Aft of the engine

From Ocean Navigator #78
November/December 1996
Things such as gear boxes, flexible couplings, shafts, intermediate bearings, stuffing boxes, stern tubes, cutless bearings, and struts are faceless names referred to en masse as a boat’s “drive train.” Admittedly, the topic of drive trains is rarely considered an exciting one; however, some or all of these vaguely familiar devices are found on every boat. Those found on your boat not only profoundly affect how the boat goes through the water, but your comfort and convenience as well. There are a number of types of drive train. Even though the straight drive is the most common, presently no one system is superioreach has its pluses and minuses.

The straight drive train is often selected by designers because of its economy. We can agree that the straight drive takes the least design effort and is the simplest to install, but it may not be the most economical to operate and maintain. Since boat owners are involved with operation and maintenance, a more comprehensive economical analysis that includes these factors as well as comfort and convenience makes more sense.

The cost of operation can be improved by better efficiency. The cost savings due to easier maintenance is more difficult to assess, but easy engine access saves time during repairs$60 to $80 per extra hour adds up quickly. Putting a dollar value on comfort and convenience is entirely subjective but is still a real part of the decision. For example, on your boat is the engine located, along with its noise, heat, and odor, near the prime living space?

Some of the common drive train options available include the straight, vee, hydraulic, constant velocity, stern, jet, and sail drives. We’ll take a look at some these drive train options.

The straight drive

The straight or conventional drive is found on an overwhelming number of displacement hulls. The advantages of this drive system are that it is simple to design and install, and, theoretically, this drive places the engine in the ideal spot for any heavy object on a boatlow and near the center of gravity.

Two obvious disadvantages of this engine location on boats using straight drives are immediately apparent. The central location is prime living space, certainly not made better by a hot, noisy, smelly engine. Second, the lower in the boat the engine is positioned, the more difficult it is to work on.

A not-so-obvious disadvantage to this system is that commonly the engine is required to be installed at an angle so that there is adequate tip clearance between the propeller and the hull. The safe operating angle of most engines is less than 20°. The sloping propeller shaft is so common on conventional drives that most marine transmissions now have a shaft connection flange that angles downward 7° or 8° to reduce the engine installation angle. This improvement allows the engines to be mounted closer to horizontal and requires less space in the engine compartment, but ignores the inefficiencies and handling problems generated by angled propellers.

Vee drive

The vee drive has nearly all the components of a standard drive, plus it adds to the system a unit called the vee gear which breaks the drive shaft into two parts and changes its direction. Using this system, the engine is mounted with the shaft running forward, and the addition of the vee gear allows the shaft to turn back on itself and exit the hull in the conventional manner.

This feature allows the engine to be mounted higher than normal and out of the main living area. Moving the engine up allows for better access and easier maintenance. When planning for the installation of this drive, the shaft between the engine and the vee gear needs to be long enough so that servicing the components of the drive train under the engine can be performed without lifting the engine.

The disadvantage of this system is the additional 1% to 2% loss of efficiency caused by the vee gear and the potential balance problems from locating the weight of the engine aft. The loss of fuel efficiency is minor and can be recovered by installing the shaft horizontally and using the vee gear to correct for engine slant.

Moving such a large weight as the engine aft and up in a small recreational boat will strongly affect the longitudinal and transverse stability of the boat, and therefore it must be addressed during design or by a naval architect during modifications to keep the center of gravity low and near the center of buoyancy. In a power boat, trim tabs can be used to overcome the some of the balance problems. In a sailboat, ballast will probably have to be redistributed.

Hydraulic drive

Hydraulic drives for boats can be considerably simpler than conventional or vee drives. The difference between this and the conventional system is that the transmission is replaced by a pump, some hydraulic lines and a hydraulic motor. Also, no flexible joint is necessary since the engine is not connected to the shaft.

Hydraulic motors come in two basic configurations: closed and open loop. In both of these configurations the engine can be set at a constant rpm and the speed of the boat regulated by the hydraulic valves. The open loop is simpler in design than the closed, but is less efficient and requires a larger oil reservoir and more plumbing.

In the open loop, speed and direction are controlled by two valves, thus some energy is wasted when the pump operates against partially closed valves. The open system requires three levers, one for speed of the engine, a second for speed of the hydraulic motor, and a third for the direction of prop rotation.

The closed system, while a little more complex inside, is simpler to install and operate. The speed and direction of prop rotation are controlled by changing the angle of the vanes in the pump using a single lever. This modification wastes less energy and requires a smaller reservoir because only the amount of oil needed to turn the shaft is sent to the motor.

The hydraulic drive offers many advantages. First, the engine can be mounted anywhere on the boat and even oriented athwartships. Since the engine is not attached rigidly to the propeller shaft it can be mounted on very flexible supports, greatly limiting noise-producing engine vibrations from being transmitted to the hull. Further, the hydraulic motor is so small that it can be mounted low down in the keel giving the advantages of a horizontal shaft. The shaft can be shorter and cheaper and alignment problems minimized, although it may be necessary to install an external thrust bearing.

