When the Covid-19 pandemic first started I had lots of free time and a little money saved from Lifeguarding at the local pool, I purchased this classic 16ft 70's Tri-hull boat with a broken engine for about 1200$. I'd never worked on engines before and knew very little about their systems. Through the process of debugging and fixing the boats problems, I got to learning the ins and outs of complex mechanical system. I learned a lot about incremental testing and how to isolate problems in an unreliable system. At one point I think I had validated every component on the engine from the ignition coils, to the carburetor linkage, only to find out that the pickup stick in the gas tank was was getting blocked at higher flow rates.
About 1000$ and 500 hours later the boat was working again! I was able to sell it for 3200$ and make a slight profit.
*Not pictured: cracked exhaust valve, and many many tows back to the dock from friendly boaters on Lake Washington.