Nice!  Very detailed system, I rather like it.  However, it's very much a feature of naval battles (where accidental collisions are possible).  I'm looking for something dedicated more to space (actually, to large fantasy airships) where the ships take up little enough space in the hex that accidental collisions simply don't happen, and where ramming is a desperation tactic against a foe who's winning.

Following is a quick kitbash.  It ends up bearing only a little bit of resemblance to your system, but your system does guide mine.  The damage of this system is much higher than in yours.  Partly, this is because of the fact that ships have more "hit points" in Nova (thanks to Armor).  Partly, this is to simulate the "desperation move" element of ramming in space sci-fi.  Modifying it to reflect naval battles likely requires reducing the damage inflicted, and perhaps making the chance of collision higher (maybe by making the damage a function of the number of hits on the attack dice, which I do not do).

If you want to increase the damage (?!), then you can make the damage dice rolled equal to the full Hull Points of the other craft.  This would greatly favor bigger vessels, and mean rolling a metric butt ton of dice for damage in many cases.  If you want to reduce the damage, one option is to change the effect of Speed.  If it's a head-on collision, take the higher of the two Speeds as the damage bonus (rather than the total).  If it's a broadside collision, take the lower of the two.  If it's an aft collision, take the ramming ship's Speed minus the rammed ship's Speed, or the lower of the two Speeds, whichever produces a lower modifier.

One final note - the attack dice/damage dice system is intended to prevent having to make complicated calculations at the table, and at the same time make hits while ramming less probable.  There are probably better systems, and I'm open to suggestions.

Ramming

A ram can only occur on a phase in which the ramming ship ends the Movement Phase adjacent to the target ship, and the target ship is directly in front of the ramming ship, or in one of the "sideslip" hexes.  The ram itself occurs in the Combat Phase.  The ramming ship declares attacks normally, but as its final attack it may declare that it is ramming the target vessel.

Normally, roll one attack die (this doesn't determine damage, just whether the ship hits or not).  Roll an additional attack die if the Speed of the ramming ship is less than or equal to its Thrust (thus, it's in much more control of its maneuvers), and roll an additional die if the ship being rammed is moving at a lesser Speed than the ramming vessel.  The attack takes no modifiers for range.

Attack Modifier/+0/-1/-2/-3/-4/-5/-6
1 AD/1/1/1/0/0/0/0
2 AD/2/1/1/1/1/0/0
3 AD/3/2/2/1/1/1/0

If at least one attack die scores a hit, then the ship is rammed.  It makes no difference how many "hits" are scored.  Each ship must make Shield rolls against damage.  The base damage is equal to the Hull Points of the smaller vessel, modified by relative speed.  Thus, a 3 Hull Point vessel ramming a 9 Hull Point vessel does a base damage of 3 dice, but the same damage is inflicted the other way around.

If the ramming ship struck the fore of the craft (the FR hex face), then add the Speed of both crafts to damage.  If the ramming ship struck the side of the craft (the PP or SS hex faces), then add the Speed of the ramming craft.  If the ramming ship struck the aft of the craft (the AR hex face), then add the Speed of the ramming craft and subtract the Speed of the craft being rammed (this might turn into a negative modifier).

Example:  A ship with 5 Hull Points rams a vessel with 4 Hull Points, striking its aft (the AR hex face).  The ramming ship has 4 Thrust and is moving at a Speed of 6.  The ship being rammed is moving at a Speed of 4 and has ECM 1.  The ramming ship gets two base attack dice - one base, plus one for moving faster than the target vessel.  The total Attack Modifier is -1, so only one attack die is rolled.

If the attack hits, each ship suffers a base of 4 damage dice (for the lower Hull Point value), plus 6 for the ramming vessel's Speed, but minus 4 for the rammed vessel's Speed.  This comes out to a total of 6 damage dice, against which each must make Shield rolls.  This could easily gut both vessels.  If it was a head-on collision, it would inflict 14 damage and likely destroy both vessels.

Ram

This is a ship system that both augments the damage a ship inflicts while ramming and reduces the damage received.  Normally, Rams are mounted on the fore, but any hex face might receive a Ram (so as to defend against side swipes).  A ship might even get multiple Rams to account for multiple hex faces.  Each Ram takes up SU equal to half the Defense Factor of the vessel, and increases ORAT by the ship's Thrust Factor.

If a ship rams or receives a ram on the hex face protected and the opponent does not have a ram on that side, the ship will inflict more damage and sustain less.  For the ship with the Ram, rolls of 3 or 5 on the Shield roll inflict no damage.  For the ship without the Ram, rolls of 3 or 5 will inflict damage.  If both ships have Rams facing the correct direction, they cancel.

Putting aside all of the arguments for and against ramming I figured I would jump in 

What about something like:
(OPTIONAL)
Ships that end their movement in a hex occupied by another vessel may declare that they are attempting a Ram Attack rather than going through the usual process of moving to an adjacent hex.
Rams are Successful on a die roll of 5 or 6, modified as follows: Add a positive modifier based on the Thrust Rating of the Ship attempting to ram: for Thrust 1-3: +0, 4-6: +1, 7-9: +2, 10+: +3. Subtract a negative modifier for the ship being attacked: for Thrust 1-3: -0, 4-6: -1, 7-9: -2, 10+: -3.
Damage is always equal to the lowest current Hull value of the two ships involved.

This should favor higher thrust ships either succeeding in Ramming or avoiding a Ram attack and should favor larger/more intact vessels for damage purposes.

Just my 2 cents.
Erik

There are plenty of examples of Ramming from a variety of source material, so I am in favor of including it, but I think any Ramming rules should only be used sparingly at best and conditionally otherwise they would probably be abused. The Ramming arguments go back for generations around these parts 
Cheers,
Erik

In one of the novels written in the STARFIRE Universe, by David Webber, there was a race of aliens that had special ships equipped with a ram that rammed enemy ships causing double the damage, and then troops boarded the rammed ships.  They also had missile/boarding pods with good range.  The designers of STARFIRE later published a suppliment with these ships & the ramms.  8-) 
Whopps, gotta go; it is c7am and I need to head off to work (Public School Teacher here)
Cheers
Steven Gilchrist