Sink drains very slowly. Swift challenger 2014

T Gordon T

My kitchen sink drains really slowly. I have followed the drain pipe and underneath the van there is a  'u' shaped dip in the flexible drain hose which must be causing the problem. Is this designed to be a water trap to stop drain odour coming back into the van or is it a flaw that I could resolve by shortening the pipe? Anyone else come across this problem? 

Thanks all