Having a quick glimpse at some photos of the schooner, I reckon you could fit the naval 81mm mortar mount on the stern of Clipper City for some additional firepower.
The USN Mark 2 Model 1 mount incorporates a breech-loading 81mm mortar and a .50 cal M2 and was used on a number of US and allied naval vessels (such as the Attack class patrol boats of the Royal Australian Navy).
But if you don't want the .50 cal, the Mk2 Mod0 mounts just the mortar.
From what I understand, the mortar allows breech-loading (trigger fired) and also muzzle-loading (drop fired like a traditional infantry mortar).
There's two images on the following page that should be useful (plus the page itself details armament on US coastal/riverine craft during the Vietnam War)
http://www.eugeneleeslover.com/USNAV...-ARMAMENT.html
But specifically, the image I am thinking of is this one, the Mk2 Mod1 mount: -
