The Way Back by A Bertram Chandler.

The scene you're thinking of is:

Grimes checked Faraway Quest's descent, held her where she was, then turned the controls over to Carnaby. Williams already had the big telescope trained and focused and the picture was showing on the screen. Grimes looked at it. Yes, it was a ship all right. Graceless, broad-beamed, with a single mast, stepped amidships, a low poop from which jutted a steering sweep. Other sweeps, six to a side, were flailing at the water. She seemed to be making heavy weather of it.

The telepath is Mayhew:

"And there's that nong, sweating his guts out on his steering oar while his crew, at the sweeps, must be on the point of dropping dead with exhaustion. Damn it all, I hate to see a ship, any ship, in trouble! If only I could tell the stupid bastard what to do ..."

"You can, John," said Mayhew quietly.

Grimes laughed. "All right, all right, so that skipper's not the only stupid bastard around here. I forgot that you can transmit as well as receive. Do you think you can get a message to him?"

"I'm ... I'm trying now. I'm ... inside his mind. I don't like it much. He's terrified, of course. And it's not only a healthy fear of the elements, but also a superstitious dread . . . He didn't make the proper sacrifices before pushing out on this voyage, and he knows it. The wine that he poured out on to the altar was very cheap and inferior stuff, almost vinegar . . . And the goat that had its throat cut was diseased and no good for anything else

It looks as if the skipper of the ship has got the idea and drops a sea anchor, but in fact:

Ah, at last. The seamen, acting as one, were lifting what looked like a bundle of rags. "Not big enough ..." muttered Grimes. "Not nearly big enough ..." They lifted the bundle of rags and dropped it over the stern.

"And where the hell's your sea-anchor line?" demanded Grimes furiously. Then, just before a hissing rain squall blotted out all vision, the Commodore and his companions saw that the thing jettisoned was a man

I can't find any mention of tunnelling under city walls. Maybe this is from a different book?

The Way Back by A Bertram Chandler.

The scene you're thinking of is:

Grimes checked Faraway Quest's descent, held her where she was, then turned the controls over to Carnaby. Williams already had the big telescope trained and focused and the picture was showing on the screen. Grimes looked at it. Yes, it was a ship all right. Graceless, broad-beamed, with a single mast, stepped amidships, a low poop from which jutted a steering sweep. Other sweeps, six to a side, were flailing at the water. She seemed to be making heavy weather of it.

The telepath is Mayhew:

"And there's that nong, sweating his guts out on his steering oar while his crew, at the sweeps, must be on the point of dropping dead with exhaustion. Damn it all, I hate to see a ship, any ship, in trouble! If only I could tell the stupid bastard what to do ..."

"You can, John," said Mayhew quietly.

Grimes laughed. "All right, all right, so that skipper's not the only stupid bastard around here. I forgot that you can transmit as well as receive. Do you think you can get a message to him?"

"I'm ... I'm trying now. I'm ... inside his mind. I don't like it much. He's terrified, of course. And it's not only a healthy fear of the elements, but also a superstitious dread . . . He didn't make the proper sacrifices before pushing out on this voyage, and he knows it. The wine that he poured out on to the altar was very cheap and inferior stuff, almost vinegar . . . And the goat that had its throat cut was diseased and no good for anything else

It looks as if the skipper of the ship has got the idea and drops a sea anchor, but in fact:

Ah, at last. The seamen, acting as one, were lifting what looked like a bundle of rags. "Not big enough ..." muttered Grimes. "Not nearly big enough ..." They lifted the bundle of rags and dropped it over the stern.

"And where the hell's your sea-anchor line?" demanded Grimes furiously. Then, just before a hissing rain squall blotted out all vision, the Commodore and his companions saw that the thing jettisoned was a man

I can't find any mention of tunnelling under city walls. Maybe this is from a different book?