

Head/Holding Tank

Pump Out: Before the boat leaves the marina or mooring, be sure to have the holding tank pump out. If the holding tank is left full, or partially full during the winter, there is a possibility that the holding tank or plumbing fittings will fail, causing a catastrophic failure.

Clean and Flush the Head:

  • Close the Seacock: If you have a raw water flush system, close the seacock to prevent any seawater from entering. Do this after the boat is out of the water to allow any seawater to drain from the plumbing.

  • Head Flush: Disconnect the raw water intake hose from the seacock and place it into a bucket filled with -50F antifreeze. Pump the head multiple times until the antifreeze replaces all the seawater. This is important, as without this step, none of the internal components of the head will be protected from freezing.

Holding Tank: Once the head is winterized, it is important to winterize the holding tank. To do this, add a gallon of -100F antifreeze into the pump out deck fitting. The -100F will dilute with any additional liquid left in the holding tank and will prevent freezing failures in the system.


Winterization is an important step in boat maintenance, and if you take the time to do it properly, it will pay off in the long run. If you need any additional assistance with winterizing your boat, please don't hesitate to reach us for assistance.


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