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The Propshaft Alternator


The author's boat, Calypso V, powering along—and creating power as she goes.
By Denis Glennon

Picture yourself sailing as the sun goes down and a beautiful egg-shaped moon slowly comes to share the heavens with the stars. Life is easy and on this particular night, sailing is easy. You are using running lights, autopilot, full instrumentation, radio, radar, and listening to your favorite CD. You are not concerned about draining your sailboat's battery banks. The stuff of dreams? No. It is feasible, if you harness the tremendous electrical output from an easy-to-install supplementary charging systema propshaft alternator. 

Alternate Supplementary Charging Systems    The most pleasing thing about the alternate 12-volt supplementary charging systems available today is the ability to integrate wind, solar, and water energy sources. Clearly, solar and wind sources come to the forefront while at anchor, if there is sufficient sun and/or wind available to generate the surplus amps required.

Solar panels are mostly maintenance free, produce no noise, and when correctly positioned free from shadows, and capable of being gimbaled (90 degrees to the sun can deliver more than a 25 percent increase in output), they give that wonderful feeling of getting something for nothing. There is an abundance of technical information available to ensure a competent installation by any cruiser, and without the need for specialist tools or expertise. 

Wind generators have advanced in design and performance in recent years. In particular, the ever-present noise and annoying vibrations have been noticeably reduced in the latest models.  However, should you prefer downwind sailing, the apparent wind speed is reduced with a subsequent decrease in electrical energy generated. If your cruising pattern is predominantly enjoying a glass of wine in peaceful, sheltered anchorages, wind generators are not a practical proposition for delivering the extra amps needed to keep your white wine chilled at the correct temperature. Like solar power, the technical information available on wind power is copious. A competent installation presents few problems for most cruisers.

"It is possible for a cruiser with reasonable mechanical and electrical knowledge and skills to undertake a successful propshaft alternator installation. "
Water Charging Systems    For reasons that are unclear, water charging systems (towed turbine generators or submerged/submergible generators) are not prolific on cruising boats in Australia. On the other hand, permanently mounted propshaft-driven alternators/generators are commonly found on larger displacement boats, many of which are fitted with a substantial fixed-blade propeller that happily freewheels under sail. A number of major gearbox manufacturers have dispelled an earlier unfounded fear of gearbox damage due to improper lubrication while freewheeling.

Once the propshaft gets up speed, tremendous power can be generated by having a large drive pulley (around 10 inches in diameter) on the propshaft, driving a small pulley (around 1.5 to 2-inch diameter) on an adjacent, low speed alternator/generator. This is especially the case once the boat speed gets above 4.5 knots, typically generating eight to11 amps at a cruising speed of 6.5 to 7.5 knots. This output is well in excess of the five or so amps usually obtained from two large solar panels. Most gratifyingly, the eight to 11 amps are generated continuously under sail, irrespective of time, the intensity or angle of sun-rays, or the strength of the wind. The latter two are far from being predictable!

Limited technical information is available to select the optimum propshaft alternator for a given auxiliary engine drive arrangement. There is some guidance in the Australian publication The Marine Electrical & Electronics Bible by John C. Payne (pages 84-85), and in Offshore Cruising Encyclopedia by Steve & Linda Dashew (pages 521-523). Despite the absence of detailed guidance, it is possible for a cruiser with reasonable mechanical and electrical knowledge and skills to undertake a successful installation.


Calculating the probable horsepower from a freewheeling propeller may seem like a dauting task, but according to the author this shouldn't be a deterrent to carrying out your own installation.
Each Propshaft Alternator Installation is Unique    Calculating the probable horsepower from a freewheeling propeller is a little complex for most cruisers. This is due to the many variables in propeller physics, e.g. diameter, pitch, blade area, tip clearance to hull, friction in stern tube, gearbox type, aperture profile (i.e. the type of opening the propeller sits in), how free from barnacles the propeller is kept, etc. In practice, there is really no need to worry about this.

In order to capture the maximum electrical output from a propshaft alternator, there is a strong temptation to fit the largest size alternator in the space available. The torque delivered by the large drive pulley fitted to the propshaft, and the speed at which the alternator rotates will determine the level of electrical output. This torque creates new, additional forces on the coupling/shaft key and keyway, coupling retaining studs, the gearbox aft seal, and possibly on the stern tube. Higher than necessary torques may contribute to premature wear in the shaft key and/or the keyway, or in the gearbox rear seal. The rectification or replacement of either can be a very difficult and expensive task in most boats due to the awkward location. Be grateful for small mercies!

The primary objective should be to extract the optimum electrical output from a propshaft alternator without damaging existing equipment that will interface with the new installation, (i.e. gearbox seal, stern tube packing, stern tube itself, and the proshaft coupling). Generally this means (1) maximizing the size of drive pulley (circa 10 inches in diameter) that can be fitted to the gearbox/propshaft coupling, (2) maximising the propshaft/alternator pulley diameter ratio (5: 1 gives good results), (3) determining the most suitable location for fitting the bracket on which the alternator itself is to be mounted, and (4) ensuring ease of maintenance and belt tension adjustment. Notice that maximizing the electrical output is not the dominant factor.

