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Home / Blogs / Engine Replacement - Moody 31 Mk2
Home / Blogs / Engine Replacement - Moody 31 Mk2

Engine Replacement - Moody 31 Mk2

Brian Robertson describes how he replaced the engine on his Moody 31 Mk2 yacht, Tidecatcher. A shorter version of this account has been published in Practical Boat Owner magazine but for the full and complete story, read this!

I bought my M31 mk2 in September 2017 and I still remember the excitement of flying down with a friend from Edinburgh to Southampton for the day to view her. 'Tidecatcher' was being offered for sale by MS&P (Hamble) who had taken her from a customer in part exchange for a new Hanse.

Tidecatcher (or 'TC' as she is now affectionately known) was out of the water and fully rigged when we viewed her. For me this was my first real opportunity to take a close look at these boats, and I was not disappointed. Although I knew that this was the 'type' of boat I wanted I had come to that conclusion not just because of the 'Moody' reputation, but also their look. I remember being smitten when I was first shown an image of this particular make of yacht and I think Bill Dixon really hit the sweet spot when he came up with this general design which, of course, he replicated across other models of the Moody range in the 1980's/90's.

Tidecatcher was well appointed and, for her age (1987), in generally good condition. She came with a full Raymarine electronics package and an extensive maintenance record going back many years, which detailed, in particular, the recent (2014) replacement of the keel bolts and also the spline conversation for the MS2B gearbox. It was comforting to know that these two 'Achilles heel' issues had already been addressed! The Volvo Penta 2003 engine, although original, seemed to run well under test and, as an added bonus, it came with a 1 year warranty.

The accompanying survey, which MS&P had commissioned for the PX, was very favourable and, following some further negotiation, the purchase was made. I took possession of TC just prior to the end of the 2017 season. She was brought up to Edinburgh by road transport (John Shepherd yacht transport very highly recommended!!) and floated at Port Edgar Marina, South Queensferry. Following a short 'motor' to Granton Harbour she was lifted out to spend the winter in the Royal Forth Yacht Club yard at Granton, Edinburgh.

Despite the fact that the engine has served me well this past year (and that includes a 740nm return trip through the Caledonian Canal to the west coast) I am very conscious of the fact that the engine is original and likely (as it gets even older) to require regular, ongoing and potentially costly attention, with always the risk of complete failure. Looking to the future therefore and knowing that I DO wish to keep this boat for several years, I have decided to renew the engine. One would assume that the natural replacement for the Volvo Penta 2003 power unit would be the VP D1-30, but there are alternatives, such as the Beta 30.

Which Power unit to choose?

Ten or so years ago I had a Beta 16 engine fitted to my previous boat, a Seamaster 925 (these are great boats incidentally, and ones which I would highly recommend). The engine gave excellent and reliable service during my period of ownership. Quite by coincidence however, a couple of years ago I found myself in conversation with Scott Sandford (TS Marine, Southampton) who was at Granton installing a Beta 25 in another yacht at the club. When I admitted rather guiltily to Scott that I had never even checked the valve clearances on my Beta 16 in the (then) 9 years I had owned it he simply said that if you do the regular maintenance on them; oil, filters, heat exchanger, anodes etc., you should never have to check the valve clearances! Naturally, I had done all the other recommended maintenance but, never the less, his remarks were very comforting and went some way to reinforcing my own view on these engines that if you look after them they are virtually bullet proof!

The Volvo Penta D1-30 is a formidable power unit however, and not one to be disregarded, but I was aware of the fact (from another Moody owner at Granton who had installed such an engine in his yacht) that there have apparently been ECU problems with these engines recently. My associate experienced such problems on no less than 5 occasions, and, on each occasion, there was complete (and at times alarming) failure of the power unit. Although the ECU was replaced each time under warranty what made it worse was the fact that the nearest Volvo Penta engineer who could do the warranty work was in Glasgow. Fine if you live on the west coast, but not much use for elsewhere!

