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Vectored thrust for voyagers?

Jan 1, 2003

Once the province of big ships and tractor tugs, vectored thrust is becoming increasingly available for power voyaging vessels. Both Volvo Penta and Cummins Mercruiser Diesel have introduced new types of propulsion systems for power voyaging boats. The new Volvo Penta drive system, called the Inboard Performance System (IPS), and the Cummins Mercruiser Project Zeus system provide a number of compelling advantages for some types of pleasure boats.

Props have typically been installed on shafts that were inclined downward by about 7° to 10° and made long enough to provide the necessary clearance between the top of the prop and the bottom of the hull (usually at least 15 percent of the prop diameter).

Although a very workable way to propel a boat, the inclination of the prop shaft creates a number of problems. The inclination angle causes a portion of the thrust developed by the prop to be directed downward rather than straight aft. The ring of water that rotates with the usually rather long exposed prop shaft creates a drag loss (the Magnus effect). Most significant, the inclination of the prop disc relative to the flow of water causes a "P-factor" effect, an imbalance in the thrust delivered by each semicircle of the prop disc that results from the difference in the relative angle of attack of the prop blades on the "downward" rotating side of the prop from the relative angle of attack of the blades on the "upward" side of the prop disc. ("P-factor" accounts for the turning effect that is evident on virtually all single-screw vessels that use inclined prop shafts.)

The supremacy of the inclined prop shaft approach in recreational vessels has been repeatedly challenged, first by the outboard motor, later by inboard/outboard and water jet drive. All of these alternatives avoid the efficiency limitations inherent in the conventional inclined prop shaft approach and also provide steering control with vectored thrust. They push the boat in the desired direction rather than pushing ahead, and then intercept and redirect a portion of the thrust with the rudder.

While the inclined prop shaft has remained the standard for most recreational boats, the maneuvering needs of some ships led to the development of alternative prop installations such as the ABB Azipod drive, in which a horizontal propellor shaft is located in an easily steered, watertight pod beneath and outside the vessel's hull. The Azipod's prop is usually driven by an electric motor contained in the pod. A similar system, the z-drive, where the driving engine is contained within the hull, is used in tugboats.

Volvo Penta presented a very significant challenge to the status quo for recreational boat propulsion with the late-2004 introduction of the IPS drive. Offered only as a twin-engine system, the IPS closely resembles the Azipod in that the prop shafts are horizontal and steering is accomplished with vectored thrust by rotating the drive pods around their vertical axes. The IPS places the prop ahead of the pod in a tractor position ensuring that the flow of water into the prop is unimpeded. The IPS uses contra-rotating propellors, simultaneously gaining the advantage of reducing the boat's draft and recovering the swirl loss that occurs when large amounts of power are absorbed by relatively small propellors.

Precise, efficient maneuvering is one of the virtues of any drive that uses vectored thrust. The steering angle of the two drive units in the IPS is controlled via the system's computer. In a turning maneuver the system commands the drive on the inside of the turn to move through a greater angle than that of the outboard drive, an action analogous to the way in which the front wheels of a car must move to provide proper, skid free tracking. The result is a significantly tighter turning radius, especially at high speeds. An optional joystick control is available for use in docking and other low-speed maneuvers. The system computer interprets movement of the joystick so that the boat moves in the direction the control has been deflected: ahead, astern, directly to port or starboard, or any intermediate direction. Rotation of the joystick translates a turning motion around the center of the boat. All of the maneuvers are carried out by individually controlling both the azimuth angle of each drive, and the direction and speed of rotation of the props. Volvo Penta's claim that there is no need for a bow thruster was born out of a recent demonstration of the system in docking and undocking maneuvers in windy conditions.

Compared to a conventional angled prop shaft setup, the IPS drive has demonstrated a maximum speed advantage of approximately 7 percent and an acceleration time advantage of about 13 percent when installed in identical planning hulls using identical engines. A fuel economy advantage that can reach 30 percent is probably of even greater interest. The engine exhaust flows through the drive pod and is injected into the water at the center of the prop wash, resulting in an odor-free exhaust system that can be as much as 6 decibels quieter than a conventional exhaust.

Cummins Mercruiser introduced their azimuthing twin-engine boat propulsion system at the February 2006 Miami Boat Show. While the prototype system closely resembles the Volvo Penta IPS there are a number of significant differences. The contra-rotating props are mounted aft of the drive skeg in the conventional pusher position. The system is designed for installation in hulls that provide prop tunnels that reduce the distance the props extend beneath the bottom surface of the hull, and the system offers integrated trim tabs. Production installations will likely become available late in 2006.

The idea of having a boat's propellors located in pods exposed beneath the hull and especially in a forward-facing position raises the question of damage in the event of grounding. Although the Azipod drive on a large ship or a z-drive on a tugboat is rarely challenged by an accidental grounding, the same is not true for recreational vessels. Exploring uncharted and poorly charted waters, or running the Intracoastal Waterway with its shifting channel and infrequent dredging means that boats might "take the ground�VbCrLf on occasion. The IPS and the similar Cummins Mercruiser Project Zeus system must be able to deal with both modest ground contact and a serious, often high-speed grounding without tearing a hole in the hull and flooding the boat (a real problem with conventional inclined prop shaft systems where severe ground contact can tear prop brackets from the hull causing massive flooding).

The design of the IPS drive, largely shared with the Project Zeus drive (except that the Cummins system places the props in a pusher position behind the drive pod and recesses a portion of the prop in a partial hull tunnel), incorporates a short skeg that projects a short distance below the prop disc. In a minor grounding, such as might occur when the boat moves slowly toward a sloping beach, the skeg can contact the seabed before the prop, providing the operator with a nautical curb-feeler.

In the event of a serious ground encounter the IPS drive is designed to shed its lower unit, allowing the housing and props to shear off at a designed-in weak point just below the bottom of the boat. The design of the system prevents seawater from entering the hull; water can do no more than fill the gearbox that contains the 90�-angle drive for the vertical drive shaft. Repair is accomplished by removing the boat from the water, flushing the gearbox and installing a completely new lower drive unit. This method of repair can take less time and be less costly than repairing the damage, including hull damage that can occur with a conventional prop shaft system.

The price of a boat equipped with the Volvo Penta IPS or the Cummins Project Zeus drive will likely be close to that of another boat with an identical engine equipped with a conventional shaft drive. Both the new drive systems are offered as complete, ready-to-install packages. The hull must be specially prepared for either unit with a tunnel design required for the Cummins drive. The drive leg or pod is installed from beneath the boat and secured in place with a massive clamping ring. The engine is easily mated to the drive without the need for installation of stuffing boxes or prop shaft brackets. A flex coupling between the engine and the gear assembly eliminates the need to align the prop shaft with the marine gear. No additional through-hulls or exhaust mufflers are required. Cooling water enters through the lower drive unit, and exhaust gas and cooling water are expelled through the underwater drive unit.