Frequent oil changes ward off breakdowns and extend the life of your boat engine(s) by thousands of hours.
Engine manufacturers typically recommend an oil change every 100 hours, and at least once a year. Even if your engine manual allows for a longer interval between oil changes, changing the oil more often will extend the life of the engine.
One hundred engine hours between oil changes is fine for gasoline engines, but diesel engines are much harder on their lubrication. You should change the oil in a diesel engine about every 50 hours. The shorter interval is especially important for sailors who use their engines for only an hour or two per outing. Dirty oil sitting idle becomes destructive to the internal surfaces of the engine. If your boat will be idle for a while — over the winter, for example — it is imperative to remove the contaminated oil and fill the engine with clean oil.
Unfortunately regular oil changes are more often the exception rather than the rule, mainly because changing the oil in a boat engine can be a difficult and messy job. While it takes only a minute to drain the oil from an automobile engine, the oil drain plug in a boat is most often either out of reach or non-existent. And even where the plug is accessible, there is usually inadequate space beneath the engine for a container to catch the draining oil.
The trick to hassle-free oil changing is to find a method that works for your engine installation. Here are some possibilities.
Through the Drain Plug
A foil roasting pan from the supermarket may fit beneath your engine to provide a container to catch the old oil. Check the pan's capacity with water first to make sure it will hold all the oil. Foil pans tend to buckle when full, so put a piece of thin plywood under the pan to keep the bottom rigid. Spread an oil-absorbent cloth beneath the pan to catch the inevitable splash or spill.
You can drain the oil into a plastic garbage bag if you band the mouth of the bag to a tin can with both ends removed. A pineapple can works well in restricted space. Hot oil tends to seep through some plastics, so use two heavy-duty bags, one inside the other.
Replacing the drain plug with a hose fitting and a length of hose allows you to "decant" the oil into any container you can get lower than the engine sump. Kits containing hose and fittings are available, or you can assemble your own with a threaded hose barb and oil-resistant hose. Be sure the threads on the barb are the same as on the plug. Clamp a second barb to the free end of the hose and fit it with a cap (or a plug). Rig a hanger to keep the end of the hose well above the oil level, except when you are draining the oil.
Where space lower than the engine is inadequate for a container, use a similar hose connection between the drain plug and a brass piston pump to pump the oil out of the engine. Mounting the pump permanently to an engine-compartment bulkhead makes oil removal as easy as giving the handle a few strokes. If you don't object to the expense, substituting an electric pump--one intended for hot oil — reduces oil draining to flipping a switch.
Through the Dipstick Tube
Many marine engines simply don't have a drain plug, or it is too inaccessible even for a hose connection. Draining the oil from these engines requires a pump to suck it out through the dipstick tube.
On some engines the dipstick tube is threaded. The supplied oil-change pump screws directly to the dipstick tube and the oil is removed with a few pulls on the handle. This works fine where there is good access. Otherwise, you can mount the pump to a convenient bulkhead and connect it to the dipstick with a hose with threaded fittings — essentially a very short garden hose. Mount a short length of PVC pipe fitted with a male hose adapter next to the pump to provide a place to "park" the hose when not in use.
Where the dipstick tube is not threaded, you have to use a pick-up tube inserted through the dipstick tube to extract the oil. Most pick-up tubes furnished with oil-change pumps are plastic tubing of unnecessarily small inside diameter. Replacing the supplied pick-up tube with thin wall brass tubing — available from most hardware or hobby stores — can changes this common method of oil removal from agonizing to amazing. Buy the largest tube that is a slip fit into your dipstick tube, typically not smaller than 5/16 inch. An 11/32 tube slides easily into my Yanmar diesel. Be sure the tube is long enough to reach the bottom of the engine oil pan. Connect it to your pump with a length of rubber hose. Again, mounting the pump to a bulkhead eases the process. If you do this, also mount a length of PVC pipe, capped at the bottom, for parking the pick-up tube.
Run the Engine
Before you begin an oil change, the engine should be hot. Warm oil is easier to suck through a rubber hose or brass straw, but the reason for running the engine is more essential than that. You are changing the oil because it is contaminated with abrasive and chemical impurities. But the dirt in oil is just like dirt in water; leave it undisturbed and it settles to the bottom. Suck out the cold oil and much of the dirt stays behind, immediately contaminating the fresh oil and defeating the whole purpose of the oil change. Run the engine to get all of the contaminants in suspension so they come out with the oil.
Use a socket wrench to remove a drain plug. As soon as the plug is loose enough to turn by hand, remove the socket from your ratchet and put it on a short extension. Used like a nut driver, this will let you remove the plug without dropping it into the container.
If you are pumping out the old oil, the most convenient receptacle is probably a plastic milk jug. The small opening will keeps the outlet hose under control, and the capped jug is convenient for transporting the old oil to the reclamation receptacle in the marina or at a nearby service station. If your pump isn't mounted to a bulkhead, wrap a thick towel around it to keep from burning your hands when you start sucking hot oil through it.
Change the Filter
Regardless of manufacturer's recommendations, change the oil filter every time you change the oil.
Virtually every other type of filter you will encounter sits vertically so the fluid it contains does not spill when the filter is opened, but for some reason that totally eludes me, engine designers mount oil filters at an angle, horizontally, even upside down. You can guess what happens when you open them. For all but the upside down variety, you can contain the spilling oil by slipping a freezer bag over the filter and unscrewing the filter inside the bag. Bag it before you break the seal.
Most oil filters are the "spin-on" variety. You need a strap wrench to remove them. Strap wrenches grip the canister when pressure is applied to the handle. If the wrench slips, take it off the filter and reverse it; it only works in one direction. Coat the gasket of the new filter lightly with oil before screwing it in place. Hand tighten it until the gasket makes full contact, then tighten it another 3/4 of a turn.
Some older engines are equipped with cartridge type oil filters. A center bolt typically holds the filter housing in place. Release the bolt and remove the canister. Empty it and discard the old cartridge. Clean the housing with diesel fuel (or kerosene) and insert the new cartridge. If a separate sealing ring is included with the cartridge, carefully pry the old seal out of its seat by pricking it with a straight pin. Coat the new seal with oil and push it into position. Reinstall the canister.
Pour in Fresh Oil
The last step in every oil change is pouring in the fresh oil. Your engine manual will give you the viscosity and API rating for the oil recommended by the manufacturer. Screw-top containers make it simple to pour the oil into the filler opening on top of the engine, provided there is ample room to turn up the container. If not, you can avoid a lot of irritation by using a funnel and a piece of hose to get the oil from the container to the engine. Rig a wire hook to the end of the hose to hold it inside the filler opening.
Make generous use of oil-absorbent pads when draining and filling engine oil. Remember that even a sheen on the water from your bilge pump discharge can cost you dearly.