The Beta website has a ready reckoner which gives advice on what size of engine may be suitable for a particular craft. When you speak to different people about suitable engine size I know that you tend to get differing opinions and I am aware that water line length; the weight of the craft and design hull speed are all important factors. Engines used to be installed on the basis of a '3HP per tonne' formula, but I believe the generally held view now is that it should be 4HP per tonne. The M31 Mk2 blurb confirms that the water line length of the boat is 7.75m and the displacement is 4.5t so, on that basis, one could argue perhaps that a 20HP engine would be sufficient?

I don't think that I could necessarily be classed as a petrol head but I am certainly of the view that I would rather have more HP than not enough and I was always impressed by the fact that the M31 came with a 28HP engine by design, and I wasn't really wishing to diminish that. The slightly smaller Beta 25 would seem then to be a sensible option but, having said that, Beta rather interestingly offer 'Volvo Penta 2003 engine feet' to suit their Beta 30 engine. Additionally, Beta offer a 'TMC 60 A' gearbox option, with a 7 degree down angle, specifically to match the similar down angle on the MS2B gearbox fitted as standard to the VP2003 power units installed in the Moody 31.

So, as this is something which Beta has obviously put some thought into, it would appear that the Beta 30 is a natural replacement for many yachts currently installed with the VP 2003 power unit, and specifically Moody 31's. Armed with this knowledge I made contact with Beta head office to seek further information on their Beta engine range.

Following communication with the Beta UK sales team, all of whom were very helpful, I felt that I had sufficient information to make a decision on which power unit to opt for. The price difference between the Volvo Penta D1-30 and Beta 30 power units was fairly negligible so I naturally considered the reliability issues and also the installation and ongoing servicing. I already had confidence in Beta from my previous installation and their 'self-servicing warranty' was naturally very attractive; as was the additional 'Southampton Boat Show' discount which they offered. On the other hand, Volvo Penta parts and spares are known to be expensive and the ECU problems and warranty issues which I have already referred to were rather worrying.

With all things considered I decided to go with Beta and I placed my order in Late September 2018. The unit I ordered was the 'shallow sump' option with 'VP 2003 engine feet' and the 7 degree down angle gearbox. Whilst these extras came with additional charges, I had already factored them in when comparing the costs of the Volvo Penta and Beta engines. With particular regard to the 'VP 2003 engine feet' the additional charge is well worth it for the amount of hassle it saves you. In addition, and not to be overlooked, as the Beta 30 engine rotates differently to the VP2003 power unit the cost of a new propeller should be included in the calculation (a new suitably matched propeller which I purchased through Beta cost just over £300).

The engine lead time on the new engine was 6 weeks but I was not idle whilst waiting for the delivery of my new purchase and in addition to reading up on the extensive technical blurb (easily accessed on the Beta website) I set about making moves to remove the existing VP2003 power unit from Tidecatcher.

Removing the existing engine

I first of all disconnected the prop' shaft and removed the clamp coupling from the gearbox flange. I then decided to remove the PSS shaft seal assembly (I replaced this last year so that came off easily) and installed a split mandrel to hold the shaft absolutely central in the stern tube. This is critical to ensure that the shaft is held in a central position whilst the new engine is aligned. The mandrel was easily turned on my workshop lathe from a piece of aluminium bar.

Having disconnected the shaft; fuel lines and electrical cables etc. removing the engine from this position is almost a direct vertical lift out through the hatch, so it really couldn't be more straightforward. Take time during the decommissioning to accurately label all the disconnected wires as this could save much time and head scratching when reconnecting them to the new engine. I actually lifted the engine out and onto the cabin floor with the use of a timber beam across the hatch opening from which I suspended a strong but lightweight lifting pulley arrangement. Having lifted the engine off the bearers I simply slid a plank in beneath the engine sump and then dragged the engine back into the cabin. Actually lifting the engine out of the boat was done another day with the help of a HIAB mounted lorry and we lifted the new engine into the cabin in the same visit.

