That's a lot of sitting. There are three types of smoke: White, which is unburned, vaporized fuel. Black, which is partially burned fuel. And blue, which is engine oil. One other possibility is steam, which is white, and is water/coolant vapor. I suspect your "grey" smoke is engine oil and that would concern me if I were you.
"Rebuilt engine" is a relative term and can mean 1,000 things to 1,000 people. If you have a bill of services or materials, that is good but probably doesn't tell the whole story of what was or wasn't done during the "rebuild" and certainly doesn't shed any light on the expertise of the people involved in doing the work.
The fact that the boat did a 100 mile trip with a good report at the end would indicate that the engines are probably sound and that, perhaps, your "grey" smoke is occurring because the piston rings and cylinder liners may have pitted a bit while sitting and may wear back in with more use.
Why both engines? Well, they were both probably set up by the same guy and, like I said, you don't know how good he was/is at tuning a Detroit Diesel. I don't know the 53 myself so there may be some things that marv could cover with you that are specific to those that I may be unaware of... having only experience with the 71, 92, Series 50 and Series 60 engines.
Here's a story I heard while attending my first 71 tune up class over 35 years ago:
There's an American bus company called Greyhound that used Detroit Diesel engines exclusively for many, many years. Their engines always had a distinct sound while sitting at idle and, at one time, almost anyone in America was familiar with the "rump-rump-rump" of the bus while it sat at stop lights and bus stops. That sound was the "hunt" we are talking about now.
When the tune-up instructor mentioned that distinctive sound during his explanation of how the buffer screw worked on a Detroit Diesel, every mechanic in that class nodded his head that he had heard it but none of us knew why it was that all of those engines made the same sound.
The instructor went on to explain:
Someone in the bus company discovered that, other than compensating for the governor "hunting" at idle, the buffer screw really had no effect on how the engine performed. They also discovered that the use of the buffer screw caused the engine to use a small additional amount of fuel keeping the idle stable. So, they reasoned, if they didn't use the buffer screw, they could save a considerable amount of diesel, company wide, since all of the buses spent many hours per year idling in terminals and rest stops. We're talking many thousands of gallons per year.
They contacted Detroit Diesel and asked if eliminating the buffer screw would cause any harm to the engine. Detroit Diesel answered no, it would not. So, a corporate decision was made to eliminate them and subsequently, a whole continent got used to the sound of the Greyhound bus sitting and grunting and "hunting".
That was when I learned that you could actually have a smooth idling engine, without the buffer screw, if you "ran the rack" correctly and used matched sets of injectors. All the time I spent tuning those babies up, I strove to have a smooth idle prior to setting the buffer. I was usually successful and would only need to "buffer" a small amount to compensate idle fuel delivery from each injector. Obviously, the old Grayhound mechanics didn't use matched injectors or pay close attention to the rack settings.
So, things to look for are;
Exhaust valve adjustment. Improperly set valves could cause this particularly if the adjustments are different for several cylinders.
Injector height. Same thing as exhaust adjustment. If the injectors were timed "willy nilly" and aren't all the same height, then the ignition timing will be off and the idle won't "settle down" properly.
Do these engines have buffer screws and are they set correctly? I have found many to have loose jamb nuts and backed off.
Are the injector sets matched for fuel volume? If the engines were rebuilt, then they would have installed reconditioned injectors and may not have gotten matched sets. There are tags on each injector that should list idle fuel volume. Depending on where you are and who you are dealing with, matched injectors can sometimes be hard to obtain.
Are the components of the governors within wear limits? Even the slightest amount of wear on fuel control components can make it difficult to get a "good tune" on one of these two-cycle engines.
Bottom line? you need to get a bit dirty or have a qualified Detroit mechanic look those babies over. Hope this helps,
jimmyd