If you have done most of your sailing on school or charter boats, you will find that the seacocks are rarely touched. One of the reasons for this is that if students handle equipment with which they are unfamiliar, they can cause more problems than might be caused by leaving them open.
When a yacht is left for any length of time, the seacocks should be closed. The reason for this is that a pipe may fail, this would allow water to flood the vessel and she will sink. Although this is unlikely to happen to you, it does happen every year (there should always be 2 stainless steel jubilee clips securing each pipe the the seacock).
If the engine cooling inlet is closed, it is useful to have a system to ensure that the engine is not started until it has been reopened, a label that can be hung on the starter switch will usually do the job.
The only seacocks that should not be closed when the vessel is left for any length of time are the ones for the cockpit drains, if these are closed, rainwater will flood the cockpit until it flows into the cabin through the main hatch. Because these cocks are not normally moved they will seize solid in a few months. To prevent this open and close them a few times at no longer than monthly intervals.
On older vessels with a narrow beam, you may find that water flows back up the heads or through the sinks when the boat heels. In this case, it is essential that a routine of closing the seacocks when at sea is implemented.
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