Since shaft speed is independent of engine speed, the engine can run at optimum speed and the speed of the shaft controlled by the hydraulic motor. Finally, once the basic hydraulic system is in place, other hydraulic equipment can be added easily. For example, a hydraulic anchor windlass, a bow thruster, an emergency high-capacity pump, or even a second propeller can be powered from a single engine installation.

The major disadvantage of this system is the cost. The pump and motor may cost three to four times as much as the transmission. A minor consideration is the extra care that needs to be taken to see that the hydraulic oil does not get to the environment. This means a sump and drip pan under each separate hydraulic motor, manifold, or major connection point. All connections should be made with quick disconnects that allow the removal of hose lines without oil leakage.

Even though boat-yards or marine repair facilities are often unfamiliar with hydraulic equipment, service of this gear is only of minimal concern. Hydraulic equipment has been the mainstay in large construction equipment for 50 years. Even in remote places it is not hard to find people who repair heavy logging, mining, or earth-moving equipment. Further, since very few D9 bulldozers are driven into town for repair, you can also expect mechanics to be mobile and to show up at the boat with all the necessary tools and parts in their trucks.

Constant velocity

Constant velocity (CV) drive is a marine application of the constant velocity joints used in the automobile industry. In the marine application the joint incorporates a thrust-bearing into the hull and uses two constant-velocity joints to take up misalignment and vibrations.

The major advantage of the CV drive is that can take considerably more misalignment than a conventional flexible joint. Since the CV drives can take as much as 16° between the engine and shaft alignments, or even parallel misalignment, they are commonly used on light, high performance boats, where flexing of the hull is a constant and considerable problem. The actual misalignment the shaft can tolerate, however, is dependent upon shaft RPM and thrust, and it can be as little as 3°.

In the recreational boat market CV drives are basically used to reduce engine noise. With the CV drive, the engine can be mounted on soft rubber mounts, which reduces the engine vibration and the noise caused by these sympathetic vibrations through out the boat. The CV drive shaft can be used with vee drives, jet drives, or even a remote outdrive to reduce engine noise.

The disadvantage of these joints is that they require extra space and cost. Ten to 20 inches of extra length, depending upon the horsepower of the engine, between the engine and the stern tube is required for their installation. The cost of these units for a typical boat is more than twice that of a standard flexible coupling.

Sail drive

Sail drives are similar to outdrives in appearance in that an inboard engine is coupled to a drive leg; but, unlike outdrives, the sail drive is non-steerable. The power is transmitted to the propeller through two 90° bevel gears. These units are only currently available for engines under 30 hp and have been used on some small fin-keel sailboats. They are again simple for the builder to install, requiring no shaft or stuffing box. The drive can be positioned with limited restriction and throws prop wash over the rudder. The sail drives are often used in boats that would normally use an outboard motor. The sail drive frees the helmsperson from wrestling with both an outboard motor and the tiller during complicated maneuvers. The major disadvantage of this drive is that it is difficult to seal. The rubber gasket is now in the bottom of the boat and under considerably more head than the stern drive gasket. Any failure in this gasket floods the boat with large amounts of water very quickly. Some drives incorporate an extra seal and a warning device in case the first seal is breached. The regular replacement of these seals is not a trivial job because they are often located under the engine. Like the outdrives, sail drives are made of light alloy and require adequate cathodic protection. Finally, unlike the outboard usually found on these boats, the engine cannot be removed for maintenance.

Jet drive

The jet drive is most often used in small power boats or as bow thrusters on large boats. This drive uses a large pump to direct a high-velocity stream of water. Deflectors can change the direction of the stream for steering and reverse. The drive has few internal parts and therefore has a low initial cost and low maintenance costs. The similarities between the mechanics of jet drive and jet propulsion are few, however. Jet propulsion compresses air, heats it by combustion to compress it further, and vents the hot, highly compressed gases through a nozzle to provide propulsion. Water is essentially incompressible, and it is not heated or burned in a jet drive. A jet drive is actually a pump system, and the propulsion is derived by increasing the speed and quantity of water flowing through the nozzle.

The advantages of the jet drive are that it is mechanically simple and more efficient than a propeller-drive craft. The lack of propellers also makes the drive safer to operate around people in the water. Since there are no protrusions below the water, a jet drive has less drag than conventional drives, can operate in shallow water, and cannot snag lines on crab pots, anchors and the like. The jet drive is a simple system without a gear box, propeller, strut, packing glands, and cutless bearings. Once thought to be fuel hogs, they have been improved so that they are now more efficient than outdrives. An emergency stop with a jet drive can be made by simply shifting from full forward to full reverse without changing engine rpm.

The disadvantages are that they are not often used on engines with less than 100 hp, the intake can become clogged with debris, and the nozzle is easily damaged because it protrudes beyond the stern of the boat. These drives also have a reputation for being noisy.

Whereas jet drives are traditionally found in fast planing hulls, a few have been used on house boats and catamarans. Even though their simplicity and lack of underwater protrusions make them attractive for possible use on displacement sailboat hulls, few have been used in this application. It seems to me that the system could be modified and used on a modern canoe-bodied hull.

All drive train choices convey advantages, but those must be weighed against their associated disadvantages. Depending on your boat, you chose the best trade-off.

Don Dodds is the author of Modern Seamanship, published by Lyons & Burford, and lives in Portland, Ore.

By Ocean Navigator