Backup propulsion

Plankton may thrive while drifting aimlessly across oceans, but a dead-in-the-water posture is a power-boater’s worst nightmare.

Ever since I was a kid and learned to carry oars in my skiff, even though my trusty Seahorse usually purred like kitten, I’ve liked the idea of a secondary means of propulsion, and it came as no surprise to discover that many naval warships, cruise liners and workboats of all description have rallied to the call and found ways to pack auxiliary propulsion units in their bilges.

Today’s technology ranges from thrust-producing devices with just enough oomph to make minimal way in a calm to high-power diesel-electric engines that can be run by their propulsion generators or from the hotel-size ship’s system generating equipment. In fact, when it comes to the increasing popularity of diesel-electric power plants, the closer in kilo- or megawatts the propulsion engine is to that of the ship’s system energy needs, the better.

Let’s take a wide-angle look at the options available to recreational power-boaters favoring a single-screw approach to voyaging.

Pony diesel One of the simplest alternatives is an offset-mounted small diesel and conventional drive train that’s about the size of a sailboat auxiliary. The horsepower of the engine varies with the windage and other design characteristics of the vessel, but a starting number would be 10 to 20 percent of the main propulsion engine’s horsepower.

The drive train is a conventional shaft and stuffing-box arrangement connected to either a feathering or folding prop. The thrust developed is offset to one side, and the water flow over the rudder is reduced dramatically, but in most installations it at least affords a modest amount of maneuverability. In tight-handling situations, a bow thruster will be worth its weight in gold. This is especially true aboard longer low-displacement vessels with a substantial run of keel contributing to directional stability. If the boat’s designer hasn’t already defined such a secondary power option, consult a naval architect prior to tackling a new installation.

An example of such an auxiliary propulsion system aboard a 45-foot single-screw trawler might include a naturally aspirated (56-hp) Yanmar 4JH4E diesel, a PYI dripless stuffing box and a feathering Max-Prop or Jensen Flex-O-Fold folding prop. The upside includes standalone engine-to-prop duplication, simplicity and the potential value added in bracketing the front of the engine to hold a pair of 12- or 24- volt DC alternators. This secondary electrical energy source gets along well aboard a 12- or 24- volt DC-dominant boat with plenty of battery bank capacity. Those averse to prolonged genset running could use such an auxiliary propulsion system as an alternative battery charger. The downside of the system includes the need for extra bilge space and room for the drive train. There’s also some drag induced by the running gear and the costs associated with maintaining another drive train.

Separate tiller arms

In the most popular types of steering systems for small sailing vessels, cable over sheave, jacketed cable, or a geared tie-rod system (Whitlock or Edson), if the cable parts, the quadrant slips or some other component gives up, the boat can be steered with the autopilot if a separate tiller arm is used. Remember, bronze and aluminum components are galvanically incompatible. Rudderstocks may be bronze, stainless or sometimes aluminum (particularly on an aluminum vessel); quadrants, and tiller arms may be aluminum or bronze.

Ensure that metals that are unfriendly to each other, particularly aluminum and bronze, are not installed so they remain in direct contact. Water dripping off a bronze tiller arm onto an aluminum quadrant or hull could cause severe galvanic corrosion over the course of months or years.

A final word on drive-unit installation: It is critical that the drive unit, if linear, have more travel than the tiller arm. The drive unit should never act as the rudderstop or shorten the travel of the rudder.

If there is to be only one display, it is probably best to install it near the helm so it can be adjusted, monitored and disengaged easily. If installing multiple stations, or a hand-held remote, the main unit should go at the nav station and the remote at the helm (I have installed repeaters in the skipper’s bunk on many a cruising vessel). Most remotes use a multiprong plug (with the trend toward wireless accessories, some remotes are now unencumbered by this electronic tether), and thus more than one location can be chosen for remote operation.

The electrical consumption of the drive unit can be prodigious, especially in a seaway, and thus the desired voltage drop of no more than 3 percent must be achieved. The calculation for this formula must include a “there and back” figure for the electrical supply cable. If, for instance, the drive unit is 20 feet from the main power distribution panel, the voltage drop formula would use 40 feet of cable for the overall run.

If in doubt, consult the ABYC Standards and Technical Information Reports for Small Craft (available at or by calling 410-956-1050), or one of the many marine electrical books that contain reprints of the voltage drop calculation tables. In addition to performance and efficiency issues, the life of an electric motor may be noticeably shortened by low-voltage scenario, and undersized cabling will surely exacerbate this condition. Make the connection between the drive unit and power supply via an insulated marine-rated terminal block to facilitate the difference in wire size between the drive unit and vessel side of the cable run, as well as for ease of serviceability and replacement.

When making electrical connections, include service and drip loops in each wiring run, and heat shrink connectors offer the greatest degree of strain relief and water resistance. The over-current protection for the entire autopilot system should be a circuit breaker as opposed to a fuse, and it should be easily accessible and well labeled for the purposes of resetting. It also should be dedicated to the autopilot, sharing no other loads.

Most installation manuals are fairly specific about where and how to mount the fluxgate compass. Needless to say, the farther this piece of gear is mounted from large iron masses – such as the engine and anchor/rode, or electric motors, pumps and high-current cabling – the better. Post placards in lockers and storage spaces adjacent to the fluxgate compass, prohibiting the storage of ferrous objects (one can of beans can ruin an autopilot’s entire day, and yours). Temporarily install the fluxgate until sea trials are complete. This will enable you to ensure it’s well placed and not affected by any of the aforementioned objects.

Swinging the compass is a critical part of the installation. Follow the manufacturer’s instructions meticulously. When doing this, it is best to energize all loads that will normally be on when sailing, both day and night. Electric current sets up a magnetic field and sometimes will affect performance and accuracy of the fluxgate, or any other, compass.

By Ocean Navigator