Select Page

The basic principle of a diesel engine is less complex than that of a petrol engine. No spark plug or ignition system is needed, making the basic diesel engine a comparatively straightforward system that results in fewer faults and has lower maintenance costs than a petrol engine.

A diesel engine works as follows. A piston traps a quantity of air in an enclosed cylinder, then rises up the cylinder and compresses the air. As the air is compressed into a small space it heats up to a very high temperature, of about 600ºC. Diesel fuel is injected under high pressure into the small space. The fuel ignites when it comes into contact with the very hot compressed air. Bang! The resulting expansion of gases pushes the piston back down to the bottom of the cylinder, creating a powerful force which is used to drive a crankshaft. As this happens, the piston is pushed back up the cylinder and the exhaust gases escape through an exhaust valve and then the process or cycle begins again. This is known as the four-stroke cycle – literally four strokes of the piston – consisting of induction, compression, power and exhaust.

Valves

In order to get the air into the cylinder and the exhaust gases out of it, a combination of valves open and close through the cycle at precisely the right moment to either allow air in or exhaust out. When they are closed the valves need to be as tightly shut as possible to prevent the air, mixture and gases from escaping at the wrong moment. Strong springs called valve springs are used to hold the valves closed; the valves are opened by other lever type components called rockers. All the opening and closing of the valves is controlled by a camshaft which is connected to the crankshaft. The correct timing of the valves opening and closing is crucial, as is the gap or clearance of the valves.

Things get a little more complicated depending on the design of specific engines, number of cylinders and variations of ways the camshaft is driven – some use chains, some use gears and some use belts. It is probably best to hang onto the principle here rather than get too bogged down in the detail.

Transmission systems

Most inboard engines of sailing vessels use one of two ways to connect the engine to a propeller, either a conventional propeller shaft with a stern gland or a saildrive transmission system. Many mid-range power boat designs have sterndrive systems:

  • Conventional propeller shaft system – a coupling attached to the rear of the engine gearbox connects horizontally to a propeller shaft. The shaft goes in a straight line at a shallow downwards angle to the stern through a stern gland and exits the hull to the propeller. The stern gland prevents the ingress of water while allowing the propeller shaft to turn the propeller.
  • Saildrive transmission system – a saildrive has a transmission system leading from an inboard engine which has a horizontal output shaft. This connects with the saildrive transmission which has a system of gears that drives an intermediate shaft vertically downwards through the hull to more gears that drive a horizontally mounted propeller. This system is sometimes referred to as an S-drive or Z-drive transmission due to the way the shaft is configured.
  • Sterndrive system – sterndrives are also known as outdrives and inboard/outboards. As the names imply, these systems have an inboard engine which connects through the transom to a drive system that drops down via a shaft and gearbox to connect with the propeller. Unlike a saildrive, sterndrives can steer a boat by pivoting, similar to how an outboard engine works, with no separate rudder required for steering.

Evolving technology and regulations

Boat engine technology and regulations have evolved considerably in recent years. It is a complex subject, made even more complex by the different marine exhaust emissions standards in force around the world, resulting in widely different rules that now apply for both commercial and leisure vessels in different territories.

As an example, at the time of writing a mechanically controlled diesel engine may comply with the EU Recreational Craft Directive (RCD), but would not meet the US EPA domestic marine standard as it does not have electronic control and a common rail fuel system. While the DIY maintenance of mechanically controlled diesel engines is within the grasp of many boat owners, common rail technology is much more complex and requires professionals to carry out the maintenance and troubleshooting of these engines.

Diesel engine technology

Common rail technology is a digitally controlled very high pressure fuel injection and sensor system. This results in greater engine efficiency and reduced emissions than with mechanical systems, which is of course of environmental benefit and therefore a good thing. However, in the event of a breakdown out at sea, this is not such good news unless you happen to have a professional engineer aboard with electronic diagnostic testing equipment to troubleshoot any problems that might arise.

These advanced engines are not DIY friendly unless you are a qualified engineer, but this is the way the world is going and if marine diesel engine technology is to survive well into in the future then it will have to meet the increasingly challenging emissions regulations needed to protect the environment. It will also need to compete with the rapid advancement of alternative technologies, in particular electric drive systems that do not produce exhaust gases or carbon emissions during use.

Wooden Hull Repairs

While wooden boat hull maintenance is mostly straightforward, it is always a good idea to take expert advice on any repair job needed doing to a wooden boat, unless you have done the job before and know what you are doing.

