By Craig Johnston Oct 19, 2024

Why Winterize?

Our boats have several systems carrying water that can freeze and cause damage ranging from burst hoses and cracked pumps to cracked engine blocks, and in the extreme, boats sinking at the dock.  A little prevention each fall can prevent time-consuming and expensive repairs next spring.  While boat owners in the Northeast or Midwest face extended periods of cold weather that often calls for haulout, storage on the hard, and draining of water and coolant, we in the Pacific Northwest typically leave our boats in the water.  But since weeklong spells of temperatures below 25°F usually occur each winter, serious damage can result without protection.  We generally winterize in early December when the boat may sit unused for several months or as soon as extended cold weather is forecast.

What Systems Need Winterizing?

Any system with water in it can freeze under the right conditions:

• Engine 
• Water tanks
• Marine heads
• Sinks and drains

Protection from freezing can be had by either draining a system of all water or adding antifreeze.  What type of antifreeze depends on the system, as toxicity and the effect on rubber and plastic parts is different for each.  And of course there are other maintenance tasks best done in the fall, such as changing the engine and transmission oil.  Dirty engine oil contains acids and combustion byproducts that are best not to leave in place over the winter.

Engine

Most of our boats have diesel engines that have both freshwater and raw water systems.  The freshwater system should contain a solution of either ethylene glycol (EG) or propylene glycol (PG).  EG is used in most cars but is toxic to humans and pets. PG is non-toxic (it is found in everything from toothpaste to many prepared foods) but also more expensive.  Both are considered hazardous waste if they have been in the engine for years, but neither is actually very polluting if dumped unused into the river.  

Your freshwater system coolant should be changed every 5 years or every 1000-10,000 hours. (A glycol refractometer can be used to determine if the coolant is functional.)   Drain from the low points on the engine block and also the drinking water hot water tank heating loop.  There are numerous YouTube videos to illustrate.  The used coolant should be captured and recycled as it contains lead and other pollutants.  Refill with a 50% solution of EG.  It is important to avoid air pockets that can cause engine overheating and damage;  fill through the radiator cap, run the engine, and refill until the level is stable.  As an added precaution, I keep the shore power on to the hot water tank, on the theory that there will be some convection flow of the heated coolant back through the engine.

The raw water system takes in river or sea water, passes through the heat exchanger to cool the freshwater coolant, and then injects it into the exhaust riser.  Unlike the freshwater system, the raw water system uses a rubber impeller pump that can be damaged by alcohol or PG, so EG is also recommended for the raw water system. Super-conscientious boaters change their impellors once a season; while not necessary, inspecting it yearly is recommended and fall is a good time to do that

To winterize the raw water system, first close the intake thru-hull seacock, then open the strainer and clean the strainer filter.  The next step is to run the engine while adding the ethylene glycol full strength.  I do this by pouring the antifreeze directly into the strainer.  If it is convenient to remove the intake hose, you can put it into a pan or bucket filled with antifreeze.  Stop the engine when antifreeze starts coming out the exhaust.  This ensures that the engine, hoses, and wet muffler all contain antifreeze. Put a tag showing that the intake is closed somewhere it can’t be missed.

Drinking Water System

For our boats wintering in the river, it is not too likely that the water tanks will freeze.  There is more of a problem with the pressure pump or a foot pump.  On Sequoia, I turn off the pressure water each time we leave the boat. For winterizing, I turn on all of the taps including the shower at the stern until no water flows, then loosen the strainer next to the pump to allow the water in the hose to drain back into the tank. (We had the hand-held shower crack by leaving water in it over the winter.)   If you have an easy way to drain the water lines at a low point, that is even better.  If you drain the water system  including the hot water heater, take care to turn off the AC power to the heating element, perhaps taping the breaker switch in the “OFF” position as a reminder.  While some boaters put non-toxic water system antifreeze (which is based either on alcohol or PG) in their tanks, it can encourage bacterial growth unless full strength and is a pain to flush out in the spring;  I don’t recommend it.

If your boat has a watermaker that is pickled, the pickling chemicals will protect the RO membrane.  But it is best to drain the raw water intake lines and filters.  When pickling our watermaker, I put new filter elements into dry housings, so they are fresh when the system is restarted.

Head and Black Water System

The head pump is fairly well insulated from the water outside the boat and can freeze and crack if left full of water, as happened to George Stonecliffe on Julia Max last winter in Portland.  Prior to winterizing the head is an ideal time to de-calcify the hoses with a marine head treatment or mild acid such as CLR.  Then, after flushing the chemicals from the head and holding tank, pump everything dry.  I use regular automotive EG antifreeze  (I used to use RV water system antifreeze until I learned that it damages the joker valve).  I pour antifreeze full strength into the head and pump with the intake closed until it fills the hoses.  I then close the seacocks and put a reminder tag somewhere I won’t miss it in the spring.

If you have a shower sump, make sure it is drained or pumped dry.

Sinks and Drains

An open through-hull for your sink drain will have constant contact with the water outside the boat and is unlikely to freeze.  But hoses  may have a low spot and a burst hose could sink you boat.  I recommend closing the sink drain seacock(s) and filling  the hose up to the sink with antifreeze.  Here is a case where the best product to use is RV antifreeze (water or PG based).  Again, create a tag as a reminder.

Winterizing the Interior

With our wet winters it is important to keep interior moisture down to prevent mold and mildew growth.  On Sequoia we have four dorade vents and several hatches that can be kept cracked open to allow a good flow of fresh air.  We also deploy several of the low wattage warming fans to keep air moving.   Other suggestions:

• Clean and dry salty harnesses, life vests and foul weather gear
• Remove any opened food packages, excess clothing, and linens
• Clean and air out the fridge and freezer
• Space cushions and mattresses to allow air circulation

Doing these chores will only take a short day at the boat and will make for a much happier opening day!