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Why all the fuss about pod drives?

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Pod drives, which proponents call the most advanced marine propulsion system in the world, are gaining serious market share in the mid-size powerboat market. Boatbuilders and customers alike are making these new drive units on the must-have equipment for the next generation of powerboaters with the need for speed.

A pod drive transmits a marine engine’s thrust to the water using an integrated transmission, external pod, and propeller(s). The technology was invented in 1950, came into practical use in the shipping industry in the 1960s, and has been proved over millions of miles propelling ships across oceans. The yachting world came aboard with its own twists, bringing these drive systems into the 21st century—downsizing them and making them more versatile, responsive, and silky-smooth by integrating full computer control. Pod drives for yachts were introduced in the USA by Volvo at the Miami International Boat Show in 2005 and Zeus drives followed in 2007.

Why all the fuss about pod drives? Here’s a quick look:

  1. On a mid-sized yacht, pod drives replace (and allow the boatbuilder to eliminate) much of what’s been long considered essential equipment: rudders, conventional steering systems, large propellers, long prop shafts, big mufflers, stuffing boxes, shaft couplings, transmissions, and transmission coolers, and trim tabs. Even bow (and stern) thrusters.
  1. The footprint of a marine diesel engine and pod drive is much smaller than that of a conventional marine engine installation. Each drive is mounted using soft but heavy-duty mounting, which absorbs much of the vibration. Pod drive lower legs are made of bronze and use nibral or stainless steel counter-rotating propellers.
  1. Hydrodynamically, a pod drive is more efficient because its thrust is parallel to the yacht’s hull (with no down angle like conventional running gear) and each pod’s two propellers rotate in different directions, canceling out much vibration and all rotational energy.
  1. Pod drives on twin-engine yachts are independently steerable, meaning each can direct its thrust in a different direction. That and integrated computer control (standard with pod drives) have brought a revolutionary improvement in yacht handling and maneuvering.
  1. Pod drives offer greater speed and range, which translates to the need for less horsepower and less fuel. Back-to-back tests are few, but manufacturers are suggesting fuel savings between 10 and 30 percent.

When a pod-equipped yacht is underway, its engines and drives are controlled by its drive-by-wire software. This allows the skipper to use a joystick for close-quarters maneuvering which makes docking easier, more intuitive, and less stressful. Joystick control means the skipper can move the yacht sideways, twist it within its own length, or dock it with ease. Buying a new twin-engine boat with pod drives costs 10 to 15 percent more than an identical boat with conventional running gear, but fuel savings over the life of the yacht can help make up for that.

Before you decide to rush right out to place an order, consider the downsides. One is that you cannot add pod drives to your present boat. A yacht has to be designed specifically to accommodate pod drives because the engines and drives must be mounted all the way aft, and hulls designed for conventional engine installations don’t have the necessary balance, added buoyancy aft or bottom configuration needed to accommodate pod drives.

The good news about the back-of-the-boat location for engines and drives is the noise, vibration, and heat from the engines is well aft of crew spaces, so the ride for captain and crew is smoother, quieter, and cooler. And with no engines beneath the main saloon deck, it’s easy to reclaim that valuable space for other purposes.

Another potential downside: maintenance will more likely be difficult with pod drives than with a conventional drive system. While pod manufacturers and distributors are working hard to deliver good service for owners, with these systems on the market for only few years you shouldn’t expect every boatyard to have mechanics or techs trained and experienced in pod maintenance. And maintenance intervals for pod drives are shorter: 25 hours (for a new system), then every 250 hours or 1 year, whichever comes first—fluid changes, zincs, prop shafts, seals, and more.

Though others are getting into the game, Zeus and Volvo IPS are the two names at the top of the list whenever you research pod drives, and it’s clear that they have built the lion’s share of pod drives aboard yachts today. Cummins MerCruiser chooses not to provide exact numbers, though it points out that Zeus drives are currently being installed in more than 100 different yacht models across nearly all major manufacturers. Volvo reports that more than 16,000 IPS systems are in use worldwide, roughly 45 percent of those in the USA.

In the Volvo IPS system, the propellers face forward, meaning they work in clean, undisturbed water, with exhaust exiting each pod from the rear. Volvo’s Marcia Kull, North America VP for marine sales, notes that IPS is a single system designed, developed, manufactured, and supported by one company and one dealer network. Ms. Kull reported that as of January 2013 Volvo had over 150 IPS-certified technicians in the United States.

Zeus drives employ aft-facing propellers with the exhaust exiting through the propeller hubs. Zeus places high priority on protection: all installations include hull tunnels to protect the drives, with additional further protection from the lower leg ahead of the props on each drive. According to Cummins MerCruiser Vice President Rob Mirman, Zeus aims to have all pod service handled by Zeus-trained technicians. However, Mirman was not able to say how many U.S. technicians are currently Zeus certified.

As a longtime Maine cruiser, I couldn’t help but wonder how pod drive boats would fare in the lobster pot-strewn waters of Maine where I have to work hard to avoid pots, so I went to an expert for answers. Bentley Collins of Sabre Yachts told me that after five-plus years of operating Zeus-equipped boats in Maine, he is not aware of any entanglements. “The sports car-like handling of these boats makes pot avoidance easier—almost fun,” he said. “On occasions when I have overrun a pot or toggle, there has never been any entanglement. The counter-rotating props don’t suck the pot into the stream of the prop the way a single prop would, and there is virtually no space between the props for the warp to be sucked into the vortex.”

But there’s bad news if you’re a single-screw skipper: pod drives are a long time coming because these systems are at their best in twin-engine yachts, and manufacturers today are simply not focused on pod drives for single-engine boats.