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Diesel Boats and Yachts: A Mechanical Maintenance Guide for Before You Hit the Water

Posted September 16, 2020 by Tyler Moore

For any boat owner, it is important to have a functioning understanding of the critical systems that keep your vessel running. To go out on the water is to have trust in the operational reliability of your watercraft and if that trust is misplaced, you may end up with significant issues that could leave you and your vessel “dead on the water”.

To ensure this doesn’t happen, we have created an essential marine diesel maintenance guide to help you navigate the critical systems within your boat. This guide will help provide boat owners with a better understanding of their vessels and guide you in performing routine essential maintenance to ensure your time on the water goes smoothly.

In particular, marine diesel engines are commonly seen as workhorses that can accumulate tens of thousands of hours and be seen as “just breaking in”. Diesel engines are the engine of choice for many tugboats, trawlers, yachts, ships, and more. Diesel is energy dense in nature, allowing users to store more energy in less liquid volume for power-hungry vessels without requiring a larger fuel tank.

Because of their reputation for reliability and longevity, it is no surprise that most commercial and fleet crafts are powered by on-board diesel engines.

To best maintain reliability and engine longevity, proper essential maintenance must be done to ensure that points of failure are kept in-check, as to not cause costly engine failure, or worse- leave you stranded on the water.

Marine Diesel Fuel System Maintenance

The fuel system is arguably the most critical component in your boat. Without a functioning fuel system, your vessel would be inoperable. The fuel system is responsible for managing and moving fuel from the fuel tank to the engine, providing the engine with a steady flow of clean fuel.


It is important that before you go out on the water, that you make the effort to check all visible fuel lines for any damage that may be of concern. This includes any leaks, cracks, chaffing, or other visible damage in the fuel lines. Damaged fuel lines may be of working order; however, this wear and tear could become detrimental to your craft should it become something more.

Next, you should check the condition of your primary and inline fuel filters, cleaning and replacing the filter elements as necessary. Clogged filters can not only reduce fuel flow to the engine, but by taking a look at your filters and being mindful of filter change frequencies, you can gain a better understanding of the quality and condition of the fuel within in your tank.

In your diesel fuel tank, you should be sure to keep it “topped off” as to reduce headspace for condensation and water to develop within the tank. It is also important to use a fuel additive/stabilizer, such as AFC Fuel Additives, to improve performance/efficiency while ensuring the fuel doesn’t go bad from sitting for extended periods of time.

Diesel fuel is especially susceptible to the development of "algae" or "the diesel bug", which is microbial growth that proliferates when water contaminates the fuel within a tank. This contamination can become detrimental to fuel quality and result in engine failure when the fuel falls out of OEM recommended fuel quality standards.

Various types of visible contamination within a diesel fuel tank.

We recommend for boaters to test their fuel periodically. For quick results, we suggest using FUELSTAT to get test results in as little as fifteen minutes. By understanding the composition of your fuel through fuel testing, you will gain an in-depth understanding of your fuel quality and if there are any concerns around contamination.

FUELSTAT fuel testing kit, which provides results in as little as fifteen minutes.

If your vessel is equipped with a fuel polishing system, which removes water and other contaminants from your fuel periodically, it is important to check/replace the filters and drain the fuel/water separator.

Diesel fuel polishing system with fuel/water separators in a marine installation.

Fuel polishing systems are used to mitigate fuel contamination, and routinely cycle through the stored fuel in the tank as it filters out dirt, sludge, water, sediment, and more. Fuel polishing systems are a great addition to any diesel vessel, as diesel fuel quality in marine vessels are a large concern in the industry.

It is also important to check the functionality of your fuel injectors, as recommended by many OEMs. Ensuring your fuel injection timing and fuel injection spray patterns are where they should be is critical to keeping the engine running as intended.

Marine Diesel Engine Maintenance

Fluids in the engine and transmission should be checked regularly. While checking fluids, it is important to not only check that the fluid level is where it should be, but also the fluid itself should be checked for irregular inconsistencies that may be a result of bigger internal issues.

Engine room in a marine vessel featuring multiple diesel engines.

Fine metals in your engine oil or discolored coolant can be symptoms of internal mechanical issues you may not be able to diagnose yourself. By keeping yourself familiar with the characteristics of your engine and transmission fluids, you become more likely to notice issues that may arise so that you can address them in a timelier manner. Developing these maintenance habits may reduce the likelihood of a catastrophic engine component failure.

Be sure to check your water intake/sea strainer and ensure that it is free from debris that can disrupt the water intake from operating as designed. This water intake system is designed to keep the engine from overheating and is a crucial part of the mechanical operability of your engine.

All belts should be inspected for wear, damage, or looseness that could lead to belt system failure. Be sure the belts are tight so that they do not slip off while under operation.

Flame arrestors, which visually look very similar to the air filters of the automotive industry, should be replaced as necessary to ensure that optimal air flow is able to be achieved. For replacements, only use Coast Guard approved flame arrestors which meet marine safety requirements.

Your propeller should also be regularly inspected for damage. Bent blades, chips, cavitation, and cracking are common as propellors often come into contact with rocks, seabeds, debris, underwater structures, and other marine surfaces during regular use that may cause damage. Damaged propellers can drastically reduce efficiency or suffer complete failure and become inoperable while on the water.

Visible cavitation on a propeller blade that can reduce efficiency and lead to further damage.

While inspecting the propeller, it is important to clear any debris, such as fishing line, that may be caught or wrapped around the propeller. Oil buildup or leaks from the propeller are also a sign of other more significant issues that you may want to have checked out.

Miscellaneous Marine Maintenance

After the critical mechanical systems are inspected, you should take the time to inspect other miscellaneous components that may be failing. Zincs, also known as sacrificial anodes, are an often-unsung hero in protecting critical metal components on your vessel.

Zincs are used to disrupt the conduits established when you have two different types of metals that are electrically connected and in contact with seawater. These electric metallic connections leave any underwater metals very susceptible to corrosion.

Two zinc anodes around the propeller assembly to reduce corrosion of critical components.

By adding Zinc, you are prioritizing the corrosion of the Zinc instead of the other metal components exposed to the seawater. Zinc anodes should be replaced as necessary so that critical mechanical components of your vessel do not experience this type of rapid corrosion.

All visible electrical connections on your vessel should be checked for corrosion on a regular basis, as electronics and electrical wiring on marine vessels are especially susceptible to this type of wear. Fuses should also be checked, replacing blown fuses as necessary. All valves should be checked to be fully functional, and operability of the bilge pump(s) should be assured before heading to the open water.

The Importance of Regular Marine Maintenance

Marine vessels, such as boats and yachts, are very susceptible to the elements without a routine and regular maintenance schedule. When neglected, failure points can develop that lead to costly replacements, vessel downtime, or worse.

By prioritizing maintenance and performing routine inspections of critical mechanical components, you can gain confidence in the operational reliability of your vessel.

Through being mindful of these possible failure points of your vessel, you should be able to solve irregularities before they turn into an emergency while out on the water.

For your use, we have developed a Marine Mechanical Maintenance Checklist PDF which you can download here and use to help guide your future marine maintenance procedures.

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