I also have the same problem with my B40. Having been through this with my boatyard the problem seems to be with the lower rudder bearing, basically there are a number of small needle rollers in the bearing housing and the rudder stock goes through these.The bearing is made by Jefco of Denmark. They are not a tight for and so the rudder can move both side to side and forward and backward. With the boat out of the water it is possible to move the rudder in all directions. I changed the bearing a couple of years ago but the problem did not go away for long – seems there is not much you can do about it.
    I did try tightening up the chains to get rid of any play in them but it didn’t help. There is also a very small amount of play in the rudder gearbox but I suspect this is not the cause of the problem.