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 counter-rotating 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.
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), and Xenta (VMA Plus). These systems don’t control the rudders; they control the bow thruster (and stern thruster, if installed) and the engine shifts and throttles as the walking effect of the props move the stern from side to side.
There’s a big difference in responsiveness between a convertible sportfisherman with big props and an express cruiser with small props in deep tunnels, and that’s where stern thrusters come in. Since much of the prop wash from a small propeller bounces off the sides of the tunnels, preventing their little bit of side force from walking the stern over, the stern thruster picks up the slack, with the engines providing just fore and aft control.
Pods with a joystick
Pods have a big maneuvering advantage over inboards by virtue of their ability to turn from side to side. When the pod turns, thrust is redirected, not deflected, so control is more crisp and precise. The pods also act as rudders, so you get a combined thrust-and-rudder effect, which results in more immediate response and better control.
There are differences between brands of pod drives, too. Comparing IPS and Zeus, IPS boats generally walk sideways faster; the Zeus system’s flat-sided tunnels can interfere with prop wash directed to the side. On the other hand, IPS boats tend to heel more in a turn, which makes sense because the pods, mounted at 90 degrees to the hull’s deadrise and projecting farther away from the boat’s center of gravity, create more of a couple. If pods make the boat heel too much at high speed, the manufacturer can back off the turning angle. However, it’s best to get the hull design right to control the heel, so the boat turns like a bicycle.
The joystick that started it all is the Hinckley JetStick, which was developed by Tom Serrao and introduced in 1998 for the company’s waterjet powerboats. Waterjet boats with joysticks are wonderfully maneuverable and can be more agile than pod boats. However, they also have little resistance to sideways movement, lacking running gear, so they take more skill to handle in close quarters. It’s hard not to show off when driving a jetboat with a joystick, but it’s also easier to get into trouble if you get ahead of yourself on the learning curve.
The most maneuverable boat I’ve run — with the best response in close quarters, including positive control of the bow in a strong crosscurrent — is the single-diesel Back Cove 37 with variable-speed Side-Power DC bow and stern thrusters. For precision control, there is just no substitute for 200 pounds of variable sideways thrust in the bow and stern. The boat I ran had individual controls for the two thrusters but no joystick; with a joystick, it would be the best system possible, in my view.
This boat was also far more maneuverable around the dock than a twin-inboard convertible with big props and rudders. However, when playing a fish at 4 or 5 knots, the tables are turned, as the thrusters ventilate and lose power.
The next-best-handling system, in my experience, is Volvo Penta’s IPS. It spins and moves the boat sideways faster than any pod boat I’ve run. The absence of tunnels, which can obstruct thrust, is a big reason for this agility. The Zeus system’s prop pockets also reduce buoyancy and lift at the stern, which can result in increased bow rise when coming on plane. This is countered with self-adjusting trim tabs as part of the package.
How quickly can you actually learn to drive a pod boat? In a lot less time than it takes to run a conventional inboard boat well. Pod drives won’t take all of the apprehension out of close-quarters maneuvering, nor should they; we all need to stay on our toes. But they make running a boat much more enjoyable and eliminate much of the tension when docking. They are certainly great for anyone who is moving up to a larger boat and is worried about docking.
Test the joystick before you get near the dock to make sure you have control. As with conventional controls, use the least power required to get the job done, applying short bursts and using the low-speed maneuvering mode to avoid building up too much speed and momentum. Of course, this should be balanced against the need for enough headway to control the boat. Focus on driving the boat’s pivot point, whether turning close to the end of a dock or backing into a slip. Pay attention to what’s around you and the distances to the nearest objects, and the bow and stern will take care of themselves.
Make sure the helm ergonomics work for you. I prefer the joystick in my right hand — I’m right-handed — and positioned so I can hold it and comfortably look aft. If your boat has a searchlight, you should be able to control it with one hand and operate the joystick with the other.
Lastly, learn how to drive a boat using conventional controls, even if it has a joystick. It’s a skill you should have to more fully appreciate what you’re asking of the joystick. You’ll take more pride in a job well done docking without the stick — and for good reason.
April 2015 issue