The "Marinizing" Process - Continued

Marine motor manufacturers (those who are in the business of selling ready-to-go "marinized" engines) often make additional modifications that aren't as easy for the individual converting his own engine. For example, cams may be changed or re-ground in order to change torque and horsepower characteristics so they will be more suitable for boat use. Carburetors may also be changed or modified, since on a boat the carburetor may operate at half or full throttle most of the time, which may be unlike an automotive carburetor that must operate over a variety of settings.

Fig. 3-13: A typical "bobtail" marine engine conversion kit. With this type of installation, a marine transmission is not used. (Courtesy of Glenwood Marine Equipment)

For those who want to convert their own gasoline engine to marine use, the following will outline the process. Note that this process is basically the same one that a marine motor manufacturer will use in "marinizing" a motor that will end up as a so-called "marine" motor. Basically, the conversion process involves adding, deleting and changing parts, many of which are external or "bolt-on" in nature. A typical marine conversion "kit" may consist of:

  1. Water cooled exhaust manifolds
  2. Marine water pump
  3. Carburetor tilt shim or angle plate (perhaps)
  4. Carburetor flame arrestor or "backfire" trap
  5. Brass freeze plugs (for saltwater use)
  6. Engine mounts
Fig. 3-14: A completed "bobtail" conversion of an automotive V-8. This engine will be used in a stern-mounted v-drive installation without a marine transmission. Note the hoses connecting to the exhaust elbows from the top of the engine block, and the water pump driving off the end of the camshaft. (courtesy of Nicson Engineering)

In addition to these components, depending on the engine, other parts might include an oil filter relocation kit, oil coolers, water cooled exhaust risers or elbows, various cover plates, various hose and plumbing assemblies, various brackets and pulleys, starter shaft extensions, lifting rings, and other items. In some cases, the oil pan will be replaced because of shape or material. Ordinarily, automotive pans are stamped sheet metal, which can corrode quickly if in contact with seawater in the bilge. Heavier cast iron pans are better in this respect. Also, a pan of greater capacity may be required for marine use, or a reshaped pan may be necessary due to the boat's structure interfering, or because the shaft angle is so severe that the oil sump will be poorly located to provide positive lubrication.

Fig. 3-15 & 3-16: Typical water cooled exhaust manifolds as used in a marine engine conversion. These manifolds are made from light weight cast aluminum, shown in traditional "log" and "header" styles.

1 - Water-Cooled Exhaust Manifolds

In the typical engine used for shore-side purposes, the exhaust manifolds are cooled by air alone. However, in a boat where the engine is often confined within closed spaces, this heat is hazardous. Also, the heat may cause the engine to overheat and hamper engine breathing. Consequently, the exhaust manifolds must be cooled, and this is done with water jacketed manifolds. These are perhaps the most obvious aspect of the marinized engine. In some limited uses, especially for competition boats where weight is important, and the engine is not completely confined, air cooled manifolds (or "dry stacks" as they are called) may be used (see Chapter 15). In normal boats, however, this practice is not safe.

2 - Marine Water Pump

The automotive-type engine uses a water pump that is the recirculating type. It is not intended to pump water, but to circulate it instead. On the other hand, the marine water pump must be capable of pumping water into the engine's cooling system. Furthermore, it must be able to stand up to marine conditions, such as salt water, without corroding. Most marine water pumps are of the "Neoprene" hard rubber impeller type that can stand up to sand and silt. The water pump may be fitted on special brackets or fitted in the allotted space for the automotive type it replaces. The pump may be driven with pulleys and vee belt, or directly off the engine cam. Usually the marine pump runs at a considerably slower speed than the motor.

3 - Carburetor Tilt Shim

In many marine motor installations, the engine is often set at some angle from the horizontal. This is quite unlike, for example, an automotive installation where the engine is basically level most of the time. Since gasoline engine carburetors are intended for level operation, the tilt shim is used to bring the carburetor back to its correct plane of operation. Tilt shims are available for many different angles depending on the installation angle. They are also available for a wide range of carburetors and number of barrels or throats. If the engine is mounted horizontally, a tilt shim is not necessary.

