Beyond routine maintenance


Ensuring that your diesel engine works when it should is the goal of any engine maintenance program. We have to use preventive methods, however, that reach deeper than routine practice. There are several points to consider beyond routine maintenance to ensure a diesel is properly commissioned for sea.

Engine additives

All diesel fuel contains some water, and microbiological organisms (fungi, bacteria, and yeast) live and reproduce at the fuel/water interface. Hormoconis resinae (a sac fungi member of Ascomycota phylum), is the most notable contributor to the creation of biomass sheets inside fuel storage systems (sludge formation).

Adding enzymes to a diesel tank to deal with microbial fuel contamination.

The indirect and negative effects of more than 250 types of microorganisms living in diesel fuel are numerous: microbial influenced corrosion; organic acid accumulation; fuel-line flow restrictions; filter plugging; engine wear; corrosive deposits on metal tanks, injectors, and cylinder linings; and changes in fuel properties (pour point, cloud point, thermal instability).

A common and unavoidable source of water in fuel tanks is atmospheric condensation, caused by changes in temperature and humidity, via the tank’s vent. Warm moist air stabilizes in the tank by day, while H2O sweat forms on tank sides above the fuel level as the ambient air cools at night. Water droplets sink beneath diesel fuel and stay. The easiest way to minimize H2O condensation in the tank is to keep the tank full with fuel, minimizing the airspace.

Obviously, diesel engine performance becomes unreliable when the pickup-tube sucks water or biomass sludge… water won’t ignite in the cylinder, and sludge clogs fuel lines, filters, and injector nozzles. Either way, lack of atomized fuel in the cylinder means combustion doesn’t take place. Water separating fuel filters are extremely helpful to isolate water and capture sludge, but the water must be drained from the filter frequently.

If a boat hasn’t been used, and suddenly sets out in rough conditions, sludge and water will slosh inside the tank and get sucked into fuel lines in a heartbeat. This scenario is a common cause for engine failure. Fuel lines must be cleared or filters changed multiple times. Worse, a cylinder may not fire because an injector nozzle is plugged. Rather than set-off on a voyage aboard a yacht that’s been sitting idle, it’s prudent to treat the fuel first.

A fuel management methodology that combines mechanical and chemical treatment of stored fuel is most prudent. Mechanical fuel polishing (when properly administered) will completely rid your contaminated tank of harmful microorganisms. Two types of chemical additives (biocide or enzyme) are available to treat and manage microbial contamination of diesel fuel.

An antimicrobial biocide may be added to diesel fuel to kill microorganisms. When biocide is introduced to fresh fuel, biomass particulates (sludge) won’t grow, or may be scarcely visible. If a tank shows signs of sludge contamination, the biocide “kill” will dramatically increase dead-biomass particulates in the tank resulting in frequent filter changes. Heavily contaminated tanks should receive a combined treatment of fuel polishing and biocide to completely ensure stored diesel is ready for use at sea.

Enzymological treatment is an alternative to biocide use; it offers biochemically engineered results that improve diesel engine combustion, and minimizes harmful exhaust emissions, while purifying microbial contaminants in fuel tanks like Hormoconis resinae. Uniquely selected enzymes promote specific molecular reactions within diesel fuel, and within its combustive-phase. Lyase enzymes begin by cleaving hydrocarbon substrate bonds in an attempt to increase diesel’s burn rate; while isomerase and transferase enzymes then rearrange hydrocarbon molecules by attaching atmospheric oxygen to a weakened carbon structure to hyper-oxygenate the fuel at the point of combustion. In effect, this decreases fuel consumption (comparative reports of enzyme use between similar engines show a 10 percent improvement of fuel economy). Enzymes also reduce the emission of harmful NOx acid-rain gas and diesel-soot because ignited fuel isn’t being forced past the exhaust valves. Emission test data indicate that enzymes successfully weaken Polycyclic Aromatic Sulfur Heterocycle (PASH) molecules in diesel fuel to reduce SO2 in exhaust, showing that enzymes render diesel exhaust safer — a glimpse of good news since the World Health Organization and IARC (International Agency for Research on Cancer) classified diesel exhaust as a carcinogen in June 2012.

The raw water strainer and seacock. Should the strainer fail the vessel can rapidly flood.

Wet-exhaust circuits and components

The most common diesel exhaust system installed aboard yachts is the wet-exhaust, where raw seawater is pumped through a heat exchanger that indirectly lowers the temperature of coolant flowing around the engine block in a closed circuit. The raw water then cools and mixes with exhaust gases to finally be pushed out of the yacht by built-up exhaust pressure.

A properly plumbed wet-exhaust circuit should not sink a yacht, nor induce seawater to back-siphon into the engine’s cylinders.

Located at least six inches above the vessel’s waterline at operating heel angles, a siphon-break should be incorporated into the wet-exhaust circuit to prevent back-siphoning. Two connection locations are found: 1) between the heat exchanger and the exhaust elbow; or, 2) between the water pump and heat exchanger; the former being more widely accepted.

Not all siphon-breaks are equal; some require maintenance, and some don’t work under certain circumstances. Aside from numerous valve fittings designed to break a siphon when needed and keep water from splashing out, the open-tube led high above the waterline is the simplest effective option. Anti-siphon valves with rubber seals tend to stick, and mechanical valves with springs and balls bust-open and scatter pieces.

