Oh, the turnaround only needs to be instant instant to be guaranteed. In practice it just needs to be less than, well, however much you've accidentally time traveled, which like I said depends on how fast you're moving and how far back you send your message. If you move back ten milliseconds and the turnaround takes two, then (I'm going to refer to ansible nodes as Janes, do you mind?) B-Jane is still going to get the message 8 ms before ship-Jane sent it - as in, if B-Jane then sent it instantaneously to ship-Jane ship-Jane would get the message before she sent it, though the duration wouldn't be 8 ms it'd be something like 8/gamma.