If you do a search with Mr Google for "Whanganui River Steam Boats" you will find information on the boats which were built in England by Yarrows for A C Hatrick & Co with props operating in a tunnel. One (the Wairua) is restored and runs excursions but there was at least one other. The Wairua is single prop but I am sure there was a twin prop one also. It should be possible to obtain plans from the Whanganui Riverboat Centre. Hope this helps, Ian.