MSea has said it all except the strange shape was because the "blisters" on the sides were ballast tanks.In shallow water, they could be flooded and the ship would sit on the bottom making a very stable gun platform.
They had their uses in the days when we used to "send a gunboat"
There were only two in WW2, Roberts and Abercrombie, they both lasted out the war and had busy careers.
Poor sea-boats and slow with a top speed of about ten knots.
Haddock.
