A boat with triple Volvo Penta IPS pods maneuvers like a dream.
Volvo Penta revolutionized marine propulsion in 2005 by introducing its IPS pod drive system. Joystick control was added a year later, allowing two pods to turn and shift independently and create the thrust vectors needed to make the boat move sideways and diagonally or spin in its own length. It was a brilliant idea, and Volvo Penta quickly brought the concept to market.
Compared to inboards with conventional prop shafts, pods offer increased efficiency at speeds above 25 knots (inboard drivetrains can be more efficient at lower speeds), reduced vibration and a higher top end for a given horsepower and fuel flow. What has really made these units popular, however, is their maneuverability at low speeds.
If you turn the pods toward each other (by the right amount) and put one in forward and the other in reverse (at the right rpm) the boat will “walk” sideways. Adjust their angle and thrust, and the boat will spin or move diagonally. Push the stick, and the computer figures out what to do with the pods.
Pod drives are ideally suited to these maneuvers. They have counterrotating propellers, so the props’ side force, or walking effect, is canceled. When you back down on one unit with a single prop to start a maneuver, the side force may well be working against the results you are trying to achieve. For example, when you back down the starboard engine to walk the boat to starboard, the prop will want to walk the stern to port.
Compared to outboards and sterndrives, as well as the rudders on many inboard boats, pods are positioned farther forward beneath the hull. This means that pulling the bow to one side or the other is made easier by shortening the distance component of the “force x distance” needed to create a couple, or rotating, moment. This is an important point since pulling the bow sideways against a strong beam wind or current is the pod’s weakest link.
Inboards
Conventional inboards have fixed propellers and rudders that turn to deflect prop wash. This results in inboards being very good at controlling the stern from side to side and the boat fore and aft, but they are ineffectual at pulling the bow from side to side. The exception to this is when the props and rudders are set unusually far apart and well forward.
To account for this, many inboard boats use bow thrusters to push the bow from side to side. With a bow thruster, it’s easy to walk a twin-screw inboard sideways. To walk to port, put the rudder amidships, port engine ahead, starboard engine astern, thruster pushing to port, and use mostly the starboard engine’s throttle (the prop develops less thrust when backing) to control the boat’s position fore and aft.
An inboard joystick controls the bow thruster and the engines, replicating what I just described, but it has only a single lever with which to contend. Skippers sometimes get into trouble with conventional controls when trying to keep the boat parallel to the dock while closing in on it, as well as keeping it properly positioned fore and aft. When a joystick system is installed in a boat, it’s tuned to the particular hull, so pushing the stick sideways results in the proper amount of thrust being applied to move the boat sideways.
Joystick systems for inboards are produced by ZF Marine (Joystick Maneuvering System), Cummins (Inboard Joystick), Twin Disc (Express Joystick System), Yacht Controller (Joystick Control System),