If I said that the trope of a Caledonian ship's engineer reared it's ugly head, I probably wouldn't narrow the field much.
Not Tau Zero, this was basically an instantaneous transition to light speed which everyone thought was FTL.
Got the answer anyway once you mentioned the engineer - The Mote in God's Eye
by Niven and Pournelle and sequel - the Moties try to send a ship through the Alderson Point (wormhole thingy), unaware that the other end comes out inside a sun. After several ships vanish and never return they send a slower than light ship with lightsail and laser boosters instead. Set in the Co-Dominion universe, several other writers have set stories there, and there are others not involving the Moties.
. With the Moties being blockaded into the Mote system since time immemorial (1189 CE, or much earlier) by a combination of the FTL's (Alderson Drive, but not really involving wormholery - that's a plot point for Moat
) physiological shock and the unfortunate presence of the outer parts of a red giant star to cook any Moaties who arrived there. So they took the slow route to New Caledonia and ...
Actually, given the duration of the Moatie population/ sociology problem, Niven/Pournelle's astrophysics probably don't work. Unless you add a swarm of McGuffins.
You have the conn.