1. Failure to winterize the engine: Freezing temperatures occur in all 50 states and while they are taken seriously up north, it's the balmy states of California, Florida, Texas, Alabama and Georgia where boaters are most likely to have freeze-related damage to engine blocks. It routinely occurs to boats stored ashore here. Boats left in a slip are less susceptible to sudden freezing as the surrounding water retains heat longer than air.
2. Failure to drain water from sea strainer: If your winterizing plan calls for draining the engine, the seawater strainer must be winterized or residual water could freeze and rupture the watertight seal. Sometimes you won't know it's damaged until spring launching and water begins to trickle in.
3. Failure to close seacocks: For boats left in the water, leaving seacocks open over the winter is like going on extended vacation without locking the house. If a thru-hull cannot be closed, the vessel must be stored ashore - the sole exception is cockpit drains. Heavy snow loads can also force your boat under, allowing water to enter thru-hulls that are normally well above the water line.
4. Clogged petcocks: Engine cooling system petcocks clogged by rust or other debris can prevent water from fully draining. If one is plugged, try using a coat hanger to clear the blockage or use the engine's intake hose to flush anti-freeze through the system.
5. Leaving open boats in the water over winter: Boats with large open cockpits or low freeboard can easily be pushed underwater by the weight of accumulated ice and snow. Always store them ashore.
6. Using biminis or dodgers as winter storage covers: A cover that protects the crew from the sun does a lousy job protecting the boat from freezing rain and snow. Unlike a bonafide winter cover, biminis and dodgers tend to rip apart and age prematurely by the effects of winter weather.