There is no fixed set of installation plans that apply to all boats. The best ideas for a particular installation come as you hang like a fruit-bat with your head and shoulders dangling under the cabin sole! Don’t hurry this investigation and planning stage, since the best place to eventually fit the mounting bracket will probably not be the “obvious” one you pick when you first put your head down there. Ease of access for checking and adjusting belt tension in later years should be high on the list of priorities.


Looking aft: The 10-inch diameter drive pulley mounted on the gearbox/propshaft coupling, the toothed vee-belt drive to the two-inch. diameter pulley on the alternator, aluminum mounting bracket, and 25-amp alternator, as fitted to Calypso V.

Installation of Propshaft Alternator on Calypso V    Wherever possible, Australian-made or “readily available in Australia” electrical parts are installed on our 49-foot ketch Calypso V. When things go wrong, and they will, we know we can go to the nearest marine or automotive electrician and replace or repair the ailing unit. This thinking prevailed when selecting a propshaft alternator. We chose an Australian unit, especially wound to achieve high outputs at low speeds. A kit, containing alternator, large and small pulleys, control switch panel and bracket, was purchased. The mounting bracket is made of 6mm aluminum to minimize weight and corrosion.

The large pulley seen on the above image needed some machining and drilling of four holes to fit onto the existing coupling, but this took only about 45 minutes in a local engineering shop.


Looking forward: The very effective, easy accessible, belt tensioning arrangement, the small (two-inch diameter) pulley fitted to the alternator, the vee-belt and the mounting bracket.
All of the components used in the alternator are readily available from local auto electricians. Bearings, brush holders, and slip rings in the unit are interchangeable with Bosch and Ingram alternators both of which are commonplace here and abroad.

The mounting bracket is bolted to the fibreglass structure underneath the cabin sole. It may be bolted to a vertical bulkhead running parallel to the propshaft. To avoid excessive belt and bearing wear the most important factor in the installation is ensuring the correct alignment of the two pulleys. A straight edge and eye sighting will suffice. A spare belt was fitted around the propshaft during the original installation five years ago and has not yet been required. The belt lasts longer if only used when sailing. When motoring or motor-sailing, the propshaft alternator field is switched “Off” so that the rotor spins freely with no load (also see wiring diagram below).


The general wiring diagram prepared by the author shows the shunt fitted in the supply line to the batteries. This is connected to an onboard battery management monitor that gives instantaneous readings of amperage being produced by the propshaft alternator.

For ease of use and safety of operation, a master switch, and separate warning light were fitted in a visible position on the cockpit binnacle. This master switch is an “On/Off/On” type and is also used for the auxiliary motor, thus providing a fail-safe system to prevent the propshaft alternator field becoming operative when motoring. All aspects of the installation were carried out by the author over two weekends. No special tools were required.

Placing Propeller Drag and Noise in Perspective    Theoretically, under any load, the alternator will brake the propshaft by slowing shaft rotation, causing drag and thus a reduction in boat speed. On a lightweight yacht used for racing, where a loss of even 1/10 knot can become a life threatening experience for crew, this state of mind will probably not be acceptable or conducive for successful cruising. On a medium to heavy displacement yacht, the inertia of the vessel simply overcomes the drag effect. The unsurpassed joy we get from listening to Andrea Bocelli and Celine Dion singing “The Prayer” (on repeat!), as we sail on a star-lit night, not worrying about “precious amps,” places the topic of propeller drag in perspective relative to what matters in this life.

Using the instrumentation on Calypso V (boat speed measurable to 1/100 knot) we have never been able to detect any difference in boat speed between having the propshaft alternator “On” or “Off." Remember, however, that the fixed three-blade propeller (22-inch diameter x 19-inch pitch) is freewheeling in any event. Why not capture all that available power?

If there is any downside to fitting a propshaft alternator it is associated with the small change in noise level emitted from a constantly spinning propeller. We noticed a slight increase in noise level initially, but, this was traced to the original spinning propeller noise (which we were used to) being transmitted through the aluminum mounting bracket to the fiberglass structure, through the teak sole, and hence to the inside of the aft cabin, which is the master cabin. Fitting soft rubber /flexible mountings between the bracket and the fiberglass rectified the minor annoyance.

The propshaft alternator now “purrs” along and sends the off-watch captain to sleep, happily dreaming of amps being deposited in the battery bank, and enjoying the self-sufficiency, that cruising demands. Our unsung hero, the propshaft alternator contributes in no small way to the owners and guests finishing each cruise physically, emotionally and spiritually refreshedthe way cruising is meant to be!


About the Author:  Self-sufficiency, that ability to sort out things for yourself when you are out of touch with the instant convenience of shore side advice and repairmen is a major component of Denis Glennon’s sailing philosophy. He has installed a variety of power sources on his 49-foot ketch, Calypso V. Each contributes to the overall peace of mind that comes from combining an onboard integrated, balanced energy system, with wise conservation during power consumption.  

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