Boat maintenance log

Keeping a boat maintenance log is an ideal way of reminding owners what needs to be done to a boat and when. Read on for some tips,...

Sector lights, directional lights, leading lights – how do they differ?

Sector lights, directional lights and leading lights guide vessels safely through hazardous waters or narrow channels at...

Marine engine electrical system

The typical basic electrical system associated with a marine engine includes a dedicated engine starting battery, a starter motor, a charger in the form of an alternator, a solenoid and some engine sensors and instruments.

Understanding marine sealants & adhesives

Sealants, adhesives and adhesive sealantsThere is a bewildering variety of sealants, adhesives and even adhesive sealants available for...

A five day sailing cruise of the Solent, UK

Welcome to our virtual Solent sailing cruise – a five day sail in the south of England from Bosham Quay in Chichester...

Boating emergency – how to broadcast a MAYDAY emergency call

How to broadcast a MAYDAY emergency call   How to broadcast a MAYDAY emergency call if a vessel or person is in grave...

Boat gas system maintenance

There are correct types of hose for marine plumbing, sewerage, exhaust, cooling and gas and all hoses should be checked regularly for wear and deterioration.

Sailing to windward – how to take advantage of wind shifts

For most sailors, sailing upwind is the most exhilarating point of sail as you tack your way to your destination. Sailing to windward is a bit like zig-zagging your way up a mountain road through a series of hairpin bends – great fun but also calling for concentration and hard work.

Tacking a sailing boat

Tacking is the sailing manoeuvre used to change a boat's direction through an oncoming wind. Tacking a sailing boat calls...

Essential boat engine checklist

Boat engine checklist Engine oil level check Even if you have checked it previously, confirming the engine oil level is up...

Tidal terms and definitions

Getting to grips with tidal terms and definitions can seem a little daunting, even to the most experienced sailors! Here we look at some of the key terms and definitions associated with the language of tides.

Understanding your boat’s compass

Article submitted by Mike Rossiter, Certificated Compass Adjuster. Since the magnetic compass was first used by the Chinese...

Keel design – options to consider when choosing a yacht

Keel design is constantly evolving and nowhere is this more apparent than in modern racing yachts such as the Imoca Open 60...

Top 5 Reasons Why an Inflatable SUP Should Be Your Next Yacht Accessory

In this article, inflatable paddle board expert Jason Paul gives the top 5 reasons why an inflatable SUP should be your next...

Top Ten Tips For Learning The ColRegs Boating Rules Of The Road

Colregs Boating Rules Of The Road Skippers struggle to learn and remember the ColRegs Yachtmaster and Day Skipper course...

The give-way hierarchy at sea – who gives way to whom?

Whatever their size or type, all skippers have a responsibility to avoid collisions with other boats at sea.  It is...

Pleasure craft safety equipment recommendations

Safety equipment is an important part of boat preparation and it is advisable for all pleasure craft skippers to check their vessel is...

Marine diesel exhaust checks

You should inspect the exhaust system for corrosion damage regularly, especially around the injection bend. If you have noticed the engine exhaust smoking a lot during the sailing season this can also indicate a number of potential problems.

How a propeller works

Have a look around any boatyard and you will notice quite a variety of propellers – some have two blades, some have three and others have four or more. While most propellers are completely rigid some have blades that fold.

Tools and spares for your boat

It is wise to have a comprehensive and well-organised tool kit and a supply of spares for your boat. This is both for routine...

Jester Challenge 2022 – Sailing single handed from Plymouth UK to the Azores: Part 8 – Arriving at the Azores

Jester Challenge – A modern experiment in old-fashioned self-reliance, self sufficiency, and personal responsibility. This is the eighth of a 10-part post where solo sailor, Bernie Branfield, shares his first-hand account of his single-handed, 2022 Jester Challenge, from Plymouth, UK to the Azores, in his 26′ Invicta Mk2, Louisa.

Rewiring a boat – overcoming the challenges involved

Skippers need to have a basic knowledge of boat electrics, to avoid potential problems and to be able to solve them when they happen.

Galvanic and electrolytic corrosion

Galvanic corrosion is an electrochemical reaction between two or more different metals, in the presence of an electrolyte (note salt water is a good electrolyte).

Antifouling for leisure boats – Part 4

Applying antifouling. Antifouling is best applied on a dry, calm day. It is best to apply the antifouling in the middle of the day to ensure the hull is dry and as warm as possible.