The one problem is the prop pitch. It is obvious that it is wrong, and everyone's "best guess" is a 16 X 7, but the frist "best guress" was 16 X 10.
For what it is worth to you, Tranquility Base has a Yanmar 2GMF, a 13 HP engine (15 when running flat out). Apparently this engine was an option in 1984 for the Islander Bahama 30. The Island 30 Mk I and II and the Bahama 30 all have the same hull. TB has a 13 x10 R.H. 2 blade prop (most Yanmars are RH engines). The prop seems to me to be about right or is just a smidge underpropped. Unloaded RPM is 3600 rpm. In forward I get up to about 3400 rpm and 6.3 kts or so; hull speed is around 6.68 kts so it is close.
T. P. Donnelly
S/V Tranquility Base
1984 Islander 30 Bahama