At what point is keeping an old boat engine throwing good money after bad? PBO expert Stu Davies has some tips to help you decide…

Most of us will know someone – and may have been that someone ourselves – with a rusty, rattly, oily or smoking marine diesel engine. The sort of old boat engine that no longer fills us with complete confidence in its reliability.

Many have also been in the position of thinking of buying a second-hand boat with concerns that the engine may be suspect or isn’t performing quite as it should.

First questions when assessing an old boat engine

The first thing to do is set a baseline so any decisions or spending of money are based on cold, hard facts. This entails putting the wallet firmly to one side and rapidly getting an answer to the frank question: ‘is my engine actually shot?’

If that’s the conclusion, the immediate follow-up question is always ‘do I need to buy a new engine or can I refurbish what I currently have?’

These first questions are the most important, and to make an informed decision a definitive survey of the whole engine needs to be performed. At this stage compiling a brief list of pros and cons is a good idea. In approximate order of importance these are:

  • Does it start easily?
  • Has it been breaking down regularly?
  • Is it noisy?
  • Is the exhaust elbow perforated or blocked?
  • Is it dripping oil or coolant?
  • Is it terminally rusty outside, or could a good couple of days with wire brushes sort it out?
  • Are the engine mounts broken?
  • What do the alternator, starter motor and electrics look like?

If any of the top three of this list give you the wrong answer, the next step is to start with a compression test.

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The compression test

A compression test is a good guide as to the health and state of an engine as it will show up leaky valves or worn pistons.

Consult your engine manufacturer for exact parameters for compression test results, but a usual rule of thumb is if the average compression is within 10% of its recommended level and all cylinders are within 5% of each other’s compression values, then there is not a compression issue.

It is important to note, though, that compression testers vary in their results, due to factors such as quality of seal and rpm of your starter motor, so most marine engineers value the consistency across the cylinders much more highly than the overall values.


A diesel compression tester, not to be confused with the common-or-garden variety for petrol engines

Note that you will need a diesel compression tester, a petrol compression tester will not do. Diesel testers are higher pressure gauges that usually come with many more elbows and adapters, making them considerably more expensive than petrol units. If a compression test shows up a problem then your decision has largely been made for you.

If the compression checks out OK and your issues are limited to the more minor lower regions of the list, then ask yourself whether relatively cheap new parts such as a parts4engines refurb kit along with ancillaries such as a new water pump, engine bearers, exhaust elbow, alternator, or injector nozzles could fix it.

On a positive note, having to replace an engine or get it rebuilt could be just the excuse we need when it comes to justifying investing in a new technology such as an electric drive.

Old boat engine: Case study

Using the Volvo Penta MD2020 fitted to Maximus the PBO Project Boat as the basis for our research and calculations gives us a typical repair or replace conundrum for a middle-aged engine.

The MD2020 is based on a simple industrial engine, originally built in Japan. The design was taken over by Perkins and resold with some modifications to Volvo Penta who marinised it.

Base engine parts are still readily obtainable either as Perkins parts or Volvo parts, mostly they are interchangeable and various outlets sell them at very reasonable prices. In this case, we found that a rebuild kit for the basic engine came in at about £300.


Correct servicing is crucial, and in the case of a second-hand engine, documented proof of this is key

Add another sum for a water pump and injectors etc, a new exhaust elbow, a fettle of the alternator and starter and a refurb yourself is achievable and not too costly compared with a whole new replacement engine. I’d guess £2,000 would cover the cost of a refurb done by the reader.

However, the drawback is with the Volvo Penta only parts such as intercoolers that were fitted to the engines as part of the marinisation. They are very specialised castings and unobtainable except from Volvo Penta at relatively high prices.

If the intercooler exhaust joint is corroded through, for example, then throwing thousands at one of those is probably not an option and it’s re-engine time.


Some parts of an engine are easier to access than others. Here we are checking the coolant header tank for any oil residue which could suggest a head gasket or liner problem

Buyer beware

The next option is buying a second-hand engine. Sadly the vast majority of ‘refurbished’ ones that I have had contact with seem little more than a coat of paint from a rattle can respray. My advice on used marine diesel engines is simply don’t do it unless you are buying from someone you know and trust implicitly.

The Bukh ones taken from lifeboats are a popular option often discussed on the boating forums. While they are ‘low hours and well maintained’, they are still second-hand engines when all is said and done.

The Bukh engines being talked about are very good but they have a drawback in that they have ‘wet liner’ cylinder walls. It means that the cylinder liners are a push fit into the block and they are sealed with O-rings. Over time the O-rings eventually degrade and leak and the coolant leaks past them into the sump.


Learn where the key parts are on your engine before it starts misbehaving

A friend of mine had one in his Sadler, he cosseted it, he loved it, it started getting water in the oil, head gaskets were changed numerous times, it leaked, he took it out of the boat.

He took the sump off and looked upwards, coolant dripped onto his face. The O-rings were leaking. He ordered and fitted a new Betamarine engine. His life was peace and harmony again!

Can I refurb my old boat engine?

If we plump for a refurbishment that’s not the end of our decisions. The most important aspect of a refurb is who does it. Do we trust our DIY skills on something as important as our own engine, and do we have, or are we able to borrow any specific tools that we’ll require for the job?

