Jul 122013

Last Updated on 05 August 2016 by Eric Bretscher

The starting point is always the same: marine engines tend to be expensive, second-hand automotive or transportation-type diesels can be obtained for very little due to over-abundance of supply…

Both are seemingly doing the same, converting diesel fuel into mechanical power – and noise – and look so similar in nature that the temptation is often irresistible: buy some light truck engine and fit it into a pleasure boat.

A few years down the track, I have seen countless such scenarios that had turned out just as expensive, if not more, than a brand new marine engine, still with a very inferior result.

Things tend to go wrong in a number of ways.

Transmission Coupling and Matching

First, a transmission needs to be sourced and coupled to the engine. Adapter plates often need to be manufactured, the weight of the flywheel of an automotive engine is often insufficient – when a flywheel is present at all, prompting replacement or custom machining and a flex plate is needed to take the spline shaft from the gearbox.
If that flex plate is not matched properly to the application, torsional vibrations often set in between the pulsation of the engine and the transmission, causing chatter in the gearbox and noise travelling down the propeller shaft.

A considerable amount of trial and error often takes place before these issues are finally addressed, when they ever are. The noise and rattle can be unbelievable and – more often than not – these problems are not straightforward to resolve.

Engine Mounting

The location of the engine mounts in relation with the engine block, transmission and shaft all impact on the loading on the mounts. One should generally aim for even loadings, but I have seen many installations where the rear mounts were carrying most of the weight of the package, because they were not located correctly. Not only rear mounts carry some of the engine weight, but also the gearbox and some of the shaft weight in many installations. The elasticity of the mounts needs to be matched to the package itself in terms of weight, vibration frequency and amplitude to achieve good damping.

Considerable trial and error often takes place there too, and satisfactory solutions are often never reached because complete re-engineering doesn’t take place. I have seen boat owners “pre-loading” their engine mounts by shifting them out of position deliberately – forcing them to work one against another – to try reducing engine vibration… instead of addressing the root cause: inadequate design and/or component selection.

The reality is that the engine will always move in relation to the beds, it is the whole point of flexible mounting. Poor, imbalanced mounting can increase movement.

Conversion of a Nissan LD28 diesel: a water-cooled exhaust manifold was retrofitted

Conversion of a Nissan LD28 diesel: a water-cooled exhaust manifold was retrofitted

Injection Control

Marine and stationary engines, such as found in generator plants or compressors, feature speed regulation. The throttle control, when applicable, is not linked to the injection system like in a vehicle; it sets the operating point of the governor.

The governor then controls the injection to maintain the required engine speed: under heavy load, the engine will burn more fuel at the same speed than under light load, without human intervention.
Modern electronic engines can achieve the same even more effectively through the on-board computer.

Lack of a governor on an engine in a marine application may or may not be a problem. If the gearbox reduction ratio is high enough, the shaft line and propeller’s inertia are modest, it may all go well. Otherwise the engine can stall when placed into gear.

In this case, the typical response is raising the idle speed, which is only acceptable up to a point and harder on the gearbox clutch system. Skilled operators sometimes compensate instead, applying throttle just as the clutch engages, simulating the operation of a governor. In both cases, it is risky.
Let’s consider a scenario where the boat has plenty of forward momentum and suddenly needs to stop. The velocity of the water over the blade keeps driving it as reverse is engaged, and a delay between forward and reverse fails to fully mitigate the issue. The engine often stalls as the reverse gear engages and the vessel loses all control.

Lastly, the injection system of automotive diesels is nearly always capable of taking the engine to power levels that are not acceptable in a marine application. A throttle stop needs to be used to limit the injection, otherwise the result can be dark smoke and overheating.

Fuel Circuit

Marine engines include a low pressure lift pump bringing the fuel from the tank to the engine through a pre-filter/water separator, followed by a fine filter before the high pressure injection pump.

Automotive engines usually come without any lift pump. Backyard solutions range from no lift pump (“once primed it will be fine”) to electric fuel pumps, which are better than nothing, but no match for a pump mechanically and directly driven by the engine when it comes to reliability. Short of being gravity-fed at all times, a diesel engine that stops as a result of air ingress into the fuel supply circuit will not restart without being re-primed and bled out, an operation that doesn’t happen in a snap. In the meanwhile, the vessel will be adrift.

