A very nice sketch is found in The Confederate Navy by Philip Van Doren Stern at:

http://books.google.com/books?id=PVNOiXfdXw4C&pg=PA134&lpg=PA134&dq=Porter's+Dummy+Ironclad&source=bl&ots=O_9fZmpwTg&sig=FL_qXMiOExwl4a9gwftfrZ_gutk&hl=en&ei=KOxhSu3SA4SoswO3icjxAQ&sa=X&oi=book_result&ct=result&resnum=4

A came across this from a Goggle search: Porter's Dummy Ironclad.

Also found therein is a good description in Ellet's Brigade by Chester G. Hearn on starting on page 117 at:

http://books.google.com/books?id=3TmmArYduSMC&pg=PA117&lpg=PA117&dq=Porter's+Dummy+Ironclad&source=bl&ots=wSOBpmnvbN&sig=n_YjpJ2Z5qxsnPbhdgsbUexcGbM&hl=en&ei=KOxhSu3SA4SoswO3icjxAQ&sa=X&oi=book_result&ct=result&resnum=1

See also: http://en.wikipedia.org/wiki/File:Admiral_Porter's_Second_Dummy_Frightening_the_Rebels.jpg