Starships flying too fast

[TomMazanec]

Muslim dominated future. 1980s (probably) novel. Starships accelerate to turnaround, then decelerate. But if they go too far they disappear.
It turns out that they are going so fast their relativistic mass implodes them into black holes (which is not how relativity works).

 

[Vince W]

This sounds familiar. I'm thinking it could be one of Bantam's The Next Wave books. Possibly Alien Tongue by Stephen Leigh or The Missing Matter by Thomas R. McDonough.