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Auxiliary Propulsion Unit

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Illustration by Steve Karp

The WESMAR APU can give single-diesel-powered boats the confidence to voyage farther.

Backup Plan

Put power to the prop shaft when you need it most.

Finally retired, you’re on a weeklong cruise aboard your trawler, just you and your wife. Suddenly your only engine quits, and you can’t restart it. Your peaceful cruise is over, and your wife is asking, “Are we stuck in the middle of nowhere?”

Fortunately you thought ahead—seemingly to this very moment—when you bought the boat: You ordered a backup plan, WESMAR’s Auxiliary Propulsion Unit (APU), and right now, it’s worth every cent of the $12,000 upcharge. You just grin, flip a switch, turn a key, push a button at the helm, and you’re back underway. You’re a hero.

A get-home system is nothing new, but one that is applicable to such a wide range of vessels is. The APU was invented by Ronald Voegeli, of Oak Harbor, Washington. “We licensed [it] and made it producible and usable,” says Bruce Blakey, president of WESMAR.

The APU is a hydraulic propulsion system that mounts on the aft end of a marine transmission via brackets. Within its cast-aluminum housing is a carbon-fiber belt, similar to those used on motorcycles, that is driven by two gears. An Eaton hydraulic motor driven either directly off the genset or a genset-powered A.C. electric motor, attaches to the upper gear. The prop shaft passes through the larger lower gear. When the APU is engaged, two small cylinders on the housing push it forward and rotate the unit slightly to align two drive rings, one on the front of the APU and the other on a five-inch shaft coupler on the back of the transmission, that lock onto the prop shaft. The hydraulic motor turns the gears, which rotate the belt to spin the propeller. When the APU is not in use, limit switches on the hydraulic cylinders hold it in place abaft the transmission to ensure that it doesn’t accidentally engage.


Illustration by Steve Karp

To simplify installation, WESMAR offers diagrams, such as this schematic of a hydraulic system that can provide power to the APU.

In the event of an engine failure, the helmsman simply turns off its electrical power source and powers up the generator for either hydraulic or A.C. power. (Obviously if there’s a central hydraulic system that’s driven off the engine, he’d better have one of these two options.) A helm panel includes a key that when turned provides power to the APU, a start button, and a joystick. Among the improvements WESMAR made over the original unit are the helm start and allowing the unit to shift into reverse for easier docking. The APU works on either 12- or 24-volt D.C. systems and needs a dedicated circuit breaker.

WESMAR has been installing the APU mainly in single-engine trawlers; I saw it on the Selene 45 when Capt. Bill Pike tested that boat. WESMAR says it has also received inquiries from whale-watching boats because of the APU’s quiet operation.

WESMAR offers the APU in two sizes. The 200 series has a maximum rating of 25 hp and can take up to a 2.38-inch-diameter prop shaft. Pricing starts at $10,695. The 300 series maxes out at 100 hp, takes a prop shaft of up to 3.38-inch diameter, and lists for $12,400. Prices do not include installation. Expensive? Compared to the cost of a second engine and the required running gear—not to mention the additional mechanical complexity and hydrodynamic drag—it’s a good deal. And, as they say in those MasterCard commercials, being a hero in an emergency is priceless. PMY

(425) 481-2296.

This article originally appeared in the December 2011 issue of Power & Motoryacht magazine.