Marine engines tend to return the excess fuel pumped to the tank to prevent overheating it, rather than recirculating it around the pump, albeit some very small marine diesels sometimes do operate this way. If fuel temperature becomes excessive, efficiency drops and consumption increases. An engine confined in a boat doesn’t benefit from natural air cooling and fuel cooling is often a consideration that needs to be taken into account.


An additional raw water pump needs to be installed, usually belt driven due to the complexity of any other solution, and a heat exchanger. Custom piping must be fabricated for both the coolant and raw water circuits, attention needs to be paid to galvanic corrosion considerations and the cooling capacity needs to be sufficient.
While this is usually easy enough, it is also easy to reintroduce exceedingly cold water into the block when the thermostat opens, as a result of overcooling. Overcooling can cause the thermostat to cycle constantly instead of more or less settling down in a position for a given engine load.

A raft of issues that are not immediately apparent can result, with reduced engine life as the outcome. Good marine engineers look for this and tune their cooling system.

Marine engines often also feature oil cooling, for the simple reason that natural air cooling around the block and sump is not available and loads tend to be higher. Due to the change in viscosity of the oil with temperature, oil coolers are constructed completely differently than water/water heat exchangers. In many cases, an oil cooler is required for the engine, and another one for the gearbox. Homebrewed oil coolers often represent nothing more than a good intent.

The order the cooling water circuit travels through the various heat exchangers is relevant and is frequently the object of subsequent alterations.

Air Intake

In a vehicle, the air filter arrangement provides for intake silencing. This is often stripped when the engine is installed in a boat as not really suitable and replaced with… very little. If the environment in the machinery space is well protected and the air ducts well designed, the absence of filter is seen as a detail. It is a very noisy detail as a tremendous amount of valve noise escapes from the intake, not to mention that abrasive particles can flow freely into the cylinders.

Exhaust System

The exhaust system starts with the exhaust manifold, the hottest part of the engine. In a vehicle, it is naturally air-cooled. The arrangement is totally unsuitable in an enclosed engine room, so a manufacturing a water-jacketed manifold is one of the first tasks undertaken.

It is time-consuming as it needs to fit the exhaust ports on the side of the head perfectly and not leak any amount of water, which would readily enter the cylinders. Fabricated water-cooled exhaust manifolds are notoriously awkward to weld due to the tight spaces and welding distortion compromises bolt hole alignments.
Nothing that can’t be overcome, but the result is never to par with a cast component and water leaks through cracks or weld defects down the track are not uncommon. Those tend to cause considerable engine damage.

Marinising a turbocharged engine presents further challenges as marine turbochargers are water-jacketed. Wrapping an automotive turbocharger with high-heat insulation is not an option. It will not stand the resulting temperatures for long.

Next down the pipe is water-injection in the case of wet exhausts. Water pressure, flow and configuration of the water injection point all need to be correct to obtain a good result. The water needs to cool the exhaust gases before they reach any component that is temperature-limited, typically a hose, not just flow down the line with the gases.
Manufacturing a good mixing elbow is time-consuming and usually requires experimentation to obtain acceptable operation both at idle and full load.

Then comes exhaust piping and silencing. Common issues are incorrect pipe sizing and inadequate silencing. A good exhaust system is dimensioned to provide low back-pressure and good silencing. Oversized exhaust are usually noisier then they should be. Installation guidelines are available with a marine engine, but not with a marinised automotive engine.


A frequent outcome of home-made marinisation is a noisy, rattle-prone and unreliable installation. If the boat is little used, the need to address the issues may not be prominent enough and some problems just don’t actually have a chance to develop into consequences; but if the vessel is steaming a lot of hours, the initial savings are quickly wiped out by alterations, repairs and re-engineering where a proven and well-dimensioned engine package would provide long, quiet and reliable service instead.

Such projects can be quite interesting and challenging for someone with a passion for engines and diesel power, but they can also lead to a lot of frustration.

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