Improving the engine bay access

As all M31 owners will know, access to the engine compartment is already fairly generous but when the cabin steps are removed along with the cross supporting bar and the 'box' storage unit in the aft cabin, access is even better and perfectly adequate for most of which requires to be done there. Having said that, I was greatly impressed by an image I viewed recently of a Moody 31 which had an access hatch installed to the engine compartment from the 'heads'. This struck me at the time as a very sensible and useful modification and thus I replicated it on Tidecatcher. As the image shows, this hatch gives vital portside access to the engine mountings as well as the starter motor electrics.

Modifying the engine bearers

With the old engine out of the way I set about making preparations for the new power unit. The Beta technical information confirmed that, of the 8 existing drilled engine bearer holes which the VP2003 unit used, 6 of the holes are suitably positioned to match the Beta 30 mountings (with the suitably installed 'VP 2003 engine feet'). This is why it is important to order the VP2003 engine feet when you are placing your order. That meant that only two holes had to be drilled and tapped for the new engine to sit on top of the existing engine bearers.

The M31 engine bearers are encased in fibreglass, but beneath a top layer of fibreglass there is a steel plate and it is this plate which the engine mounting bolts secure to. The thread size is M10 and, having carefully taken my measurements, I drilled and tapped the two holes as directed.

Fitting the new engine facing the challenges!

Soon after we got the new engine on to the boat we were keen to see just how well it fitted and so we rigged up the lift pulley arrangement and started manoeuvring the engine into place. The Beta 30 power unit is 20kg lighter than the VP2003 power unit, so you will find it slightly easier to push around!

Any engine replacement will throw up some 'challenges'; it is only to be expected, and the first of these challenges concerned the engine mountings and feet. I had loosely mounted the four flexible mountings on the engine bearers, bolting them to the appropriate threaded holes, but I noticed that when I lowered the engine onto the mounting spindles the front and rear spindles were being pushed very slightly apart. Accepting that the 'VP2003 engine feet' had been designed specifically for the Moody 31 engine bearers I concluded that there had perhaps been a slight error in the design calculation or indeed in the manufacture of the special feet. I therefore modified the rear flexible mounting plates to allow them to sit slightly further apart from the front ones (5mm was enough) and I was then able to lower the engine onto the mountings with no visible signs of stress.

One other 'mounting' issue concerned the front portside mounting spindle which sits so close to the engine oil filter that, once the engine has been lowered onto the mountings, it is impossible to get the top securing nut onto the mounting spindle. I had no option but to remove the filter (which I managed with great difficulty due to the close proximity of the mounting spindle) and then use my angle grinder to remove the top third or so of the spindle. I didn't like interfering with the mounting in this way but the alternative was to face this difficulty every time I wish to replace the oil filter (which will be regularly) so reducing the height of the spindle was the preferred option.

Continuing with the installation, we had positioned the prop shaft clamp coupling (minus the 'flexi' component) on the end of the prop shaft and nipped up the locking bolts. We were pleased to see that the gearbox flange had faced up to the clamp coupling flange well and was sitting about 15mm lower than it. As the engine mountings were at their lowest position this variance was perfectly acceptable.

As the Beta technical information recommends that the engine feet should be positioned as low as possible on the flexible mounting spindles, I decided to make up some simple spacers to sit between the flexible mounting plates and the engine bearers. This practice is quite common and whilst these spacers are usually made of steel I happened to have some sheet polyethylene in my workshop and decided to use that. This material, which happened to be 7mm thick; is extremely dense, completely impervious to oil and diesel, and almost indestructible. With these spacers in place it meant that I was only going to have to raise the engine up the threaded spindles a few millimetres to get the gearbox and prop shaft flanges to mate correctly.

Prop shaft alignment

Prop shaft alignment will always be one of the most critical aspects of any engine installation and even a slight misalignment will cause vibration and possibly wear on other components. The shaft mandrel (mentioned previously) was still in place so I knew that the shaft was now central (through the cutlass bearing and stern tube) and the prop shaft flange was therefore perfectly positioned. Now I had to adjust my engine mountings to get the gearbox flange to match the prop shaft flange perfectly.

