As usual, Bill's drawing is beyond praise.
According to wiki
I thought warships have round badges and the other shapes to support vessels and so? Anyone having a simple explanation?
prior to 1940 it was like this:
circles for battleships, pentagons for cruisers, 'U'-shaped shields for destroyers, and diamonds for auxiliary units, including depot ships, small war vessels, and aircraft carriers.
Thus the badge is correct shape. Round badge would be OK as well though