Solving common problems of a sailboat inboard diesel engine

To fully understand the operation of the inboard engine of a sailboat, you need to study at least the basic principles. Refer to the many resources available online, a short google search will bring up many results. Alternatively, read a book that deals with the subject. In this article we will talk only about some of the more common problems you might face with an inboard sailboat engine. For other boats, the installations and types of engines are so varied that it cannot be easily summarised here. Here we will talk about the typical 15-40 horsepower marine diesel engine of boats between 8 and 16 meters.

Interestingly, most calls to the coast guard are related to inboard engine problems. This, despite the fact that marine diesel engines are quite simple in both construction and maintenance. Distracted owners however discover that there are so many things that can go wrong. Many owners treat the marine inboard engines as the engine of a car, confident that it is enough to turn the key. In reality these are not complex engines, on the contrary, they are infinitely simpler than those of a car. The marine environment, however, is hostile to everything mechanical or electrical. In addition, we use the car every day, the sailboat inboard engine is turned once in a while. During the winter it remains stationary for many consecutive months which can be damaging if we don’t adopt the necessary precautions.

Sailboat Inboard Engine
Sailboat Inboard Engine 

Common inboard sailboat engine problems

When you turn the ignition key (or push the button) I would say there are three common problems. Flat batteries, problems with the electrical system that powers the starter motor or problems with the starter motor.

Flat battery

Modern sailboat inboard engines cannot be started by hand so finding yourself with a discharged engine battery is equivalent to finding yourself without an engine. The starter battery should be completely separated from the services electrical system. A split charger or other similar solution should prevent you from accidentally discharging it. A voltmeter is enough to keep its state of charge under control, at rest with the engine off it should be around 12.6V.

A voltage below 12.2V is already a symptom of some problem with your system. You have to understand why the battery voltage drops after starting the sailboat inboard engine. It could be a dead battery that no longer holds the charge, it will need to be replaced immediately. Or you may find that someone has put their hands on the electrical system by connecting something to the engine battery that discharges it.

In some boats it is possible to put engine starter battery in parallel with the service batteries with a 1-2-both switch. You should never have the battery banks in parallel, and it is easy to forget it so always check and double check it. Only as a measure of last resort, if your starter battery is flat you can put it in parallel with your service batteries bank to try to start the engine. In fact, the services battery bank is usually much larger than the single engine start battery. Even at a lower state of charge the service bank can save us by providing enough power to start our inboard engine.

If putting battery banks in parallel is impossible, you should carry cables such as car cables with alligator terminals on board to be able to put banks in parallel in an emergency situation. On board some cruising boats there is an additional separate battery bank for the anchor windlass. Would you be able to put the windlass battery bank in parallel it with the engine battery in an emergency situation? Study your system and prepare the solution to this problem, so that you can easily intervene in case of emergency. Remember to disconnect the switch that puts the service batteries in parallel with the engine batteries, this still remains the most common human error.

Starter motor
Starter motor 

Starter battery fully charged but the engine does not start

In some situations, even with charged batteries, the starter motor does not turn or gives the same symptoms as a flat battery. In this case the most likely cause is a corroded connection. When starting the engine, the immediate peak demand for current is very significant. If the battery terminals or those on the starter motor are corroded, the current flow will be interrupted or insufficient. First of all, try to disconnect the terminals and clean them with a brush or a bit of sandpaper.

If even after cleaning the terminals nothing happens when you try to turn the key (or press the button) there is a solution you can try. It is not practicable on all engines and depends on the ease of access to the starter motor. With a large plastic handle screwdriver you can short the positive and negative terminals of the starter motor. The problem could be with the relay that is activated by turning the key (or pressing the button) and not the starter motor per se. Or with the circuit that transforms the key turn or button press in the activation of the relay which causes the starter motor to turn. By bypassing the electrical circuit from the key or button and the relay, you can start the inboard engine with just a screwdriver. Be careful, however, there will be very strong sparks and if the screwdriver is not of sufficient size it could overheat considerably or even melt. So you must take all necessary precautions, including wearing protective goggles and gloves. Let’s say that this solution should only be attempted when the alternative is worse than the risk you are running.

