Storage in Water
If the boat must be left in the water, the thru-hulls have to be protected by closing
all seacocks and gate valves.
Leaving a thru-hull unprotected over the winter is like going on an extended vacation and leaving your home’s front door open.
Failure to close thru-hulls is a major cause of loss in the BoatU.S. insurance program. In a recent study of 40 winter-related claims, seacocks or gate valves left open caused or contributed to the sinking of seven of the boats in the sample group.
If the boat must be left in the water, the thru-hulls have to be protected by closing all seacocks and gate valves.
This boat in Maryland sank when it's plastic thru-hull was shoved underwater by the weight of the snow. the intake was broken by ice (the surveyor who inspected the damage suspected that it was already cracked) and water flowed into the boat.
It should be noted that raising and refurbishing a boat that sinks, even at a dock, is a daunting job that can keep the boat in the repair yard for many weeks over the spring and summer. Whenever a boat is stored in the water over the winter, all thru-hulls, with the exception of the ones for cockpit drains, must be closed or it could be on the bottom next spring.
And all thru-hulls, especially the ones for the cockpit drains, must be double-clamped with stainless steel hose clamps at each end. This is critical. When water
freezes it expands and will lift a poorly secured hose off of a fitting. The hose itself is also important. Lightweight hose and PVC tubing can rupture or crack. Use only a heavily reinforced hose, especially at cockpit drains.
If your boat has thru-hulls below the waterline that can’t be closed, either because they are mechanically frozen open or have broken (typical with gate valves, which is why they are not recommended), it should be stored ashore for the winter.
Seacocks are closed by moving the handle down so that the handle is parallel to the hull. Gate valves are closed by turning the wheel clockwise.
After the seacock or gate valve has been closed, remove the hose so that it drains and then use an absorbent cloth or turkey baster to eliminate any residual water, which can freeze and crack the nipple.
(Taking off the hose also assures you that the valve has closed properly.) Reinstall the hose immediately and secure the two clamps.
It should be noted that thru-hulls above the waterline are not required to have seacocks and most don’t. That doesn’t mean that these thru-hulls aren’t vulnerable.
Ordinary plastic thru-hulls deteriorate in sunlight and have been broken when they were shoved underwater by the weight of snow and ice in the cockpit, which then sinks the boat. Plastic thru-hulls near the waterline are especially vulnerable and should be replaced with bronze or Marelon (the latter is the only type of plastic approved for marine use by U.L.).