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To keep your boat engine in good working order, it is good practice to perform all servicing activities. Regular engine servicing prevents unforeseen events and failures, also serious, that could put the season at risk or compromise safety during navigation.

Engine servicing consists of two categories: seasonal works and occasional works. Seasonal works should be performed at the beginning of the season and however at least once a year, after 100 – 200 engine hours on average. These works are, in my opinion, the minimum necessary to ensure the engine functions and avoid any unpleasant surprises. On the other hand, occasional works will be performed whenever they are required, without a fixed scheduling.

Here below there is a list of both seasonal and occasional works. Then we will talk in details about the most important ones.


  • Wash with fresh water the saltwater circuit
  • Change of engine oil and filter oil
  • Replacement of prefilter and diesel filter
  • Replacement of alternator and/or timing belt
  • Check of starter electric wiring, bulbs, sensors, spindle, ignition key, back panel
  • Change inverter oil and cleaning of heat exchanger, if hydraulic
  • Bilge cleaning
  • Check raiser (exhaust pipe)
  • Check screws tightening and, in particular, silent block, propeller shaft box
  • Air filter cleaning and/or replacement
  • Check clamps and rubber sleeves
  • Replacement of impeller, lid and seal gasket (also the screws if rusty)
  • Replace anode if a direct cooling circuit is present (to be performed once a year)
  • Spray vaseline on the connectors and behind the alternator to protect it from moisture and condensate.


  • Change coolant, check and clean exchanger
  • Clean nozzle if necessary
  • Valve clearance adjust or check
  • Remove and clean turbo and relative bearing
  • Grease leverage (replace cables if necessary)
  • Check alignment shaft-line and inverter
  • Replace sound-deadening panels
  • Bench check and replacement of alternator brushes, starter brushes and relays
  • Check shaft-line for leakage (packing seal, flusher, ring seals)
  • Paint rusty parts (if any)

Cooling circuit washing

Once the boat is out of water, salt has to be removed from the cooling circuit. Generally, this is the procedure: switch the engine on, then, rinse using the hose at medium-pressure for 10-15 minutes. Disconnect the hose, operate the engine 2-3 times to free the pipe then switch the engine off.

In case of inboard/outboard and S-drive, a special flusher can be attached to the dedicated holes to draw water or, sometimes, some gearboxes have grids and a main hole. In this case, close the main hole with a cork.

If the procedure cannot be performed from outside, operate from inside removing the water filter or a pipe and connecting the hose.

What is important is not to flood the engine when operating the hose, always turn the hose off when the engine is not running.

To optimize drying, unscrew the muffler, then drain the discharge pipe.

In cold climates, in order to avoid drainage, it is recommended to make the engine draw a mixture of coolant and water (50% each). Then, it is also important to check sleeves and hose clamps. Grease if necessary.

As for the engine gearbox, every now and then pour from inside the boat towards the outside a solution of water and vinegar to descale the pipes and reduce shellfishes. If they do not dissolve, eliminate piece by piece using a small screwdriver or thin, pointed small brushes.

Replace the impeller

Draining water from the circuit and cleaning the filter are operations to be performed to avoid unpleasant surprises the next season. A thin smear of grease will protect the new impeller and facilitate installation. It’s needless to say that it isn’t a good idea to change the impeller at the end of the season when the boat won’t be used for long.

Replace the diesel oil filter

This is absolutely necessary to avoid the formation of seaweeds that will be necessarily drawn in by the engine when it is first started, clogging all the filters and increasing fuel venting.

If during diesel oil replacement sludges or rust are found, clean also the tank and purge it with diesel oil.

At the end of summer, the season when the tank is more often refilled, it is good practice to clean the tank to avoid that sludges on the bottom, shaken by diesel oil which is, in its turn, shaken by sea during navigation, enter the fuel circle.

Replacement of alternator belt

To remove the old belt, loosen the alternator fixing bolts and the tensioner, then install a new one having the same characteristics. To tension the belt properly it can be useful, before tightening the nuts, to operate with a screwdriver on the alternator. Having limited space available doesn’t help.

Oil suction

Oil change requires oil drainage or suction from the oil pan using a manual or electric pump connected to the circuit or to the dipstick. If the oil is warm, sludges are removed more easily. The replacement of the filter is generally a quicker and easier operation. All materials must be disposed of in the dedicated bins.

Changing coolant and cleaning exchanger

Coolant (50% pure water, 50% coolant) has to be changed every 2-5 years depending on the coolant expiry date, as its properties become corrosive once the coolant has expired.

The easiest way to do it is to replace the liquid by draining it through the special draining cap. Here is a trick to speed up the operation: open the cap and the water will drain quickly. Then you can fill the exchanger tank with the water-coolant mixture.

If this is also the opportunity to clean the equipment, remove the exchanger lids and extract the tube bundle. Keep in mind the precise position of the bundle. The gaskets of the lids have to be replaced while the tube bundle can be cleaned using small brushes and oxalic acids or vinegar, paying attention to carry out the cleaning quickly.

This is the basic service to perform to sail safely: keep to it and you will serenely enjoy the season and avoid the unexpected troubles that a non-properly maintained engine may cause.

Davide Zerbinati
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