Battery charged, starter motor does not engage

It may happen that when you try to start the inboard engine, the starter motor turns without engaging and starting the engine. The most likely cause is some oxidation or rust on the gear at the head of the starter motor. This is projected forward on its axis of rotation and engaging with another gear connected to the crankshaft. If you have not used the boat for a long time it is possible that moisture has caused oxidation and the gear is locked in place.

A first attempt to resolve the situation is to hit the head of the starter motor with a hammer. The method seems a bit crude but this is often enough to unlock the gear and save yourself from calling a mechanic. You should then remove the starter motor and leave the pinion and gear soaked with WD40. Then clean and dry to make sure it doesn’t get stuck again.

Water / diesel separator
Water / diesel separator

The inboard engine turns but does not start

In other cases, when you attempt to start the inboard engine, you hear the starter motor working and engaging but the engine does not start. Don’t insist too long if you don’t want to drain the starter battery and be stuck. If everything else seems to be in order and the battery delivers enough peak amps, there are likely problems with the diesel fuel circuit.

Fine diesel filter
Fine diesel filter

Air bubbles

If the problem occurs at sea and the sailboat inboard engine had recently started it is likely that you have some air in your diesel circuit. This happens as diesel fuel was tossed around in the tank during navigation. This is even more common for racing boats that sail with the tank as empty as possible to avoid additional weight. Not all tanks are designed to ensure good operation when the boat is heeled.

Solving this problem is very trivial if you know where to put your hands. In fact, priming the diesel circuit is a procedure you absolutely must be familiar with because sooner or later you’ll need to do it. There is usually a 10mm bolt near one of the injectors that needs to be unscrewed. A small pump along the diesel circuit will allow you to manually pump diesel fuel along the circuit. When you see that each pumping corresponds to a leak of diesel instead of air, you can lock the bolt fully and try again. Make sure you tighten the priming bolt well, I remember a race where I didn’t tighten it. With the vibrations of the inboard engine being used to charge the batteries during the race, it unscrewed itself. I found myself drying two buckets of diesel in the bilges in strong winds, a job that I don’t wish to anyone

Very heeled racing boat
Very heeled racing boat

Problems with the diesel circuit

Sometimes the starting problem occurs after a period while the inboard engine has not been used. Try bleeding the circuit and cranking the starter motor a little longer than usual. If it really does not start, you must understand if there are problems with the diesel circuit. For example, if the filters have been changed recently you will have to prime the circuit again, quite a bit of work. You must also check the water/diesel separator for water in the circuit.

For example, if you have left the tank almost empty, condensation precipitates in the form of water drops inside the tank. You may find yourself with a significant amount of water mixed with your diesel. For this reason, make sure you leave the boat with a full tank every time you leave the boat stationary for some time. If you find water in the separator try to purge it and run the engine again but keep checking the separator. White smoke is a clear sign that water mixed with diesel is reaching the pistons.

If you have replaced the filters but they become clogged, it is likely that the bottom of your tank has a layer of sediment. If this is the case, you may be forced to have the tank emptied and cleaned, I recommend that you have someone do it with the right equipment.

Having said all this, remember that many boats have a lever to close the diesel circuit for use in the event of a fire. It is quite embarrassing to call a mechanic just to realise that this was the problem!

Sailboat Inboard Engine
Sailboat Inboard Engine 

Problems with the alternator belt

It may happen that once the engine is started, the warning light that indicates the state of charge of the batteries does not go out promptly (even if you accelerate slightly). If so, then you must stop the engine immediately because the problem can extend to the cooling system. The problem may be with the distribution belt, the only belt on a sailboat inboard engine. This ensures that the motor shaft rotates the cooling circuit pump and the alternator. Regularly check that the belt is tight and that the batteries are properly charged when the engine is running. In fact, if the distribution belt is not tight or broken there will be problems. The pump that draws seawater for cooling will not turn, the engine will overheat quickly and you could damage it. This is in addition to the less serious problem that the alternator is not charging the batteries. As soon as the inboard engine is turned on, you should always check that the exhaust spits out water happily.

The seawater for the cooling circuit is drawn from the sail-drive.