Every 90 days, check the engine air filters to see that they are clean, and disassemble them to clean or replace as necessary. Many boats have foam filters, which are very easy to clean. Just take it off and squeeze it like a sponge in some soapy water, rinse it, squeeze out all the water and put it back. If your engine has the automotive type paper filters, you should carry two spares and replace them regularly.

CLEAN OIL

Dirty lube oil in marine diesels contains the built up byproducts of combustion, which form acids that eat engines. The lube oil used in marine engines has additives that neutralize these acids, but the additives wear out. By changing the oil, you are protecting your engine against these acids.

If you have an older engine, you should change your lube oil after every 50 hours of engine use. Newer engines can go 100 hours. Comparing this to your car, at 60 miles an hour 50 hours would take you 3,000 miles, and 100 hours would take you 6,000 miles. Your boat needs its lube oil changed just as often as your car does. Always carry two sets of lube oil filters and enough oil to make one complete change. Each time you change oil, change the filters at the same time.CAPT. ALAN R. HUGENOT

Some boats do not have engine hour clocks, so use a rule of thumb to calculate the running hours. If you operate your boat two weekends a month for 6 hours each day, it adds 24 hours a month to your engine. The same schedule for four months of summer puts 100 hours on the engine, requiring only one oil change a year. For eight months of similar operation, change the oil twice a year, after four months and after eight months just before putting her away for the winter.

So change the oil in the fall, just before lay-up, while the engine is still warm from the last cruise. Change the oil and filter while the engine is hot, and then turn it over for five minutes to let the new oil come up in the engine, and shut her down for the winter. This technique keeps clean oil in the engine during the cold and prevents those acids from eating away. Waiting to change it in the spring gives the acids all winter long to feast.

If your boat is kept in water, another good idea is to go down once a month and light off the diesel, allowing it to run for 30 minutes in gear against the dock lines. This is a great exercise for a cold rainy Saturday in January -- it almost feels like you've been out cruising. But it also prevents the combustion byproducts from corroding the engine's interior, because the lube oil, with its neutralizing additives, works up through the engine.

PROPER DIESEL IGNITION

It is surprising how many boats have trouble starting simply because there is too much resistance in the battery cables. Even though a diesel operates by spontaneous ignition, meaning pressure rather than a spark plug ignites the fuel, the diesel must be turning fast enough (usually about 600 RPM) to initiate the combustion just slightly before the cylinder reaches top dead center. Often, dirty battery terminals and salt build-up from the marine environment cause the slow cranking.

Oversized battery cables go a long way toward reducing the resistance, but so does regular maintenance.

Start by cleaning the battery terminals and the connections to the engine every 90 days. This means taking the connections apart and using a wire brush or sandpaper to sand the contact surfaces. The damp marine environment allows salt to creep in and corrode the terminals, forming an insulating barrier between loose contacts. When the contacts are thoroughly cleaned and tightly connected, protect them with a non-conducting coating or dielectric grease. This allows a metal-to-metal contact where the non-conducting grease is displaced, but the area around connector is protected from moisture and salts cannot get in. The grease must be non-conducting so it won't form its own electrical pathway and short your systems. Ninety days later, wipe off the grease and clean the terminals again.