Boat Diesel Engines

About boat diesel engines and how do diesels work? The marine diesel engine is the main propulsion source for most cruising and sailing yachts as well as motorboats and power vessels. This page will cover the basics so that all the subsystems are in context.  A boat diesel is an equipment comprising several subsystems to operate. In case you didn’t already know the diesel was invented in the late 19th century, and named after the pioneering engineer Rudolph Diesel.  

Boat Diesel Engines

In the yacht market the two main boat diesel engines players appear to be Volvo Penta and Yanmar. Other significant manufacturers include Nanni, Westerbeke and Beta. Others include Styer, Solé Diesel, Lister Petter and Perkins.  Also I have run across BMW Marine, Iveco, Isuzu and more. In the powerboat space we also have Detroit Diesel, Cummins, Caterpillar, Mercury and MAN as well as Yanmar and Volvo Penta. Just a note that many of the smaller yacht boat diesel engines are marinized engines, often from proven industrial and agricultural engines that include Kubota, Mitsubishi and Toyota.  I have owned boats that included a very old Volvo that I upgraded to a Yanmar, a Solé Diesel on a smaller yacht, which proved very reliable, and a Nanni, also proved itself. 

Boat diesel engines now have to meet more stringent emissions standards and these include the Recreational Craft Directive (RCD), the Rhine Vessel Inspection Regulations (RVIR) of the Central Commission for Navigation on the Rhine (CCNR), the Inland Waterway Craft under Regulation (EU) 2016/1628 – Stage V, the U.S. EPA Domestic Marine Tier 2 and Tier 3 standards and in many cases the International Maritime Organisation (IMO) standards. This has driven changes in how boat diesel engines are designed.  

Boat Diesel Engines Basics

Boat diesel engines work on the principle of compression-ignition, where air is compressed to a point where fuel combustion will occur spontaneously when fuel is introduced. This deals with 4 stroke compression-ignition engines.  The 4-stroke cycle comprises the air intake, compression, power and exhaust cycles. But you knew that already didn’t you?

What is the Air Intake (Suction) Stroke? At (Top Dead Center) TDC the inlet valves open and the air required for fuel combustion is drawn in via the air filter or the turbocharger if installed, as the piston moves downwards.  At the bottom of the stroke, Bottom Dead Center (BDC) the inlet valves close. 

What is the Compression Stroke? In the compression stroke the piston moves upwards to compress the air, and raises the temperature within the engine cylinder, typically to around 550ºC.  At just before TDC fuel injection takes place and after an interval ceases. 

What is the Power Stroke? When the fuel is injected via the fuel injectors, it ignites spontaneously, and once ignited, increased pressure is then generated in the cylinder, driving the piston down to BDC. 

What is the Exhaust Stroke? At BDC the exhaust valves open to expel the exhaust gases, and at the end of the stroke the valves close at TDC.

Boat Diesel Engines

The principal parts of boat diesel engines are the following, each should be looked at as separate subsystem and each requires different maintenance and troubleshooting tasks. Eventually the collective sum of all these sub systems provides the motive power for your boat. While I may sound simplistic so many boat diesel issues are because one or more systems are not looked after. Far too many people view boat diesel engines as an install, operate and forget bit of equipment, with an attitude of “when it breaks I’ll fix it” which can have catastrophic consequences. Imagine if the aviation industry had that attitude?

The Fuel System. The fuel system is the one causing so many problems, either through lack of maintenance, or use of poor fuel and so on. Understand the basics and you will have less problems.

The Air System. The air system is crucial to good performance, and unlike their land based cousins, clogged air filters are not the prime cause of problems afloat, however issues arise in lack of fresh air supply and so on.

 The Lubrication System. The engine lubrication system is crucial to the efficiency, cooling and longevity of the engine. Many people will take their vehicle for regular service and oil change but do little for the boat engine.

Boat Diesel Engines

The Cooling System. The critical system, where the boat is afloat on a mass of coolant yet issues arise constantly. From the seawater system to the closed freshwater system, they need regular maintenance.

The Exhaust System. Often overlooked, and not looked after or cause of major issues when improperly installed, in particular wet exhausts.

The Starting System. Probably the most common boat diesel engine system with issues. Either it can’t start, won't start and this will all be blamed on boat batteries and starter motors and so on. There are a number of factors to consider. Check out Part 1 about the Boat Starter Motor. Also read Boat Starter Motor Part 2.  Now all about Starter Motor Maintenance in Part 3.

The Boat Charging System. Most boat diesel engines rely on the boat engine charging system to recharge its own dedicated start battery. On most boats it is relied on to charge the whole boat battery system. In many cases the engine alternator is the centre of the boat power systems. What could go wrong?

The Instrumentation System. Normally a simple system of meters for monitoring the basics, from engine speed, oil pressure and water temperature, with associated alarms for out of tolerance conditions.

There is much to consider with boat diesel engines and Boat diesel engines need care and attention.