YSC Blog Post

Swan necks & anti-siphon valves

18 February, 2017

Seacocks connected to below the waterline fittings such as bilge pumps or sea toilets can be prone to back flooding by siphoning. If the seacock is left open or has a small leak past the seals then it is possible for the vessel to slowly flood and sink.

Having witnessed a vessel taking on water due to siphoning, I can confirm for a relatively small outlay they are definitely an invaluable safety item! Never rely on one-way valves as I have seen on boats. In my experience they will fail to seal properly after about 6 months.

Swan necks should rise above the sea level by at least 8” or 200mm when heeled over. In practice this is normally limited to below the side deck. The anti-siphon valve is a small one way valve inserted in the top of the loop that is normally closed. When you finish pumping and the water level tries to drop back to sea level and small vacuum is created inside the loop which forces the valve open and allows air into the pipe. This breaks the siphon and prevents water flooding back into the vessel.

Swan necks on their own will not prevent back flooding, you need the anti-siphon valve as well to allow the seawater side to fall back to sea level.

The small valve is often a small rubber moulding under a screw cap and it requires regular cleaning to keep it working. Never use Vaseline on these or any other rubber seals as it deteriorates the rubber, use a silicon based grease.