At this point I believe the boats of Graves' design were built on the Thames. The engines were Williams' design, this fact brought to light by mention of him making dimensions too large resulting in too heavy a structure.Yes, no condensers and meant to operate in salt water. Not much of a life expectancy there.
The boats I am familiar with are open top, though a shrap shield would make sense. They were to be armed with spar torpedoes, some of which are still around in shallow water.

Off for some Turkey...Regards this Thanksgiving of 2007.