It just so happens that over the last 2 winters I've been burning a scrapped wooden boat in my fire stove so I know how much fire you get out of a boat.
The boat in question was a 15 meter long oak on oak boat. It lasted 2 winters for 2 homes with a fire stove, and we didn't even burn it all.
There's an amazing amount of wood in a boat, particularly around the keel. The framing, keel and other structural elements are made of really heavy timber. On top of that good wooden boats are (were..) built of slowgrown oak that is incredibly dense. One large piece would last for 6-8 hours in my stove and give off a lot of heat.
It doesn't seem unlikely to me that a 25-30 meter boat would last for 15 years if you the goal was primarily to keep the fire going, only getting it really big/hot when needed.
"It just so happens that over the last 2 winters I've been burning a scrapped wooden boat in my fire stove"
Well played, sir.
Definitely (but not as well as this[1], which is pure gold). I left it open that I could be wrong, and I'll graciously accept the word of someone with real boat burning experience that my expectations are way off. ;)
1: https://news.ycombinator.com/item?id=35079