Fig. 3-17: A carburetor backfire flame arrestor is a necessary part of gasoline engine conversions for safety. Such units prevent flames from the engine during backfiring which could ignite volatile gasoline fumes that might accidently enter the engine compartment.

4 - Carburetor Flame Arrestor

An air cleaner is not required on a marine motor because it operates in basically clean air surroundings. Since a gasoline powered engine can backfire on occasion, this presents a potential hazard of fire or explosion aboard a boat, especially if there is no air cleaner. Consequently, the air cleaner is replaced with an approved-type flame arrestor that prevents this condition from being dangerous. These units are available in a wide range of types and sizes to adapt to various carburetors. Always allow ample clearance above downdraft carburetor flame arrestors for sufficient air to reach the engine.

5 - Brass Freeze Plugs

Automotive and other similar type engines have machined holes in the block or cylinder head for use in fabricating the engine. These holes are plugged with freeze or expansion plugs usually made from steel. When engines are used in salt water, these steel plugs will rust and fail rapidly, and consequently should be replaced with solid brass plugs (but not brass plated steel plugs!). Only those plugs which will be in contact with corrosive conditions need be replaced, however.

6 - Engine Mounts

Because the engine will usually fit to motor stringers or similar mounting members, special mounts are required for most conversions. Such mounts may be separate components, or built in as part of other components, such as the flywheel cover. Either rigid or rubber type flexible mounts may be used, with the four-point mounting being common.

7 - Oil Filter Relocation Kit

With many converted engines, the standard location for the oil filter could make it inaccessible. Therefore, kits are available to relocate the filter to a more convenient location. In addition to the filter, a water cooled oil cooler is frequently added in line with the filter unit. Oil temperatures often exceed those found in automobiles due to the lack of air flow.

8 - Water-Cooled Exhaust Risers & Elbows

Water-cooled exhaust elbows or risers are usually required in conversions. The elbows direct the exhaust downward so that the cooling water can be exhausted through the exhaust line without backing up into the engine, which would damage the engine. Similarly, the elbows also provide backflow protection in the event that water enters the exhaust line from the back of the boat. If the engine is located deep in the hull or so the exhaust manifold water outlet would be too low to have a proper downward slope, risers would be used instead of elbows.

9 - Cover Plates

Depending on the engine and the conversion kit, various cover plates may be required. Water cooled manifolds often have cover plates on the ends with threaded adapters where the cooling water hoses are connected. Flywheel covers are often fitted which may also serve as engine mounts, as well as being machined for mounting the starter motor. Timing gear covers are sometimes available with provisions for mounting a cam driven water pump. For safety, cover plates are often available for covering pulleys and vee belts.

10 - Starter Extensions

On some conversions, the starting motor, if left in its original position, could be subjected to moisture and splashing bilge water, or may be in a position that is difficult to service due to the angularity of the motor when installed. In these cases, starter extensions are sometimes available which relocate the starter to a more forward or higher location on the engine.


A consideration with most marine engines is the need for some type of transmission. These are discussed in detail in Chapter 4, however, in many conversion kits, they may be part and parcel to the conversion process, and therefore may require advance thought on the part of the purchaser of an engine or conversion kit. As will be pointed out later, the standard automotive transmission is not really practical for marine use and is therefore dispensed with, together with the clutch. Automatic transmissions found on automobiles also fall into this category, although there are some exceptions. Since a boat does not have brakes, usually some sort of transmission arrangement must be provided so that reverse can be engaged to slow or stop the boat. Consequently a marine transmission is frequently coupled to the end of the engine.

If a transmission is not coupled to the engine, the engine is called a "bobtail" engine. The "bobtail" engine is used with sterndrive units, jet drives, and in certain v-drive installations where the transmission function is a part of the v-drive. In certain competition boats, a "bobtail" engine without a transmission of any type is used. In other words, the engine is started in gear and the boat takes off as soon as the engine is started. The only way to stop such boats is to shut off the engine. Obviously, such a boat is dangerous and the practice is restricted to racing boats operating under controlled conditions. Basically, the "bobtail" engine consists of a flywheel housing for enclosing the flywheel, and usually to provide the rear mounts of the engine. A drive shaft or coupling is also necessary to connect the flywheel to the power transmission system, be it a jetdrive, sterndrive, or v-drive.