With regards to piracy attacks, a PCTC (Pure Car Truck Carrier) does not fit into the most vulnerable category of ships, those with low freeboards and traveling at low speeds, the so called "low and slow". Car ships generally are capable of 18 kts or more and have high freeboards. However taking into account the consequences of a successful takeover by pirates of an American flag PCTC manned by Americans and with U.S. Government cargo aboard, standard risk assessment doctrine requires that steps be taken to mitigate that risk.
The basic philosophy of ship security is (from USCG)
“hardening” merchant shipping targets. Even if such tactics cannot entirely prevent pirate attacks, they may prolong the time it takes for pirate groups operating from small craft to gain control of a target vessel long enough for naval or law enforcement response assets in the area to successfully intervene.
The most vulnerable points can be identified by taking a walk the length of the ship while alongside, the point of view of someone trying to board from a small boat.
In the case of a car ship there are only a few vulnerable points. How these points are best defended depends upon the layout of the ship. A combination of razor wire and well directed fire hoses (lashed so that can be used unmanned) greatly adds to the challenge of boarding a PCTC at sea.
Once at sea I am guided by a publication called "Best management practices to deter piracy in the Gulf of Aden and off the coast of Somalia"