So, this is what I think:
1) Boarding parties are "fired" like torpedoes at their target.
2) Range mod is +1 per 1" beyond the first.
3) Armour does not apply.
4) In order to attack, the line of sight between the attacker and the target must cross opposing broadsides (e.g., the attacker's port and the defender's starboard, or vice versa). In other words, the attacker must move alongside the target.
5) The difference in momentum between the two ships is an additional modifier to the target number:
0-2 = -1
3-4 = 0
5-6 = +1
7-8 = +2
9-10 = +3
6) A "hit" lands one boarding party on the target. A "miss" means the boarding party never made it to the target... it is gone, if not forever, then at least for the remainder of the game.
7) Once boarders have breached the hull, shipboard combat occurs.
The defending ship gets a base number of d4's relative to the number of hull boxes remaining:
1-3 = 1 die
4-8 = 2 dice
9-15 = 3 dice
16-24 = 4 dice
25-35 = 5 dice
In addition, any friendly marines/boarding parties on the ship can roll their dice as well. Obviously, the attacking boarders roll their dice.
Each roll of 4+ scores a hit on the opponent, and eliminates one enemy boarding party. If there are no remaining defenders, then each hit scores a crew casualty instead.
). Men in vacuum suits now start swarming across the tethers in both directions.