I'm looking more for something that could plausibly be used as a "coast guard" type of vessel, which gets hastily refitted with weapons when war breaks out. IE, not something designed from the ground up as a combat vessel (like 90% of the suggestions in this thread). It's going to be operated as a security boat basically by a group of policemen... it's not an MTB or missile boat making cruise missile attacks.
I think the YMS class fits the bill for this (more or less). I'm going to reduce its speed to around 7 knots, citing inadequate maintenance and the uncivilized climate.
ACE: let me know if (weight wise), this would make sense: (and any suggestions you'd have)
I believe the minesweeping equipment in the stern will be removed and replaced with a motor launch and davits and possibly a single Bofors 40mm right aft (or a 20mm Oerlikon single if the Bofors is too heavy). The forward 3"/50 will be replaced by a single 40mm Bofors (or possibly a double if this keeps the ship balanced). The wing 20mm mounts will be replaced by .50-caliber machine guns citing cost of 20mm rounds as the reason. The SG surface search radar will be retained, but by 1968 (time of the story), a component has failed and the operators were not able to obtain a replacement. The depth charge projectors will be curiously retained but the charges landed at the boat's home port. I am planning on using the landed depth charges later on in the story.
Any other ideas?