The Twin Screw Motor Vessel SOFALA I can confirm was a motor vessel with 2 engines leading to twin screws, in responce to Gordon's question. He served on her when named Vorlooper and was unsure if she was single screw or twin. Happy to put the record straight about ships built at Leith and you will be able to find out so much more about the ships built at Leith, in my second book in the series which is almost complete.
In the meantine you can read about the ships from 1850 to the end of WW1 in the new book to be published next month.
The details of the book can be found at the publisher’s website as follows-http://www.whittlespublishing.com/They_Once_Were_Shipbuilders
You can also pre-order the book from Amazon at the following link