Posted: Fri Jul 08, 2011 8:13pm

I have just cleaned and coated the fresh water tank on my 1991 Liverpool built boat. Occasionally when running the fresh water pump I get a loud bang out of the plating on the tank due to I assume a vacuum. The filling line was extended, before we bought the boat, so that it was outside the cratch cover. To stop this banging I have drilled a small hole near the top of the pipe extension. What should be in place to stop a vacuum forming and where should it be situated. I have looked in all the front lockers and as stated I have been in the tank cleaning it?

Posted: Sat Jul 09, 2011 7:45am

Often water tanks have a separate breather that terminates in a hole in the hull fairly low own so water flows from the breather when the tank is full before it gushes out of the filler. Those breathers I have seen are just a combination of plain pipe and hose with an i.d. of about 1/2". I can see nothing wrong with your small hole but I might secure a piece of gauze over it with a couple of Jubilee clips to prevent spiders getting in. It is possible that the cap on the original filler either had no thread on it or had a hole in it so it acted as a breather. I suspect your new filler is a proper deck fitting using a rubber seal around the plug. The boat is by a budget builder so we may assume the top plate on the tank is not particularly thick so if your tank is under the well-deck it may well always flex as people jump on and off the boat. Make sure the locker sides are still secured at the top. They are often tack welded and over the years rust forces the welds apart so the "floor" is no longer supported. Tony Brooks