Book about wooden sailing ships with FTL

I read this book in the 1990s; I’m trying to remember the name. It seems like a the start of a series I’d like to read the rest of now.

It’s a space opera about humans traveling our galaxy with giant wooden spaceships powered by special FTL engines. Later in the book the FTL engine breaks open it’s powered by living mercury (silver and viscous). They also use telepathy to communicate across the galaxy. The book shifts to be an alien encounter in the middle of the book. The aliens have come from another time or dimension to see if they can find the cure for a disease that is killing them.

At the start of the book, there is only one non-human character, he has more than four appendages. He doesn’t like to be touched and has a the ability to speak and sing in harmonies. There are more of this race, but we don’t meet them in the this book (and I never read any more of the series). The book talks about how this was one of the few alien species that humans didn’t wipe out, because they were great negotiators. Humans have transposed symphonies to be played by a single multi-legged alien.

Plot

The crew of the a large wooden ship is escorting a group of human rebel prisoners back to talk with the human galactic parliament. A wizard (male) uses his own magic powers to fly a small spacecraft to land on the larger ship. His small ship just has short range capability. The crew thought he was working with the rebel prisoners so they made him use all his power to get to the large ship.

The rebel prisoners escape and try to take over the ship. A fight breaks out between the rebels and the ship crew. This is when the FTL engine is cracked open. The rebels had their own wizard, but she was hidden with the regular prisoners. She attempts to use the FTL engine directly and is absorbed by the mercurial power source. Later we learn she is still alive and bonded to it.

But in the middle of the battle, some alien ships, shaped like spheres, show up and take a subset of the crew, the rebels, the male wizard, and the multi-legged alien hostage. They go through a series of environments where the aliens interrogate them for the cure to their disease. Each environment is meant to make things of that size comfortable.

The first area looks like a Star Trek holodeck that isn’t running, just a big empty room. The aliens want to look at them from an atomic perspective.

The next environment looks like the interior to the original ship, but it’s slapped together like a terrible random level in Diablo 2. There are random rooms with window and doors to nothing, hallways that go nowhere. And every room has piles of dead bodies that look like a Salvador Dali painting. Heads, legs, arms, and torsos in every and all configurations, lots or none of them, sticking out of every end. Some of the characters get cold or flu like symptoms. They aliens were interrogating them at a cellular level.

Then the characters are put into a giant indoor park. Smaller alien spheres come by to unspool the characters’ brains to see if they know how to cure the disease. The humans run away. But the wizard when he is caught asks the sphere to tell him about the aliens. The sphere unspools the wizard’s mind then refills it with all the information about the alien race. This is when we learn about the motives of the alien spheres. The wizard becomes a savant. His brain is too full, he can’t deduce any new information, but he can tell the other characters any facts about the aliens or their space ships.

One of the human protagonist lies to a sphere that he has the cure. This creates confusion and allows the remaining humans to escape on a commandeered alien ship (also a sphere, those aliens loved their spheres) using the wizard’s information to rewire it to work for them. The multi-legged alien rigs up ropes so they can fly in a ship without artificial gravity. After they escape, they realize they they need to prepare for when the alien spheres come back again.

Additional Info

• They have fire suppression on the wooden space ships by sealing the fire in force field type bubbles that wont let oxygen in and the fires will die out. The good bubbles will still let human escape so they don’t die of asphyxiation.
• Their wooded ships can’t sail past the edge of the galaxy. If a ship leaves the galaxy it just floats away, like a boat without oars. They are impressed that the invading alien ships can travel beyond the edge.