A boarding action is when either you or your opponents uses a Teleporter to beam onto your ship. While the enemy crew is on your ship they can damage systems, subsystems and kill members of your crew.
Teleporting onto a ship ignores normal shields, making it an effective weapon against enemies with high shields. However, the Zoltan Shield on most Zoltan ships will block teleportation until it is battered down.
Boarding is very useful against most opponents. Some enemies, such as Rock or Mantis ships, are difficult to successfully board due to their high HP or combat damage. It is sometimes a good idea to delay teleporting until after your opening volley, so you can target your shots at the rooms the enemies are in and deal some initial damage, making the combat faster and safer when your crew does teleport in. Boarding is especially effective against the The Rebel Flagship, as the boss's weapons are in disconnected rooms with a single human crew member inside, so once you've killed the crew member and destroyed the weapon, it will never be repaired until the next stage.
The best crewmembers for boarding are Mantis (100% damage bonus), Rockmen (50% HP bonus and immune to fire) and Crystal (25% HP bonus and lockdown ability). Human and Slug have moderate combat performance, but the latter allow you to view the interior of adjacent rooms which gives you a slight tactical advantage when you have no level 2 sensors or fight in a nebula. Bad races for boarding operations are Engi (50% attack penalty) and Zoltan (30% HP penalty).
Be careful when firing at an enemy ship that you've boarded - weapons hitting a room will damage your crew as well as the enemy! Do not destroy an enemy ship while your crew is on it, or else those crew will be lost. (Be careful of fires burning while you're boarding, as they cause additional hull damage when they damage systems, and can destroy the ship accidentally.) Be also careful when enemies are attempting to escape via FTL - if the enemy is able to jump out while your crew is on board, they take your crew with them.
Be very careful when boarding an auto drone, as their rooms have no oxygen. Only Crystal crew can survive asphyxiation long enough to wait out the 20-second cool down of a level 1 teleporter. A level 2 teleporter will reactivate fast enough to save all but Zoltan crew from dying. A level 3 teleporter cycles fast enough to rescue anyone.
Once all enemy crew have been killed, the battle is over. Winning a battle through boarding typically yields higher scrap rewards than blowing up the ship. There is also a chance that your crew will find either a hostage (who will then join your crew) or an intact weapon.
The easiest boarding ship is the Crystal B ship: with 3 crystal crew members and their "lockdown" ability you can teleport into a room and "lockdown" the room, meaning that crew can neither enter nor leave. This will let you destroy systems and sub systems or kill individual crew members easily, without their friends coming in to help. It also makes it much easier to board a ship with a medbay, as you can prevent the enemy crew from leaving combat to go heal.
Defending Against BoardingEdit
Upgrading your doors to level 2 or 3 will slow down boarders, as they have to attack each door to open it. This allows the useful strategy of venting the rooms that the boarders are in, so they suffer asphyxiation damage while trying to attack your ship. This becomes doubly ironic should they teleport into the airlock. Boarders will attempt to flee a room that has run out of oxygen (a pink coloured room), so this also helps protect the system they were attacking. It can be useful to vent empty rooms in your ship ahead of time, so that enemy boarders will begin to suffocate immediately (this also helps put out fires). Note that each time the boarders force doors open they cause air to leak out of the room they're entering.
If you are using the Zoltan ship, your Zoltan Shield will prevent boarding the ship but boarders caused by an event will still get through, with event text ending in, "You don't know how the intruders managed to get past your Zoltan Energy Shield!"
The best place to face the enemy boarding party is the Medbay.
Cloaking will also prevent the enemy from boarding your ship.