Used these in a game last night and found them to be quite handy indeed.
Some small points came up:
Can you hit the same ship more than once with a tractor beam:
(a) from two different ships in the same turn - slowing it down by two speed steps (to a min of speed 1)
(b) from the same ship in two consecutive turns
We thought "yes" to both.
Also, is there any practical difference to when a navigate token is used to cancel the effect?
Either you use it immediately to cancel the drop in speed, or use it the following turn to increase your speed again.