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July 02, 2013

Pods: Fad or Future?

Will Pod-Drives Overtake Conventional Shafts and Struts?

Big-game fishing is a pursuit rife with tradition and time-proven tactics. In many ways, fishermen — a nongendered term that embraces both sexes — trust what we’ve learned over decades of personal experience and the experience of our mentors. Our mentality could be considered the opposite of today’s kids’ in relation to new technology. We are not early adopters.

That’s the environment marine propulsion engineers entered when they envisioned a new pod-drive system, a form of propulsion for boats that could change the way anglers pursue big game. Pods have taken certain segments of the cruising-boat market by storm, and now big-game charter captains who were once fixed-shaft aficionados are embracing this new technology.

A Brief History of Pod-Drives

Volvo Penta introduced the first pod-drive system in 2004. The company didn’t invent them, but it made “pod” a household word among boaters. 

The first pod-drive was invented in 1955 and went by the generic name “azimuth thruster.” They were either diesel- or electric-powered, and some were large enough for a man to walk around inside the machinery housing. Some had forward-facing propellers. Some could be rotated 180 degrees and run adequately facing either fore or aft. Interestingly, the most notable early prop-forward applications were in ice-breaking ships launched in 1990 — an interesting defense to the argument that forward-facing props were too vulnerable to debris. 

Regardless of prop orientation, the first drives were a far cry from the watchlike beauties brought forth by Volvo Penta and, later, Zeus and ZF Marine.  

When Volvo Penta took the industry by surprise with the IPS, it had patented the forward-facing counter-rotating propellers, patented new propellers for the design and also sent the exhaust out the back of the pod, not through the hub of the props. The new props don’t ventilate, and their position alignment with the keel gave them a decided advantage by leveling the keel on the hole shot.

At Volvo Penta’s IPS rollout in Mallorca de Palma, identical inboard and pod-drive boats were pitted against each other. The pod-drive boats were clearly more maneuverable and accelerated more quickly than strut, shaft and -prop-drive inboards. 

First, hull efficiency was enhanced, because the props were always aligned with the direction of motion in the boat. Inboards’ props are angled downward, diminishing the power directed toward forward motion. Partly because of their angle of attack and partly because of their slightly forward position under the hull, pods gave more speed, better acceleration and more miles per gallon on the same horsepower.

Each pod turns independently of the other, giving remarkable maneuverability. By turning one engine pod at a sharper angle than the other in a turn, they could narrow its radius. By turning pods in opposite directions and running one in forward and one in reverse, the vessel can be pivoted more quickly and easily than in either outboards or inboards. 

The most remarkable performance advantage was one that sent every propulsion and boatbuilding engineer to the drawing board to imitate: the joystick control system. With it, fingertip operation of any vessel became possible. Instead of having to jog throttles and shift levers to effect a maneuver, simply push, pull or twist the joystick and the boat follows the -directions, pivoting in place or crabbing sideways — say, into a slip that’s just long enough to fit your vessel. 

One hardly noticed the greatest benefit to the end user: The engine systems were so compact, many boats could gain an aft cabin or at least a sleeping berth where diesel engines once thrummed away.