Test-driving vectored thrust

May/June 2007

In a recent issue we discussed the introduction of the Volvo Penta IPS and Cummins Zeus system drives (“Vectored thrust for voyagers,” July/August 2006, Issue No. 155). But what is it like to drive a power voyaging boat using these drive systems? Recently I gave both of these units a try and found that they provide impressive performance.

In fact, your next fast trawler yacht will likely go faster and farther on less fuel, and be remarkably quieter and more maneuverable than any trawler you have ever operated if it is powered with Volvo Penta IPS or Cummins Zeus system drives. Results achieved with the IPS drive on a wide variety of planing hull express cruisers, ranging in length from about 30 to 75 feet, show a typical improvement of 30 percent in cruise speed fuel economy, a maximum sustained cruise speed improvement of 15 percent, improved acceleration and a very substantial reduction in exhaust noise (often 6 dBA or better).

Many owners of trawler yachts are changing the way in which they use their boats. While living aboard remains a compelling goal, the reality of work and other responsibilities often limits opportunities to use the boat to a weekend, three or at the most four days. Under these conditions, cruising speed becomes an important consideration. An anchorage, yacht club or marina that is in easy reach at 20 knots can be a place too far at 10 knots. This use pattern is spurring a transition from the single-screw, full-displacement, cruise at hull speed trawler yacht to semi-displacement, twin-engine designs that can cruise at sustained speeds in excess of 20 knots. Replacing the conventional propulsion systems in these vessels with vectored thrust systems such as the IPS or Zeus drive will result in eye-opening performance improvements.

The driving experience

Maneuvering a boat equipped with either IPS or Zeus drives is a point-and-shoot exercise. You don’t have to know anything about how the systems work, how they are installed on the boat or, in the

extreme, anything whatever about how to handle a boat. All you need to do is move the joystick in the direction toward which you want the boat to move and it will go there, at a speed dependent on how far you deflect the joystick. Want to rotate the boat about its axis? Twist the joystick without deflecting it from its spring-loaded center position. The direction of rotation follows the direction in which the control is rotated; the speed of rotation depends on how far the control is rotated from its neutral position.

In my test of the Volvo Penta IPS and the Cummins Mercruiser Zeus drive systems they performed as advertised, including the low speed/zero speed region that often poses significant maneuvering challenges. The integration of power and steering functions through the joystick/computer control provides a totally transparent control system. The user has no need to know or concern about what the drives are doing; the boat responds quickly, goes in the commanded direction, at the desired speed and will stop with little or no overshoot.

Maneuvering in windy or tide-ridden conditions is totally unchallenging once the degree of displacement is determined with reference to surrounding objects and the appropriate amount of position correction commanded. When coupled to a specially configured GPS, the system can station-keep, holding position and heading even in the presence of the wind/current forces. At this time the special GPS interface is not an integral part of either system, however, it is an attractive feature and is likely to become an option in the future.

Both the IPS and Zeus drive systems make bow and stern thrusters unnecessary in virtually all yacht power applications. However, it is possible that the additional control provided by a bow thruster might be desirable in vessels whose forward configuration presents an unusual amount of flat plate area, such as a workboat with a high fo’c’s’le structure.

Handling a boat equipped with one of these propulsion systems is like flying a stick-controlled aircraft. After a short period of time the machine becomes a virtual appendage that, like your hand, goes where you want it to with virtually no conscious effort. It is that good!

Bottom line, I wish I had one of these systems on my 46-foot, single-engine ketch, but then I would have to have two engines €¦ unless of course someone develops a single-screw version.

The standard approach

The typical fast trawler is equipped with a pair of 6-cylinder inline diesel engines that deliver 315 to 425 hp. The running gear consists of relatively long prop bracket supported shafts, inclined at between 7° and perhaps 10°, turning three-bladed propellors. The system works. It is comfortably familiar and until recently has been universally accepted as an appropriate way to propel this type of boat. However, this system has a number of inherent flaws.

The prop shafts must be angled downward to provide clearance between the hull and the prop (a minimum of 15 percent of prop diameter is usually considered minimum). The downward angled shaft wastes a significant part of the thrust developed by the prop by directing it downward, creating a force that works to lift the stern of the boat. The inclination of the prop plane from the vertical results in what aircraft pilots know as “P” factor, asymmetric thrust developed by a propellor whenever its disc is inclined from 90° to the air or, in this case, water flow. (The blades in one half of the prop disc, the semi-circle in which the prop blades are moving downward, operate at a higher angle of attack relative to the flow of water than the blades in the semi-circle in which the blades are moving upward, resulting in an overall loss of prop efficiency since only one half of the prop is operating at an optimum angle of attack.)

The result of this effect is familiar to anyone who has operated a single-screw vessel; the boat turns either to port or starboard (depending on the direction of rotation, the “hand” of the prop), a particularly vexing problem when maneuvering astern. The steering effect is cancelled out in most twin-screw boats by counter-rotating the props, however, the inefficiency remains.

The relatively long exposed prop shafts and their brackets extract an efficiency penalty due to the drag force created as they impede the flow of water beneath the boat. The rotating prop shaft imposes an additional power loss due to the Magnus effect, rotation of a column of water that surrounds the shaft caused by the boundary layer effect. The molecules of water that are immediately adjacent to the shaft remain firmly attached to the shaft’s surface. Each successive molecule remains somewhat less well adhered to the shaft, with a net result that a significant mass of water is being constantly rotated along with the shaft, creating drag relative to the flow of water past the shaft.

Since the direction of thrust from the propellors is fixed, rudders are required to redirect the prop thrust so that the boat can be maneuvered. Unfortunately, the rudders are relatively ineffective, they typically redirect only about 35 percent of the thrust in the direction needed to turn the boat, and they create drag by interfering with water flow even when set at zero steering angle. The rudders, normally located aft of the prop, are also a major source of frustration when maneuvering the boat astern since they have virtually no initial effect and become sources of drag rather than steering aids if they are moved more than slightly off center.

Outboards and stern drives avoid many of these problems. Unless intentionally trimmed in or out, their prop shafts are horizontal, avoiding the misdirected thrust and “P” factor effects. These drives deliver thrust in the direction required for maneuvering, eliminating the need for rudders. With no exposed rotating prop shaft there is no Magnus effect loss. Overall, both outboards and stern drives are excellent ways to propel a boat. However, we rarely see them being used to power a trawler-type yacht. With very few exceptions outboards are gasoline fueled and are optimized for high-speed performance, not for the low speed, high thrust required for a trawler-type yacht. The aluminum lower units of stern drives can be problematic when they are continually immersed in seawater.

The Volvo Penta IPS and Cummins Zeus drives provide the attributes of a stern drive equipped with contra-rotating propellors. The propellor shafts of both systems are always horizontal, eliminating the prop inefficiency caused by inclination of the prop plane. There are no rudders; steering thrust is obtained directly by rotating the drive about its vertical axis. The lower units are constructed of bronze and stainless steel to withstand continuous immersion in seawater.

Throttle and steering control of the IPS and Zeus systems are via the system’s computers. The steering angle of each of the drives is individually controlled. In a turn, the inboard drive rotates through a larger steering angle than the outboard drive, just as the inside wheel of a car turns through a “sharper” angle than the outside wheel.

The boat’s turning circle can be significantly smaller than the best that can be achieved with a conventional drive system. Since all of the steering commands are executed in the computer’s software, it becomes a relatively simple operation to optimize the steering response of the system for any given type of boat and hull.

By Ocean Navigator