Two cylinders or three?
Along with the Yanmar 2GM and 3GM models mentioned in the text, diesel engine manufacturer Westerbeke makes a similar engine, the 30B,seen here, a three-cylinder diesel that develops 27 hp at 3,600 rpm. It comes with a standard 2.47:1 reduction marine gear box and is freshwater cooled so a potable water heater can be connected to it. Image Credit: Courtesy Westerbeke
Question: Can you shed any light on selecting the proper size diesel for repowering? I wish to replace the Atomic 4 in my 1969 C&C Corvette 31, and I am evenly split between the two-cylinder and three-cylinder Yanmar. In working on this I found a specification sheet that seems to indicate that the boat weighs 8,485 pounds. Inside the vessel is a large wooden "7 ton" placard. What is the purpose of the placard? Roger A. KarmesNorth East, Md.
Answer: Taking things in reverse order (and why not?) the placard likely refers to the measurement value of the boat, which is in reality a reference to its supposed cargo carrying capacity. This measurement was most likely made in the process of applying for documentation. The lower limit on documenting pleasure boats used to be five tons, which of course had nothing whatever to do with the displacement of the vessel, or anything else of value for that matter. Best forget about it unless you plan on going to England and operating as a Thames Barge, in which case it still may have no real meaning in today's world. Your choice of Yanmar engine should be dictated by the type of Atomic 4 installation in your boat.
If your engine has a 1:1 gear box, with the prop turning at engine speed, it means that you have a small prop and therefore the two-cylinder Yanmar 2GM 20F(V) is the best engine choice. If the Atomic 4 (does it really glow in the dark?) has a 2:1 gear then the boat has a larger prop and the best Yanmar choice will be the three-cylinder 3GM 30F(V) engine. The F in the engine designator indicates that it is a freshwater-cooled engine, the best choice for engine life and a real plus if you intend to connect a potable water heater to the system. While you are doing the engine conversion, please attend to the fuel-supply system.
You will need a fuel-return line to the tank and should install a Racor fuel filter/water separator between the tank and the engine. An electric fuel pump between the fuel filter and the engine is also a very worthwhile investment. It will be used primarily to re-prime the system after a fuel filter element change. Also, check to ensure that there is adequate air flow into and out of the engine compartment. A cool diesel is a happy diesel. The exhaust temperature needs to meet the manufacturer's specifications. With these small engines your boat's current electrical system is most likely fine, but check to be sure there is no corrosion on any of the terminals and that the battery or batteries are in good condition. When using the new engine, run it hard for the first 50 to 100 hours to ensure that the piston rings seat properly.
Unlike old-style diesels, these newer diesel engines want to run relatively fast, so don't be afraid of running them at 2,800 rpm all day. Sailors new to diesel engines do more harm by running their engines too slowly and at too light a load than by any other operating error. Also, and this is very important, read the owner's manuala little bit of knowledge can go a long way. And finally, even though you used and depended on it for years, try to not miss the old Atomic 4 too much!