A boarding action is when either you or your opponent uses a Teleporter to beam onto another ship. While the crew is on different ship, they can fight personnel there and damage systems and subsystems.
Teleporting onto a ship ignores normal shields, making it an effective weapon against enemies with high shields. Teleportation is blocked by the Zoltan Shield, equipped on most Zoltan ships, until that shield is battered down; however, the Advanced Edition offers the Zoltan Shield Bypass augment.
Boarding is of limited use against most opponents due to the time it takes for boarding party to destroy a system or kill off the crew. Furthermore, some enemies, such as Rock or Mantis ships, are difficult to successfully board due to their high HP and high 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. Since your boarding party takes damage when in a room hit by your ship's weapons, it's also a good idea to be careful what you target with ship weapons during boarding actions.
Boarding is especially effective against the The Rebel Flagship, as its weapons are in disconnected rooms with a single human crew member inside (unless in hard mode with advanced edition where the two artillery rooms near the shield room are connected by a 2-square room on either side), so once you've killed the crew member and destroyed the weapon, it won't be repaired until the next stage - provided the ship is not empty of crew and cannot auto-repair itself over time. Note, however, that if your crew is still in the ship after you end a stage, the crew will be lost when The Rebel Flagship warps away.
The best crew members for boarding are Crystal (25% HP bonus and devastating lock-down ability), Rockmen (50% HP bonus and immunity to fire) and Mantis (50% damage bonus). Lanius (drain oxygen from room) can prove effective against non-Lanius combatants and meeting enemy Boarding Drones without fear of the hull breach that the drone caused. Human and Slug have moderate combat performance, but the Slug allow you to view the interior of adjacent rooms, which gives you a slight tactical advantage when you have no level 2 sensors or are fighting in a nebula. The least effective races for boarding operations are Engi (50% attack penalty) and Zoltan (30% HP penalty).
Typically, the most practical choice for an offensive boarding party is between Rock and Mantis characters. The "Classic" version of FTL had no Clone Bay and since the life of your crew members is so much more important than dealing some extra damage, Rocks easily won out (especially as they also have much easier time picking off shields and engines from unpressurized automated ships). With the Clone Bay, Mantis can be seen as the superior choice. Crystal are still the best because of Lockdown, but since they are next to impossible to obtain unless started with, it is about as useful information as "Zoltan Shield is probably the best augmentation you could have".
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.) Also, be 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.
Worse yet, while Clone Bay will recreate crew members died aboard or indeed with the enemy ship, ones that flew away with escaping one are truly gone (since apparently Federation only allows cloning of confirmed-KIA, and missing or AWOL people are still living).
Be wary of ships with Cloaking capabilities, as you can neither teleport through reinforcements nor recover critically injured crewmen while their cloak is activated, and without level 3 sensors it is difficult to say when the cloak will end. This is especially dangerous when paired with enemies' upgraded Doors, as your crew cannot run around the ship to avoid damage until the cloak runs out. Thus destroying the cloak system before or during boarding is a worthwhile tactic.
Be very careful when boarding an auto drone, as their rooms have no oxygen. Lanius are perfect against drones, or working beside them. 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. With 4-man teleporter ships like the Basilisk, unless you have a level 3 teleporter and no Zoltans in your boarding party, it is not recommended to beam in a full party, as you'll have to recover them from separate rooms, meaning the teleporter must cool down twice.
Once all enemy crew have been killed, the battle is over. Winning a battle through boarding typically yields used to yield higher scrap scrap rewards than blowing up the ship, but it seems that the reward is now same or lower, especially in Advanced Edition (this change is an open topic), but there is also a chance that your crew will find either a hostage (who will then join your crew) or an intact weapon.
A very effective 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. On that note, it is sometimes best to teleport into the medbay itself, use "lockdown", and completely destroy the system; this prevents enemies from potentially escaping once the "lockdown" barrier fades or is destroyed, as they have no place to run to. However, beware of System Repair Drones, as they will repair the system regardless of whether or not they themselves are under attack, and if not destroyed quickly, can repair the system from right under you.
With the enhanced versions, when controling an enemy crew member via the Mind control bay, you can teleport it into your own ship while mind control is active. It can be a useful tactic against ships with a Medbay.
Another useful trick, if the player has a stun bomb, is to use your other weapons to damage a small (2-space) but critical room, such as the piloting system. When enemy crew rush into the room to repair it, fire the stun bomb into the room. The enemies will be stunned and your boarding crew can teleport over and dispatch two enemy crew easily. If the teleporter is upgraded, the boarders can then be retrieved, or allowed to roam the ship.
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!" While it is assumed the opponent has a Zoltan Shield Bypass in these instances, these events still occur without the Advanced Edition enabled.
The best place to face the enemy boarding party is the Medbay, provided it is online. Be careful against Mantis, as their attack bonus can still gradually wear down crew, especially if the medbay is at lower levels. The Engi Medbot Dispersal augment can also give an edge to inexperienced combatants facing off against similar species, and in general makes protecting critical systems easier instead of constantly having to flush enemies into the medbay.
Medbay alternative, Clone Bay, is a bit more radical in it's terminal anti-boarding function: if you are in danger of being overrun and expect to loose a copy of your dudes anyways, announce full-vent. Beauty of it is that non-Drone non-Lanius boarders will be so busy running around choking to death they wont have spare time to actually damage your systems. You will also want to set your Bay to 1 (or even 0 if you have Backup DNA Bank augmentation) so that fresh clones arrive later, to a clean, empty, and probably already pressurized back ship.
Cloaking will also momentarily prevent the enemy from boarding your ship, or likewise from beaming their boarders back.