William Smart posted a discussion

I have a ST40 Bidata installed on a 26ft GRP bilge keel yacht. When motoring in shallow waters the depth reading will start to reduce and within 20 seconds is reading zero meters even though I know there is 4 or 5 meters of water under the keel. After a minute or so the instrument will recover and slowly start counting back up to the correct depth.Any ideas on why this is happening would be welcome. I sail mainly over soft mud when berthing which is when the problem seems to occur.See More