Another valve-type that leads “vented” water directly overboard or to the bilge seems innocuous and convenient, but can become dangerous if the end of the discharge pipe drops below bilge-water, or is submerged beneath the waterline — creating a siphon that the unit should prevent. These vents are often installed in confined dry-areas near electrical equipment or inside cupboards where you’ll add insult to injury if water splashes out of the vent. Thus, two workarounds must be performed in the field to create the siphon-break for this valve-type: 1) add a small air hole at the top of the discharge pipe (may still leak water); or, 2) run the discharge pipe into a larger diameter pipe with airspace between the two.

Raw-water strainers are located before the water pump to strain-out sucked-up debris. The strainers are often located below the vessel’s waterline and sometimes have their lids held down by hand-tightened wing nuts. If this is the case, the watertight integrity of the vessel is sustained by a couple of wing nuts! If proper tension isn’t applied to these wing nuts, or should any part of the strainer corrode and fall apart, the strainer can leak enough water to sink the vessel. The easiest precaution is to close the raw-water seacock when away from the boat, or plumb the strainer above the waterline if space is available and the hydraulic head doesn’t overwhelm the water pump.

Some engines have zinc anodes mounted within their wet-exhaust circuit to manage electrolysis in that specific system. These zincs should be replaced at least annually or corrosion will attack the heat exchanger or the exhaust mixing elbow. It is common for the mixing elbow to corrode and blow exhaust into the engine room or become plugged. A plugged exhaust mixing elbow will create excessive back pressure and hinder an engine’s performance.

Corrosion inside the heat exchanger will eventually cause an engine to run too hot — a problem that must be addressed soon or the engine’s life will be shortened. If the outlet pipe feels warm, then water flow is good and heat is exchanged properly. If the outlet pipe is hot, then water flow is poor. If the outlet pipe is cold, then water flow is good but the internal piping of the heat exchanger is likely corroded. Failed heat exchangers should be replaced rather than repaired.

Raw water that exits the exhaust mixing elbow flows into a water-lift which muffles engine noise and collects raw water that wasn’t forced out of the exhaust port after the engine was shut down. Acting as a reservoir, the water-lift’s volume must be sufficient to collect all non-vacated water; otherwise the engine cylinders may become submerged by back-wash. Excessive back-pressure in a wet-exhaust system may be caused by an improperly specified water-lift that constricts free flowing exhaust, thereby restricting the intake of fresh air required for diesel combustion. If an engine seems starved for power, check the exhaust circuit for blockage including the possibility that the water-lift unit was improperly specified.

Water pump impellers should be replaced annually, and spares should be carried. Impellers still function even with one blade intact. Visually inspect the flow of water spilling from the exhaust port every time the engine runs to establish a general feel for normal flow. If the flow decreases at any stage, it’s likely that impeller blades have broken off. Change the impeller immediately at any sign of failure. Water pumps themselves may fail once or twice during an engine’s life. The first sign of a failing water pump is water leakage from its shaft-seal on the back of the pump. Change the water pump at the first sign of leakage.

Ancillary equipment, such as a dripless shaft-seal, may require a water-injection hose to be connected to the wet-exhaust circuit. Depending on the manufacturer’s installation guide, a T-connection is made in the raw-water circuit to create a positive flow of water to lubricate the shaft-seal while the engine is running. Because the seal is connected to the stern-tube below the waterline, a back-siphon will be induced if a siphon-break isn’t installed appropriately in the circuit. Typically, the T-connection must be located “up-stream” of the siphon-break, and common sense dictates that the T and break should be located between the water pump and the heat exchanger, not between the heat exchanger and the exhaust elbow which is the widely accepted plumbing installation on yachts. If a professional installer of after-market ancillary equipment isn’t careful, your installed dripless-seal can put seawater in the engine cylinders.

Sea trial

The sea trial for me is a practical and analytic evaluation of a vessel’s operation in the open ocean, where the organized testing of each system and the co-dependence of systems is the only way to ensure a yacht’s seaworthiness. The sea trial however, is a double-edged sword because you ultimately have to be at sea to identify what works and what doesn’t — many problems with yachts just don’t occur at the dock, they only haunt us “out there.” Hopefully, any problem with a yacht, especially with its diesel engine, that makes it a challenge or danger to operate can be identified and fixed at sea. It’s the continuous operation of an unproven vessel at sea under “real” conditions that allow problems to rear their ugly head.

A proper sea trial is more extreme than a yacht survey. Brokered yacht sales will often evaluate a yacht underway for an hour or so in protected sea and wind to check basic operation (Does it power on/off? Do the gauges work? Does the engine show signs of leaking oil? Etc.), but this effort barely scratches the surface to uncover complex problems related to a vessel’s operation. Yacht owners and captains are left to prove a vessel themselves. I’ve seen many new or re-commissioned yachts fail during offshore sea trial conditions following a “good” survey report; for example: falling off a wave as you normally would at sea and hearing each bulkhead separate ever-so-slightly from bow to stern as the hull twists; ball-bearings falling out of mainsheet blocks; seized engine due to improperly plumbed raw-water circuit; clogged fuel system due to dirty fuel; improperly specified propellers; high-water alarms and pumps that don’t activate; etc.

Every change to an engine’s components or maintenance protocol (even by marine service professionals) should be scrutinized so the change-effect to the vessel’s operation will be understood, and subsequently tested for system-integration compatibility during a sea trial. By taking time to validate and understand the service work performed, we take steps toward a sea-ready engine system.

Paul Exner owns Modern Geographic Sailing Expeditions ( and provides coaching and consulting for offshore sailors. He has logged 22,000 miles teaching sailing aboard his Cape George 31 cutter Solstice.

By Ocean Navigator