Five or six years ago, in a big little book of profiles ( Warships of the Worl, I think -600 Destroyers, Battleships, Aircraft carriers and Submarines ) I ran across reference to 'Rendell gunboats' which
I had never heard of before. In terms of design, they came across as something akin to a sea-going*
sturmgeschutz. One big gun forward, not much in the way of armor, and mainly intended to be cheap
and plentiful. At the time, I couldn't find much on the web about them, but that has changed in
recent years. Below, an nicely illustrated and informative article from Big Bad Battleships.
EDIT: Some years later, the original link has gone dark, but fortunately, the Wikipedia article is a good
starting point.
https://en.wikipedia.org/wiki/Flat-iron_gunboathttp://www.cityofart.net/bship/hms_flatiron.html