July 28, 2005 - HOW WE GET A CHARGE OUT OF SAILING
(With a Prop Shaft Alternator)
by Michel & Jane DeRidder

After putting up with an annoying gear whine for several years, we wanted to replace our V-drive with something quieter. Perhaps we could use a cog belt to make a belt drive transmission? Cog belts are used in industry in countless ways, and today many engine manufacturers use them as timing belts to drive camshafts and other running gear. We checked to see what was available. But it wasn't until we were en route from a spell in the Caribbean and Europe and about to head back to home territory in BC from the Virgin Islands that the belt drive plans became reality.

For the long homeward trip, we decided to replace the 35-horse DA120 Isuzu engine with a larger one, a 75-horse DA220 Isuzu this time round. While we were at it, why not change the transmission? We looked into it, ordered sprockets, a timing belt and bearings to make the transformation.

The Mark One belt drive transmission, much quieter than the Walter V-drive, was with us for a couple of years before ideas bubbled up as to how to improve on it by making a quick dismantling feature just in case we should ever have to replace the belt in a hurry one day. Devising and redesigning the front bearing supports through the center of the belt once again took many months of cogitating and sketching. (It always takes longer to dream up and draw experimental innovations than to build and test them.)

It was a good time to make the change, as we'd installed a Hundested variable pitch prop not many months before and we wanted to change the direct drive to a reduction gear ratio, now that we could feather the prop under sail.

When eventually Magic Dragon was again in the San Francisco Bay area, we looked up Doug Duane, a Kiwi master craftsman we had met there some years before. Doug had sailed out from New Zealand on White Squall with Ross Norgrove in the early sixties. He was grounded in San Francisco with four offspring (including triplets) and had begun building a boat of his own. Doug agreed to do the welding for us. We supplied the drawing. As a result of Doug's expert welding we ended up with a neat, strong, simple solution.

Old belt.

The cog belt in 1971 was a stainless steel cable-reinforced one. Later we got a spare belt that we stashed in an easy-to-get-at hideyhole, this one a newer type, Kevlar fiber-reinforced. The original belt took us safely and quietly for some 3000 engine hours over the ensuing years before breaking on a windless approach to New Caledonia's east coast reef in 1979. A quick replacement - timed by stopwatch at twenty-three minutes - surprised even the one who'd planned it all so many years before. More surprising yet, the fiber-reinforced belt was still like new in 2003 when it was replaced with the spare after 24 years and some 9000 hours running!

It was only recently in Dragon Time - perhaps seven years ago - that the designer cum gungineer realized he was missing a good chance to use the belt drive system to charge the batteries under sail by incorporating one of our spare alternators. Our power demands are much higher today than they were originally, what with our laptops, radios, and GPS, as well as battery charging for cell phones, digital camera and so on. Once again, this project was weeks or months in the design stage before its actual emergence.

Our engine is coupled to the belt drive with a "Capitol" mechanical reverse gear box. It has a cam-driven oil pump on the propeller shaft side so it can run in neutral without the engine and still get pressure oil lubrication. The belt drive with its 1:1.72 reduction ratio, when driven by the propeller, gives us the opposite effect, speeding the primary shaft by the same ratio. With a 7" V-belt pulley driving the 2" alternator pulley coupled to the belt drive overdrive, we get a 1x6 ratio between the propeller and the alternator - a suitable speed increase to get the alternator to charge the battery.

We tested it on a winter escape from NZ to Tonga. In June the sun is low and often blanketed behind the sails when heading northeast, rendering the solar panel next to useless. We found it such a luxury not to have to run an engine just for battery charging when on passage. Why hadn't we done this years before?

Once sailing speed reaches six knots or more, we adjust the variable pitch prop for maximum shaft revolutions before switching on the alternator. We are able to charge at over forty amps if sailing speed is sufficient and the battery is hungry enough. The 24" Hundested propeller doesn't do that work for free, however, and we do lose speed while charging. If the wind drives us at barely 6 knots, we fall down to 5 knots. If we have wind power to do 7 or 7.5 knots, we lose half a knot. But while we were running before a gale with the rolled genoa alone driving us at 8 knots and more, the alternator was putting out some 42 amps without slowing us down much, if at all. Interestingly, as it turns out, the best propeller pitch setting is identical to our best setting for economy cruise under power, so that the system should be feasible with fixed propellers too, as long as the reverse gear box can be run in neutral by the prop shaft.

We can also charge batteries with what we call "a Double Whammy" using both alternators if we are going only a short distance under power and need to charge quickly. To accomplish this we can switch the propeller alternator in parallel with the engine alternator so both of them charge at the same time, giving us twice 60 amps, or a theoretical 120 amps.

Even at anchor, we can charge the batteries with a Double Whammy by setting a zero pitch in the prop and running the engine in gear while charging the battery.

In theory, we could keep our battery charged by going for an afternoon sail and letting the prop churn up amps...In practice, we run our little generator when the battery gets low, unless we are sailing somewhere and the wind is cooperating. But on windy passages we've found it very useful to be able to charge the battery when needed without having to run the generator or the engine.