In reply to kermit_uk:
I think there is a fair bit of confusing regarding ferries here.
To get from mainland Scotland to Orkney with a car you can either use Northlink from Scrabster to Stromness which will cost you £36 per adult return and £110 per car return. It is slightly cheaper to use Pentland Ferries, Gills Bay to St Margarets Hope - £32 per adult return and £76 per car return.
The car ferry from Houton (mainland Orkney) to Lyness (Hoy) is £27.20 return for a vehicle and £8.50 per adult return. Foot passenger only ferry from Stromness to Moaness (Hoy) costs £8.50 return.
Should you be planning to take a car from Houton to Lyness, please please book well in advance - it's booking up very quickly. And next week there is a large event on Hoy, the Commemoration of the Battle of Jutland, and the island will be mostly shut down on the 31 May with most of the roads closed - making it difficult to get around.
Good luck.