How long does an engine last? The short answer is that age is no indicator. Heavily used commercial road vehicles, even small ones such as taxis, can be expected to run 500,000 miles, perhaps 12,500 hours, with perhaps a change of big-end bearings but possibly not.
Few auxiliary yacht engines come anywhere near this. Power boats will certainly do better than yachts but it is rare for one to last 10,000 hours without complete overhaul.
Diesels thrive on hard, constant work that keeps them up to working temperature for most of the time. What they don't like is frequent start-stops, brief periods of operation, rarely reaching design operating temperature, living in a cold, moist atmosphere and infrequent oil changes. Which is what they get, at least in yachts.
Given that hours run is insufficient guide to judge engine condition, how can the practical, non-specialist owner judge whether his engine is in good condition, would benefit from some maintenance or needs a total overhaul?
Much can be learned from observing the type of smoke emitted by distressed diesel engines.
Black smoke from exhaust Cause is incomplete fuel combustion. Can be due to injector problems, high load, poor compression and valve wear. If the smoke has always been present it could be that the propeller is over-sized. If it comes on fairly suddenly, check for hull fouling, something wrapped around the keel or some foreign material such as net, polythene or rope around the propeller.
Blue smoke from exhaust Cause is burning engine oil that has found its way into the combustion chamber. A little is normal on start-up until the pistons and rings warm and seat fully, but if it continues the likely cause is wear of oil control rings and/or bores.
White smoke from exhaust Two possibilities: either unburned fuel or steam. Holding a cool mirror or glass bottle in the flow can help to identify which it is. Note that Bukh engines are renowned for steaming from a couple of minutes after starting.
Oily grey smoke coming out of the open oil filler cap and crankcase breather hose in significant amounts with the engine warm and running, possibly with a smell of diesel fuel. Shows that combustion pressure is passing down the bores into the crankcase. Likely causes could be broken or worn rings, badly worn pistons, and possibly worn or deeply scored cylinder(s). Best confirmation of worn bores, or at the very least broken oil control rings, is that oil consumption will be high. An engine in good condition will consume a small amount, perhaps 0.5 to 1.0 litres per 100 hours; higher figures are suspect.
More than normal black smoke and a smell of diesel from the exhaust Smoke most likely to be evident immediately after starting the engine and on acceleration. The power output of the engine may well reduce as the symptom advances. This condition probably indicates worn valves
Large amounts of blue smoke from the exhaust after periods of tick-over, disappearing soon after revs are increased, probably indicates worn valve guides.
Little puffs of white smoke which smell of diesel from the exhaust on starting from cold. Almost certainly the engine has a compression problem.
Vyv Cox is a Chartered Engineer, holding qualifications in both metallurgy and mechanical engineering. Now retired, he worked for 30 years in the petroleum industry, responsible for the diagnosis of engineering failures. Originally based in Anglesey, with wife Jill he cruised extensively between Tobermory and the Scillies. Relocating to Holland they cruised most regions from the Baltic to Biscay in their Sadler 34. They are now meandering eastwards across the Mediterranean and have currently reached Greece.
