I will try and be polite, do a little clarifying and be concise. Yes I am aware that Border Force uses Cape class, in my mind they are slightly modified Armidale class, I was just being a little lazy, my apologies for that
There is an ideal size for a ship, obviously you can go larger say Arafura class, or small Cape class, I am just suggesting that longer term, the ideal size might be around 1100t, which is about four times the displacement of what Border Force uses.
With regards to drones, I dont think I explained it fully. My thoughts were that technology progresses to such an extent, that once the drone sub is set, it operates independently without regards to communicating with home. Technology is progressing so fast that it should be possible for Artificial Intelligence to detect an enemy, determine what ship it is and sink it all without need to communicate. Obviously this requires a very strong and robust software package, so that once the sub is unleashed it torpedoes only the enemy and gets it right every time. Although some have their doubts as of now, in decades to come this is how things will work.