Cazatou,
My apologies for not responding sooner.
Depends what you mean by Merchant ships. Now a days many merchant ships fly under foreign flags and have mixed nationality crews. Therefore laws apply.
Believe it or not the merchant ships may be armed, but most nations require export licences (for the weapons) for every country that the vessel might visit.
UNCLOS does state that every
State may seize a pirate ship or aircraft, or a ship or aircraft taken by piracy and under the control of pirates, and arrest the persons and seize the property on board. I suppose a non military vessel might be issued with warrants that allow them to carry out anti piracy activities, but there might be some difficulties as a seizure on account of piracy may be carried out only by warships or military aircraft, or other ships or aircraft clearly marked and identifiable as being on government service and authorized to that effect.
One of the problems is do you mean small arms (shotguns, non automatic weapons and pistols) or something slightly more lethal? E.G. Bofors, missiles etc. A number of yachts (increasing in number as well) are carrying small arms to defend themselves.
There are some flags that prohibit the carriage of arms or the use of violence. There are also some insurers that will not accept it, and a ships insurance will be void if weapons are carried.
You will probably have read about Blackwater and their proposals.
Blackwaters' solution of vessels with armed persons on board is interesting. The Djibouti government have given Blackwater permission for the paramilitary operations to be based in the Port of Djibouti. Security of the ships weapons whilst in port are to be provided by Djibouti Navy. US Embassy officials have expressed concerns about the legality of the plan.
Mind you, would you wish to be protected by Blackwater? Their tendency to indiscriminate violence and the killing of civilians would, and does, scare me.
I also question the legality of Blackwaters solution.
For your interest read
this
Possibly the best solution would be for the UN to pass a resolution allowing for "active" defence of ships by use of weapons. Only problem is controlling this. I also do not believe that use of military might is the total solution.
Cazatou, hopefully I have answered your question sufficiently. If not let me know. I could waffle on a lot more.
Hval