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Will an auto diesel fit in my boat?

From Ocean Navigator #85
November/December 1997
Question: I have the opportunity to buy an early-80s vintage Mercedes 240D very inexpensively. I'd like to install the engine (a four-cylinder diesel, model 166) into my 24-foot wooden motor cruiser, a converted lobster boat. I now have a freshwater-cooled 305 Chevy gas engine.

A Mercedes 240D diesel would need to be fitted with proper cooling, exhaust, and transmission systems for marine work.
   Image Credit: TB Ocean Navigator Photo

How do I couple the Mercedes engine to my existing transmission? What are the extra parts I will need? None of the marine stores seem to know anything about Mercedes marine engines and therefore have no advice for me. The car has only 120,000 miles, a pittance for a diesel Mercedes, so it seems like a great possibility for my hull, which is in great shape.

Carlos Cantone

Peaks Island, Maine

Answer: In principle, a diesel engine is a diesel engine, so an automotive diesel will work fine in a boat. And, in fact, many marine diesel engines are nothing more than "marinized" car or truck engines. However, when using a car or truck engine in a boat, the marinizing process is sometimes complicated. There are three principle issues that must be addressed: the cooling system, exhaust, and transmission.

As far as the cooling system is concerned, you have to go from the car's air-cooled radiator, to a heat exchanger. The heat exchanger has to be built for the marine environment, with particular attention paid to ensuring that metal components are compatible with each other and not subject to galvanic interaction (corrosion). The heat exchanger will also need to be sized to match the engine. If an exchanger from a marine engine with a comparable, or higher, hp rating is used, this should suffice. There is then the need for a raw-water pump on the seawater side of the heat exchanger; this is generally a rubber impeller-type pump, driven off the crankshaft pulley.

This is not the end of the cooling system changes. The engine will want a water-cooled exhaust manifold and a wet exhaust (a dry exhaust is too noisy). Unfortunately, the water-cooled manifold will need to be bolted to a custom-built marine cylinder head. The standard head on an automobile head won't do. (The Mercedes block in question was formerly marketed with a Volvo marinized head, according to Bill Sweetman of Orr's Island Boat-works, a Volvo/Yanmar sales and service facility in Orr's Island, Maine. Production of this engine stopped some years ago.) The wet exhaust requires the installation of a water-injection nipple on the discharge elbow from the exhaust manifold, with the raw-water discharge from the heat exchanger plumbed to this elbow, and a water-lift muffler set below the exhaust elbow. Some careful engineering is needed to ensure that water from the exhaust piping and muffler is also sized to match the power output of the engine, once again using a comparably rated, or higher-rated, engine as a model.

An automotive transmission is no use in a boat. A marine transmission is needed. The present transmission in Mr. Cantone's boat might work, but it must meet these requirements:1.The transmission must fit the engine flywheel housing(it might be possible to use an adapter between the engine and the transmission). The transmission must have a drive shaft that can be coupled to the engine's output shaft.2.The transmission must be rated for the engine's output.3.The gear ratio (reduction) must match up with the needs of the engine.

There are then other, lesser issues such as the fact that the engine will not have a suitable instrument panel, so one will have to be fabricated. At the least, an oil-pressure gauge and temperature gauge are needed. Unless these can be matched to the existing sending units, the sending units will need changing. I would also strongly recommend an hour meter (to keep track of engine use and maintenance intervals), and probably a tachometer. The ignition (cranking) circuit may need some changes. Some re-wiring of the engine harness may be needed. Finally, a marine engine has a governor, which holds the engine speed to a set level irrespective of the load on the engine (the equivalent of a cruise-control in a car). To be quite honest, I have never thought about this aspect of a conversion before, but I suspect some engine or fuel-injection pump modification may be needed to produce the desired result. However, it is not essential. Without a governor, the engine will just vary its speed somewhat as the load varies.

Clearly, marinizing a diesel engine is something that has to be carefully thought out. For many engines, this thinking has already been done by someone else, with the result that there is an off-the-shelf package that can be bought to do the job. This makes it pretty straightforward. But when it comes to Mr. Cantone's Mercedes diesel, he would have to build a custom cylinder head and a custom conversion kit for his transmission, since marinizing kits for this particular engine are no longer available. He would probably spend more on this conversion than he would buying a new marine diesel.Contributing editor Nigel Calder is the author of several books. His latest is Cuba, A Cruising Guide, published by Imray, Laurie, Norie & Wilson. Send questions to "Asked and Answered"; mail: Ocean Navigator, P.O. Box 569, Portland, Maine 04112; e-mail:; fax: 207-772-2879.