I am very new to this trailer sailer game and have aproblem with the keel on my boat, ( or maybe they all do this

When under sail and tacking the keel seems to slap from one side to the other and it is a bit disconserting.
Any help you can give would be great I am in Adelaide so if you know of anybody over here who can help that would be great too.
thanks a lot.
Flying Scotsman (Rob)