Rough idea for the to-hit mechanic.

A ship launching a boarding action rolls D4/D6/D8 based off the quality of the unit.  A "1" is a bungled roll and results in the troops missing the target, and tumbling off into space, never to be recovered.

A standard miss just means conditions weren't right to deploy, or whatever.

The formula is:
1/2 Die Type +1 for every inch beyond the first inch (yeah, even shorter range bands than torps).  You don't want boarders covering a big expanse of space.

AND

+1 for every difference between the attacker and defender's speed at the end of the Reactive Movement Phase.

The reason for the difference in speed being the modifier is that the speed of the attacking ship, by itself, doesn't matter a whit.  Acting in a vacuum, its troops would simply step outside and inherit its forward momentum.  But if their target is moving drastically faster/slower, it makes the action all the harder.

So a ship launching a D6 boarding unit (like the Japanese Fukuryu) would need to roll a 5+ if attempting to board a vessel 3" away if their ship had matched its speed/velocity ( 3+ 2 for range +0 for speed variation).  If the ship was moving one faster than the target, they'd have to roll a 6 to hit (3+2+1).

How crew casualties would be applied (continuing or one time shot only) to the targeted vessel, I have no idea.

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A standard boarding party is d4, while specialized units get d6 or even d8.

Then, make an "attack" just like a torpedo, except:

1) Armour is not counted;

2) Apply momentum modifiers for both the attacker and target;

2) A "miss" means the boarding party is not "fired" -- i.e., its tethers didn't attach to the target or somesuch, but they can try again next turn; and

4) A "hit" means the party managed to board the enemy ship.

Then, conduct shipboard combat during the End Phase of each turn.

Dan
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This sounds kinda neat.

It allows for some variation between boarders.  Different units/elites/whatever.

The only thing I would change (if it makes sense to you) is that a miss wastes the boarding party.  I'd treat them as expendable, just like torpedoes or missiles, and a miss sees them tumbling off into the void, having missed their intended target.  Kinda grim, but we're supposed to be depicting combat tactics of a race that is taking its first steps into deep space.  I picture guys in clumsy environmental suits, hurling themselves into the darkness in the hopes of reaching that nearby enemy vessel.

Just imagine how ruthless, and daredevil, such a military force would have to be.

Matt
PS -- I know I'm a heathen for bringing it up, but my main point of reference for ship to ship board actions (which I really liked by the way) was Battlefleet Gothic, where a ship could attempt a boarding action if within a VERY short distance of its target.  From there, factors such as size, armament and other things would determine the results of a contested roll between the two players.  But I like the approach you've introduced, because again, it separates average boarders with true specialists  I'd start with the route you've mentioned here.

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Boarding shouldn't allow for controlling a ship.  The grunts with explosive charges and rifles don't know how to drive, and aren't present in sufficient numbers to man the engines/bridge/weapons. 

Crew casualties are ticked off, and the thing's left floating dead in space -- just as it would be if hit by some of the kit described in TMW