Once my spacers were in place, I raised the engine 'feet' up the mounting spindles just about 6mm or so and then pulled the shaft flange forward to get it to mate into the machined grooves on the gearbox flange. After some adjustment I did manage to get the two flanges to mate fairly easily, but in order to ensure perfect alignment I opted to use feeler gauges at the 'north, south, east, west' points of the flanges.

By careful adjustment of the mountings I was able to get the four measurements spot on and, having done this, I then tightened up the engine mountings and repeated the measurement. Often the tightening of the mountings can fractionally alter the position of the engine (and therefore the position of the gearbox flange) so the installer must be prepared to re-check the measurements as often as necessary until perfection is achieved. Spend as much time as you need to on this exercise however as it is critical that proper alignment is achieved.

Many manuals will recommend the use of a dial gauge to confirm alignment accuracy and I certainly wouldn't argue against this. However, if the feeler gauge measurements are achieved as described above then the installation should be accurately aligned.

Anti-syphon Valve

Worthy of consideration is whether an anti-syphon valve is necessary on the seawater cooling circuit. There is quite a science involved in this (which I don't necessarily wish to delve into now) but suffice to say that strictly speaking, and to avoid the risk of sea water coming back into the engine through the water cooling circuit, the exhaust manifold at the rear of the engine (which the seawater cooling outlet discharges to) should be a minimum of 300mm above the sea water level. Although it is interesting to note that Moody didn't install anti-syphon valves as standard I calculated the manifold on my VP2003 to be only marginally above the sea water level. I concluded therefore that it would be prudent to install an anti-syphon valve in the sea water inlet circuit and this involved some simple 18mm copper and rubber hose plumbing work.

It is important to note that water may ingress an engine from either the water inlet side or from the exhaust side, so this led me to look closely at the exhaust system.

Connecting the exhaust

Whilst an anti-syphon valve, positioned at a suitable level, will protect an engine from water ingress from the inlet side, exhaust water ingress should be prevented by having a suitable 'fall' into the exhaust water trap, along with a sufficient loop in the exhaust pipe before discharging through the hull. The Beta website has illustrations which clearly show the critical dimensions relevant to exhausts and seawater levels and having a good understanding of the potential problems associated with this is highly recommended. Beta guidelines state that this 'fall' into the water trap should be a minimum of 300mm.

The standard water trap on the M31 is cleverly positioned beneath the cockpit locker floor and whilst this is a good place for it and it keeps it well out of the way, it is still easily accessible. This standard installation had clearly worked well with the original engine for many years.

What concerned me with my existing installation however was that, whilst the flexible exhaust pipe did appear to drop from the exhaust manifold to the water trap inlet, it didn't drop as much as 300mm. Furthermore, there appeared to be a dip in between, which was clearly going to hold water when the engine was switched off. This concerned me somewhat and I decided to look for ways to improve the installation.

Although it called for a fairly major alteration to the original installation I decided to move the water trap closer to the engine and my measurements confirmed that it would fit into the small compartment in the aft cabin, close to the stern tube. This did necessitate the alteration of the angle of the stainless steel inlet to the water trap however but, whilst I realise that not everyone has access to welding facilities, the alteration required is fairly basic and most engineering workshops would look upon this as a simple task.

I 'glassed a couple of hardwood bearers to the inner hull for the water trap to sit on and I mounted the base of the trap on rubber packers to give it a bit of flexibility to allow for engine vibration.

I was then able to connect the engine exhaust to the water trap with a short length of 50mm flexible exhaust hose and I joined the two sections of hose beneath the cockpit locker (where the water trap used to be) with a short piece of stainless tube (easily purchased on the internet).

All in all, this alteration was fairly straightforward and I was pleased that there was now a clear 'fall' from the manifold into the water trap. This was clearly an improvement on the original installation and it now conformed to the Beta engine installation guidelines.

Fuel lines

Both the diesel engine feed and return connections (8mm compression) are positioned conveniently at the rear of the engine, near the top of the bell housing, and 8mm copper pipework can be laid easily to and from these points. I made up a simple bracket to support these pipes as I led them to the existing diesel feed and return pipes on the port side of the engine compartment.


