Please allow me to rant a little bit, but also offer a systematic approach to problem solving. The inspiration ;-) for this post came from a current thread about the OP's house batteries not charging. After 10 posts, no one offered a diagnostic approach, only suppositions (and as always, quite a few extraneous posts). I posted a sequential procedure that I hope is correct and will lead the OP to find the solution.
But back to the general topic. Most of us have been burned at the hands of mechanics who simply change parts until the problem goes away. Sometimes it takes 2-3 different parts until the problem is resolved. Almost certainly 1 or 2 of those parts was perfectly good. And even worse, maybe the immediate problem was solved, but because no one understood the underlying cause of the problem, the failure happened again.
So when you have a problem whether it is engine, electrical, hydraulic or whatever, stop to think about ALL of the things that could cause the problem. Develop a list and write it down. Then think about how you would confirm or deny that each possible cause was really the cause.
Usually these potential causes will form a chain starting with where the problem is apparant (the dead house battery) and work its way back to where the situation is known to be good (the output of the engine alternator). Electrical problems are often straightforward and linear in their chain up the line.
Engine problems are more complex. An overheating engine has many potential causes, some in a chain (the raw water flow path) and others sort of one off (a fouled heat exchanger, a bad thermostat). But all have simple, low cost diagnostic steps that can be explored.
We all have horror stories and this one wasn't mine, but I was a bystander waiting for it to be resolved so I could close on the boat purchase. After replacing the exhaust elbow, the raw water pump, the heat exchanger, etc and some $2-3,000 later (on a $6,000 engine) the problem was found to be a collapsing raw water suction hose that some previous mechanic had stepped on repeatedly and crushed. The $20 hose was replaced and the engine ran at wot without overheating. A simple bucket test to measure raw water flow out the exhaust would have pinpointed this problem.
So, think about causes and a logical way to eliminate them rather than follow a hit or miss approach.
David
But back to the general topic. Most of us have been burned at the hands of mechanics who simply change parts until the problem goes away. Sometimes it takes 2-3 different parts until the problem is resolved. Almost certainly 1 or 2 of those parts was perfectly good. And even worse, maybe the immediate problem was solved, but because no one understood the underlying cause of the problem, the failure happened again.
So when you have a problem whether it is engine, electrical, hydraulic or whatever, stop to think about ALL of the things that could cause the problem. Develop a list and write it down. Then think about how you would confirm or deny that each possible cause was really the cause.
Usually these potential causes will form a chain starting with where the problem is apparant (the dead house battery) and work its way back to where the situation is known to be good (the output of the engine alternator). Electrical problems are often straightforward and linear in their chain up the line.
Engine problems are more complex. An overheating engine has many potential causes, some in a chain (the raw water flow path) and others sort of one off (a fouled heat exchanger, a bad thermostat). But all have simple, low cost diagnostic steps that can be explored.
We all have horror stories and this one wasn't mine, but I was a bystander waiting for it to be resolved so I could close on the boat purchase. After replacing the exhaust elbow, the raw water pump, the heat exchanger, etc and some $2-3,000 later (on a $6,000 engine) the problem was found to be a collapsing raw water suction hose that some previous mechanic had stepped on repeatedly and crushed. The $20 hose was replaced and the engine ran at wot without overheating. A simple bucket test to measure raw water flow out the exhaust would have pinpointed this problem.
So, think about causes and a logical way to eliminate them rather than follow a hit or miss approach.
David
....he really does care that the average boater gets the truth about our vessels. He still follows what is going on. 
