The service between Yell and Unst and Yell and Fetlar is no longer free. This is an excerpt from the councils ferry webpage.
Bluemull Fares Reintroduction 1st April 2013
On 1st April 2013 fares are to be reintroduced on the Bluemull Sound service. The fare payable will be the same as the other “return fare” routes. This is a temporary arrangement until the implementation of the fares review.
However if your journey originated on the Mainland of Shetland (you have had to use two ferries on the same day or after 1800 from the day before) then you will only be charged the one fare on Yell Sound.
Please retain the ticket you were sold on the Yell Sound ferry for presenting on the Bluemull Sound ferry.