Getting SPECIFIC and ACCURATE answers to your questions would more likely require a really good fortune teller than a diesel mechanic. The variables involved are truly immense. But, I will try and paint as rosy a picture as I can with the small little crystal ball that I own. Be aware though that IT was made to make music, not predictions
Question 1 is really a tough one. These engines were famous for getting over 1,000,000 miles in trucks. That's where most of my Detroit Diesel experience is centered. However, in a boat it's a bit different because that duty has been equated to running uphill with a full load at all times. There is no "coasting". So, the equivalent 25,000 to 30,000 hour interval between overhauls cannot reasonably be expected. Someone with more experience with these engines in marine application may be able to narrow it down a bit more for you but my guess would be that a realistic expectation would be somewhere in the neighborhood of 8,000 to 10,000 hours best case scenario. This would include, of course, EXCELLENT maintenance and care.
Question 2. Again, my experience lies primarily with on highway applications so this is definitely a "hipshot" from me. I believe that there is a fuel calculator somewhere that takes many parameters into consideration but I don't know how to access that for you. The first thing to know is that these engines are not very "thermally efficient" as compared to today's diesels and, even in their day, were considered "big eaters". I believe the last inline block was produced in the early 1980's so you are dealing with some old, albeit RELIABLE, technology. A 6 miles per gallon average would have been considered "good" back then. Of course on a boat, you can pretty much DOUBLE that calculation to the negative and I would think that, even at moderate cruising speeds, you might only expect 12 to 14 gallons per hour at best on a boat of your size. Like I said, others may have more accurate assessments for you but that is mine.
Question 3. Here's where I would put my opinion right up there with pretty much anyone. Anything to watch out for....
YES! You may fall in love! Yes, L.O.V.E.! That is, if you can get past the noise, the leaks, and the appetite, You will find that the term "bullet proof engine" was coined on the Detroit Diesel 6-71. These things will take more abuse and mismanagement than any other engine I've ever had the pleasure of being around. They are truly hearty "souls" that if, once started to get you away from the dock, they will get you back. Time and time again.
About the only things that they crave besides fuel is clean filters. The fuel system MUST be looked after or you will surly have trouble. The injectors will tolerate NO WATER so buying good quality, CLEAN #2 diesel is a must. Maintaining your water separator which may or may not include the primary filter along with the secondary filter is key to trouble free operation. There are gauges and electrical switches that can and should be utilized to flag clogging filters. So, with proper oversight, that should not be an issue.
Proper fuel pressure is maintained by a little device known as the "restricted fitting" in the fuel return line. Find it and know what it does as well as keep a spare on board. They DO wear out. Maybe not in your lifetime but if you are into spares, like me, a replacement fuel pump, filters, injector, and a restricted fitting will get you home.
There are "built in" leaks. These are the blower box drain tubes. Locate them and check them often. They MUST be free to allow air to escape the air charge box so that it does not fill with oil and damage the cylinders. On vessels, these drains are usually put into a container of some sort and it is much too easy for a skipper to forget about. DON'T.
Keep this in mind no matter WHAT you operate. 70% of ALL ENGINE FAILURES begin in the cooling system. A neglected cooling system will bring you trouble fast. The 6-71 is no exception. It will not tolerate being overheated. NUFF SAID there.
Learn to check your injector timing and valve lash. You don't need to become a DD tune up whiz but you should know how to check for trouble. The shop manual for these engines is still easily obtained and you should keep a copy on board along with some basic "special" tools for doing this "homework" unless you have a proven, reliable Detroit mechanic on retainer. It could literally save your life.
The way this baby uses her fuel comes down to this: You buy a gallon of diesel. You fire her up. She gives you back "motion on the ocean" and a BUNCH of heat. It breaks down like this;
1/3 of the heat is moving the boat
1/3 of the heat is being taken from the engine block through the cooling system
1/3 of the heat is being swept away from the engine through the EXHAUST.
The exhaust.
As you can see, heat rejection for your new baby is done, in large part, through the exhaust. Make sure it is adequate enough to do the job. Also, keel cooled engines exit the cooling water out the exhaust as well so, on a boat, the exhaust is VERY important. Check it out thoroughly or hire someone to do it for you.
That's about all I have except to say that the obvious oil with filter change is always preeminent along with "pre-flight" inspection before taking you and your loved ones or customers offshore.
I wish you luck and happiness with your new vessel.
