can some one help me about the ferry mode when does it count.on the vosa site says you have to have a bunk/and the other one does it count going from dover/calais and calais/dover because when you are on dover/calais does it count or does it count if you were going on a 24hour crossing to santander because i get told differant storys 
You cannot use it on Dover / Calais or Eurotunnel.
You cannot interrupt a 24 or 45 hour weekly rest, so forget ferry mode for now!
There is no such thing as ferry mode.
You just use normal rest mode, or break, on a ferry. The ferry option on a digital tacho is simply the electronic equivalent of the pen you use to write on a chart to explain why you have interrupted your rest. That’s all it is, it is not some special kind of rest period.
As Wheel Nut said you cannot interrupt your daily rest on the Dover Calais crossing so you wouldn’t make use of the ferry option then. On the 24-hour crossing to Santander there would also be no point in using it as you will get more than a full daily rest period on the crossing.
You would only use it when you are taking part of your full daily rest period on land and part of it on a ferry or train. In that situation you put the tacho on rest mode as normal then select the ferry option just before interrupting your rest to board the ferry. You then put it back on rest once on board.