In Gtpnf.94832$ki.57113@pd7tw2no Gary writes:

phasmatis wrote:
A friend of mine has a Beneteau 345 moored in Corfu, it was due to be
lifted out for the winter this morning but unfortunately he had a phone
call from the Marina to say his boat had sank! The boat was stern on to
the pontoon and it was only the ropes holding the stern end out of the
water. There are no other boats on the pontoon as they have already
been lifted out.

The boat has now been pumped out and is afloat again and on inspection
there is no obvious reason why the boat would take on so much water to
sink the boat in a 12 hour period since it was last viewed.

The evening before the boat sank a large power craft powered at speed
through the Marina and created a very large bow wave, the only
explanation as to the cause that has been offered is that the bow wave
caused the toilet to overflow and created a syphoning effect which in
turn filled the yacht with water.

Anyone heard of this before?

Yup, my cousins bilge pump started syphoning and sank his boat.

My neighbour in the club marina had a 30 footer, where the builder was
being smart and had joined the bilge pump and kitchen sink outlet. If
you left the kitchen sink plug on after you had pumped the bilge, you
had a perfect siphon. Of course it happened once. He was lucky that
someone noticed that hes freeboard (usually about six inches higher than
mine, was about a foot lower than mine and came to the rescue.

Another case was with the toalet pump in another boat. So if you make a
sifon, once it will work. Just wait long enough.

- Lauri Tarkkonen