The potential saving for taking the DIY route is a hefty one. But even if we pay someone, it’s still a hefty saving over a new engine given there’s no need to change the infrastructure of the boat. The rebuilt engine should slip back into its old bearers as though nothing has happened.


An exploratory wire brush can tell you if corrosion is just surface rust or could be a deeper problem

Do I need a new engine?

So let’s imagine we’ve looked at all the facts and decided it’s time for a new motor for Maximus. Where do you go?

I’ve always been very impressed with Betamarine, who sell superbly converted Kubota engines. They are a basic diesel engine (used in farm and industrial machinery worldwide), designed to comply with the latest regulations and emissions and should easily fit your existing engine bearers.

A plus point for boaters is that they use mechanical fuel injection and are not electronically controlled. Talking with a representative, he was quite adamant that they don’t want to go down the route of common rail engines with all the complexity that goes with them. They’re happy that the Kubota technology allows them to comply with existing and forthcoming emission rules.

Several of my friends have done refits outside the UK and say that Betamarine are really helpful wherever you are in the world. They also get it to you, no hassle.

Another plus point for boat owners in far-flung locations is that they let you self-certify the initial oil changes and first service so that their warranty is valid without having to take it to a Beta dealer.

One to fit Maximus, which has a sail drive, would cost about £5,200 including VAT and the bell housing to fit it to the sail drive. An exhaust adapter would be another £175. Offerings from other manufacturers would be in a similar price bracket.

Fitting a new motor would be relatively simple and within the capabilities of many of our readers, otherwise a fitting fee would have to be factored in.


The old Bukh in Ed and Carolyn Phillips’ Super Seal was ultimately replaced with an e-propulsion electric pod drive

Or maybe go electric?

There is another option of course, and that is using electric motor power. It’s a costly option, though, and electric, while progressing fast, still cannot yet compete with energy dense diesel for certain applications.

A case in point for this in our own cruising experience was our crossing of Biscay, which involved motoring for two days to avoid the remnants of a hurricane. This would just not have been possible with battery power.

Solar power and lithium-ion batteries have been the big mover in this field along with electric power pack manufacturers offering regeneration capabilities when sailing.

Similar to regenerative braking in electric cars, the freewheeling propeller is used when sailing to charge some power back into the batteries.

The latest generation lithium-ion batteries have impressive discharge and charge capabilities, plus the fact that much more of the storage capacity can be used than with older battery technologies has been a game changer. Victron do a very informative booklet on the official Victron website.

Into the blue

Serious bluewater sailors do use electric power, but often the distances they are dealing with do not encourage motoring anyway, and the concept of delaying a passage for a weather window is one that these sort of sailors are already familiar with. Examples can be found on the Oceanvolt website.

One of Oceanvolt’s motors and sail drives is fitted to Sailing Uma of YouTube fame, of which more in a later article.


Ed and Carolyn Phillips fitted an all-electric pod drive power system to their Parker Super Seal

Shorter distance cruisers and weekend or day sailors have converted to electric with a great deal of success, especially in smaller, easily driven boats. A good article by Ed and Carolyn Phillips on fitting an all-electric power system to their Parker Super Seal.

However it did come with a hefty price tag, with the parts cost alone nudging £7,000 for this DIY installation – the batteries costing a significant amount.

Hybrid power

There is another option which I quite like, a true hybrid where a generator is piggybacked on to the main diesel engine and the prop shaft. The diesel is used as normal, it generates power when using the engine to drive the propeller and when the engine is switched off and in sailing mode it is driven by the propeller to generate electricity.


It pays to check all the obvious things first with engines. If she’s running hot, is your seawater strainer clogged?

The unit can also drive the propeller shaft alone using power off the batteries. A good recharge system combining solar, wind and regen off the prop plus lithium batteries allows silent green electric power with the option to use the diesel if you get into difficulties. the backup of the diesel also sets the mind at rest when it comes to range anxiety.

A company manufacturing these generators is Betamarine package their products already fitted to Beta engines. Betamarine tell me that a hybrid package like this to suit Maximus would cost in the region of £17,000 plus the cost of batteries and fitting.

One of these Betamarine systems was put together by for a wooden boat called Tally Ho, who’s owners have a YouTube Channel, and you can see their engine package in the episode below.

There are therefore a lot of options when it comes to what ostensibly sounds like a straightforward question: whether or not to replace an engine or work with what we already have.

Parts availability and cost are usually the factor that decides it for most boat owners in the end, as once a design gets so old that critical parts are no longer available, there really can be no choice.

PBO Project Boat Maximus options

A ‘back of an envelope’ calculation over a cup of tea is as good a place to start as any, and we came up with the following options for a medium sized boat such as Maximus fitted with a 20hp engine as standard.

  • Rebuild, using parts from – about £2,000 plus labour
  • Betamarine straight diesel swap – about £5,000 plus fitting (If you have shaft drive, you get a new gearbox as well)
  • A hybrid system from Betamarine – about £17,000 (plus battery plus fitting etc.)
  • A true electric system – £15,000 (plus fitting)
  • For Maximus, a sail drive unit from Oceanvolt, similar to Sailing Uma – about £11,000 (plus battery, charger, propeller and installation)

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This feature appeared in the July 2022 edition of Practical Boat Owner. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a magazine subscription to Britain’s best-selling boating magazine.

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