Let me begin by saying that I am not a mechanic. You should seek-out the services of a good one; but finding one is not always easy. If you are in an area with a concentration of yachts or fishing boats, then you may have a number from which to choose. Many boaters are not so lucky.
What I am about to convey is based upon my thirty years of tending to diesel engines as the owner of a cruising sail boat. I have a professional do much of the recommended servicing but I do as much of the regular maintenance as I can, and that is the point of this article. The further you voyage from your home port, the greater is the need for you to be familiar with your engine and its maintenance requirements.
Make sure that you have the Owner's Manual for your engine; and that you read it. A Shop Manual, intended for more serious repairs, is well worth the expense. It will assist a mechanic in completing his work when required.
Of course, you should have the tools you need for the job. If you do not, ask your mechanic to help you prepare a list of what to purchase. Stow them aboard in a dry, accessible place. I have a preference for tool drawers near the engine space. Square tool boxes are hard to stow on (round) boats. A set of sockets with a good quality ratchet, box-end wrenches that match your engine (standard and/or metric), a pry-bar and an oil- filter wrench should be part of your kit at a minimum.
A basic inventory of spare parts for your engine is something that you need too. My experience when cruising is that it is often possible to locate a mechanic to solve a problem if you have the parts to allow him to do so. At least you should have the following on board: alternator and water-pump belts, zinc anodes, water-pump impellers and gaskets and oil and fuel filters. Needless to say, you should have extra lubricating oil, transmission fluid and coolant aboard too. The spare parts you require will be determined by a variety of things including the availability of parts in the area in which you intend to cruise. Longer cruises in remote areas require a much more complete list including spare injectors, starter, alternator, water pump, fuel lift-pump and so on.
A Parts Manual for your engine will not only assist you in assembling your spare parts inventory but it will help you to figure out how the engine is put together.
Every operating day I inspect my engine and the engine space. I note whether there are any signs of fluid leaks. I have a heat exchanger so I check the coolant level in the header reservoir too. If there are no leaks, the coolant level should remain constant; but note that the coolant does expand and that the level will increase when the engine is hot. The manual for my engine recommends checking and/or changing the zinc in the heat exchanger monthly. It is important to locate and to replace the zinc(s) as recommended in your Owner's Manual as electrical currents between dissimilar metals (electrolysis) can shorten the motor's life.
I then check my lubricating oil level; transmission oil level; and I look at the bowl of my primary fuel-filter/water-separator. If there is any sign of sediment or water in it, I drain it off. Lastly I check the tension of the alternator and water-pump belts. Make sure that you have the tools (box-end wrenches & pry-bar) and the know-how to tighten the belts too.
A job that is well within my capability is changing the oil in my engine. There is nothing that you can do that will add more to the life of your engine than keeping its' oil clean. I change the filter every time I change the oil. This is not a place to economize. If you have never changed your engine's oil before, have your mechanic do it when you can watch and have him show you the tricks specific to your motor. I ordered my engine with an oil-change pump built-in. This makes extracting the oil a cinch. There are also vacuum pumps available that make it easy to do the job too. I have one and I use it to change my transmission fluid annually so as not to get any in the bilge. Warming the engine prior to pumping the oil is an obvious help.
A proper fuel filtration system for your engine should, at a minimum, consist of a (primary) fuel filter/water-separator which is installed in the fuel line on the fuel tank side of the engine; in addition to the (secondary) one that is usually mounted on the engine. Every one-hundred hours or so, I change the element in this primary fuel filter. Changing it at sea, under duress, when the engine has either stopped or is slowed due to restricted fuel-flow is no fun at all. The ideal arrangement is to have duplicate primary filters piped in parallel. This allows for an easy switch from one to another by means of manipulating valves should one filter become contaminated.
Once a year I have the lubricating oil for my engine analyzed. Employing proper procedure in taking an oil sample is critical. My technique is to take the sample half way through an oil change when the oil is hot, circulated and I am sure that I am not getting any sentiment off of the bottom of the oil-pan from which the pump draws. Again your mechanic can offer you advice on this procedure specific to your engine.
Where I live, I buy 'sample kits' from a local lubricants plant. The kit contains a small sterile bottle into which the oil sample is placed, labels and a larger container to prevent leaks. I then drop the kit off to the lab and a report is faxed and then mailed to me. If I do it regularly, the report form has columns to show past tests and the current oil content. From the report, and the indicated contents of the oil, much can be learned about engine wear and any impending problems. Your mechanic can help you interpret the results. Most major cities have oil analysis facilities. Again, your mechanic can help you access this service.
As a yacht broker I can tell you that a written record of good engine care, backed up by reports of annual oil analyses, will dispel many engine related fears of potential buyers of your boat. This is all about elevating the 'comfort-level' of buyers. This kind of documentation is worth its weight in gold when it comes time to sell your boat.
Another form of documentation that is equally valuable is an engine log or record-book. It need not be fancy. I use a hard cover note book from Staples. In it I record oil changes, servicing, and so on, in separate sections. I note who did the work and the engine hours at the time the work was completed. This journal is a great place to record information of interest about the engine too: serial numbers, engine hours when you bought the boat, supplier phone numbers, parts numbers, you name it.
If you do not have an hour meter on your engine, have your mechanic install one: NOW. Without it, almost everything about your engine is a question mark. It's like owing a car without an odometer. At the time of sale, a potential buyer of a boat with an engine absent an hour meter is going to assume the worst in terms of the remaining useful life of the motor because of the lack of any evidence to the contrary. It will also help you in the timing of maintenance too.
What I have talked about here is not about replacing your mechanic. It's about developing an organized approach to monitoring your expensive engine, taking charge of its maintenance, doing the basics yourself and being more independent. Aren't these the principles that should apply to the maintenance of the rest of your boat too?