First, scrub clean and dry the freshwater tank well. For water tanks, make sure it is empty, vacuum out every last bit, and then let it dry. With no water, there is no possibility of growth, and all spring commissioning requires is a fill-up.
Winterizing agents should never be used in freshwater tanks or hot-water tanks. Doing so will greatly increase the chances of biological growth, which can result in foul-smelling, bad-tasting water. If your boats water system does not have bypass fittings that allow you to add glycol to waterlines, install them. The addition of a few simple fittings can reduce the annual process from hours to minutes for the cost of a few jugs of glycol.
For the freshwater tank, add a T-fitting that allows you to tap into the waterlines downstream of the tank. Add two valves; one to shut off the upstream water source (tank) for winterizing, and the other to close off the antifreeze tap for normal use.
For the hot-water tank, drain as much water as you can, and install a water heater bypass kit; some tanks already have these.
You can't judge your antifreeze concentration by its color. You need to measure the freeze point of the glycol dribbling out the far end, ideally with a pocket refractometer, a tool that can read the true freeze point of propylene glycol (PG) or ethylene glycol (EG).
Exercise all of the valves in the system. Lime and corrosion will build up and jam even the best valves if ignored too long; seacocks can use some grease, and sea life is a problem. Many valves trap water in the ball cavity (if closed) or above and below the ball/gate/plug in small voids. This trapped liquid can easily crack the valve. If you are draining your plumbing system, rather than adding antifreeze, always leave the valves open.
Remove any incompatible plastic parts. Clear plastic filter housing covers made of acetate and unidentified plastics can craze and crack after exposure to PG or ethanol for extended periods. EG is a good choice for engines and blackwater systems as there are fewer compatibility problems. Do not use EG on potable water systems; it is acutely toxic to mammals.
Dont create a petri dish. Anything less than 20-percent glycol invites biological growth. If you leave the glycol in the plumbing hose runs from September to June, certainly the bugs are going to grow and your water system will become foul. However, if you delay winterizing until the first frost and break winterization as soon as the water gets turned back on, the boat will have spent the entire winterized period in the refrigerator and growth will be minimal.
Pump out the holding tank and add one half-gallon of glycol concentrate for extra measure. Drain the head or add glycol. If using glycol in a head with neoprene parts (Jabsco), use ethylene glycol; propylene glycol stiffens neoprene parts and ethanol stiffens nitrile parts. Again, winterization valves can make the job much easier, allowing the pump to move the glycol through the head.
If you use the head after it is winterized, simply flush with a jug of -50 burst-point glycol using the dry flush setting. Ideally, the glycol concentration in the bowl and pipes is above 30 percent, otherwise you are going to grow a garden of mold and bugs by spring if left unattended.
If the engine is sea cooled, just treat it as a potable water system. However, for flushing and filling, use engine-specific products, which we will be comparing in an upcoming article. If the engine is glycol cooled by way of a heat exchanger, treat the sea-water side as you would a potable water system with engine-specific products, preferably an extended-life coolant found in auto-parts stores. An extended-life coolant should also be used in the coolant reservoir and closed-loop circuit of a heat-exchanger. This glycol can easily last five years. Our chemical preference for engine coolant and winterizing is ethylene glycol.