The ABV control panel, which rather conveniently is almost exactly the same size as the Volvo Penta one, fitted neatly into the cockpit aperture and the harness was laid through the bulkhead into the engine compartment. This then just left a simple 'multi-plug' connection to the engine wiring harness. The earth wires (and there were a few of them) were reconnected to appropriate engine bolts (bell housing etc.) and the engine 'live' and 'earth' were connected to the starter motor.


As mentioned earlier in the text the Beta engine rotation differs from the Volvo Penta and a new propeller was therefore required. Beta provide a propeller design and supply service, which is done through Paul Bell of First Marine Solent. Whilst my existing propeller was 16" diameter I noted that this left only a gap of half an inch between the propeller blade and the boat hull. According to my experience (and indeed this is confirmed on the beta website) the gap between hull and propeller blade should be a minimum of 10% of the propeller dia., this to reduce 'tip noise'. I had clearly witnessed this rather alarming noise on a few previous occasions when motoring and when I first heard it I thought I had run a bearing in the gearbox!

To give me sufficient hull clearance, my new propeller had to be no larger than 15" dia. so Paul suggested a 2BRH 15 x 9 0.36 Htb1 propeller to suit both engine power and boat hull. The new propeller was manufactured (to suit my existing shaft taper) and supplied by First Marine Solent.

Finishing off

With the raw water inlet pipe connected at the front of the engine and a general tidy up of wires and cables etc., all that was left to do was to refit the PSS shaft seal and prepare the engine for trial.


Beta Marine's Installation and Commissioning instructions are fairly clear and naturally involve a series of checks which have to be carried out before attempting to start the engine. With these all done carefully, and in an air of excited anticipation, we started the engine up for the first time without too much trouble. The Beta manual instructs that the fuel is bled up to the fuel filter prior to starting the engine. After a bit of coughing and spluttering during start-up we went through the normal process of bleeding the injectors and, having done that, the motor then began to run sweetly. Naturally, this was all done with the boat out of the water. Before the full commissioning can be completed however sea trials are required and these had to wait until the boat was 'lifted in' in mid-April.

Once in the water Beta Marine commissioning instructions require a series of engine tests whilst on the pontoon. Following that, the craft can be taken on 'sea trials', taking the engine slowly up through the rev. range. Our sea trials were carried out into a gentle breeze, but in a relatively flat sea, and the following speeds (SOG) were achieved:- 1500 rpm 4 kts, 2000 rpm 5 kts, 2500 rpm 6.2 kts, 3000 rpm 6.8 kts, 3600 rpm (max) 7.3 kts. By my calculation (square root of LWL x 1.34) the design hull speed of the craft is 6.7 kts, so the speeds achieved on the sea trials were respectable. No vibration of any note was noticed during sea trials, so the time taken during the engine alignment process was well spent!

During the trials the engine and craft performed faultlessly and, back in port, further checks revealed no leaks from either the engine cooling system or stern gear.

Now halfway through the season and with nearly 20 hrs on the clock the engine continues to start and run without issue and I am confident that all is well.


So, with the project now at a close, what is the general conclusion? Having never attempted anything like this before I must confess to an air of profound satisfaction upon the successful installation of the engine. Putting aside the considerable financial saving on being able to complete this project myself there are additional benefits; such as the technical knowledge which one gains about one's own engine and how it operates. This is knowledge which every boat owner should have; and it's not being over dramatic to say that this knowledge could save your life one day!

Yes, it is useful to have some engineering aptitude; access to basic engineering aids such as a welding set and drills/taps/dies etc., and also to have a friend with a lorry mounted HIAB! But, other than that, this is a task which could be undertaken by anyone with a modicum of DIY skill. If you prepare carefully; plan each step and give yourself plenty of time, it is an exercise which can prove extremely worthwhile. It was for me anyway!


Last updated 14:27 on 1 August 2020

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