Gunboat
In the age of sail, a gunboat was usually a small undecked vessel carrying a single smoothbore cannon in the bow, or just two or three such cannons. A gunboat could carry one or two masts or be oar-powered only, but the single-masted version of about  length was most typical. Some types of gunboats carried two cannons, or else mounted a number of swivel guns on the railings.