[dogscoff]

HOw about if mines gradually drift off/ break down. You need to keep replenishing the minefield for it to remain effective, and if your enemy can somehow get a single ship or small fleet through and harass your minelayers, then he might get a chance to send a full fleet through.

Of course, he probably won't know *exactly* when your minefield expires, since (a) he might not have seen when you laid them and (b) mine expiry could be slightly randomised, and/or affected by tech levels and/or maintenance.