When we are anchored for any period of time, our solar panels and wind generator pretty much keep up with our power needs. On a passage, however, our requirements are higher. The additional electronics — auto pilot, nav instruments, AIS, radar, and so on — all require power. We usually have to run the engine an hour or two each day to keep the batteries charged. There are a number of reasons why we dislike doing this. On a long passage, the amount of fuel required just to charge the batteries starts adding up. If we are on a significant heel, we have to alter course or reduce sail before and after running the engine. Using the engine at low rpm and with a light load is hard on the engine. In addition, it’s annoying to disrupt that perfect broad reach on a warm, starry night by having to crank on the engine.
Several of our cruising friends have had good success with propeller shaft generators. In fact, our friend Eric on Fiona has sailed more than 300,000 nm with his prop shaft generator, and has nothing but praise for it. If we could generate another continuous 2 or 3 amps while we were sailing, we probably wouldn’t have to run the engine at all. Adding one to Nine of Cups had been on our to-do list for several years, and when we were in Durban, South Africa, a few months ago, I decided to take on the project.
What does a prop shaft generator do? We have a fixed-blade prop. When we’re sailing, the water moving against the prop causes the prop shaft to rotate. (We actually have a prop shaft brake to prevent the shaft from rotating when the engine is off, but it can be disabled.) By adding a pulley to the shaft, mounting a generator or alternator next to it, and connecting the two with a belt, we should be able to use the rotation of the shaft to generate power as we sail.
That’s the theory, anyway. The rest is just details. One of the more important details is which generator to use. There are three types of generators/alternators that can be used for this application and each has its own advantages and disadvantages.
Brush-type DC Motor – The most basic DC motor that has been around since the late 1800s, has a rotating coil mounted inside several permanent magnets attached to the outer housing. If a battery is connected to a 12-volt brush-type DC motor, it will spin. Conversely, if the rotor of a brush-type DC motor is spun, it will produce a DC voltage. If the motor is big enough and it spins fast enough, it can charge a battery. The advantages are that it is inexpensive and simple to implement electrically. It has several disadvantages, however. Since it has brushes that conduct a large current, the maintenance requirements are higher; it generates electromagnetic interference (EMI), which may be a problem with an HF radio; the maximum allowable rpm for this type motor varies widely, but is typically 1500 to 6000 rpm; and it is more difficult to keep cool.
Brushless DC Motor – This type is the reverse of a brush-type DC motor: the permanent magnets are attached to the rotor and windings are attached to the housing. Since the windings don’t rotate, the need for brushes is eliminated. The advantages of a brushless DC motor are that it requires less maintenance than a DC motor with brushes, generates little or no EMI, and is more efficient. The disadvantages are that it is more expensive, generates a 3-phase AC output that requires a diode bridge to convert to DC, and the maximum allowable rpm are usually 3000 to 6000 rpm.
Alternator – A typical automotive or marine alternator is also a candidate for a prop shaft generator. It overcomes some of the issues of a DC motor. Since it’s meant to be coupled directly to an engine pulley, the maximum allowable speed is typically greater than 10,000 rpm; they are made by the millions, so the cost is relatively low; the output is easily regulated by varying the field current; they are very efficient; and they are self-cooling. There are a couple of disadvantages as well. Since they are meant to run at high rpm, unless the windings are rewound with finer wire, the output at low rpm is quite small. The biggest disadvantage, however, is that an alternator requires a typical field current of 3 to 5 amps. When an alternator is connected to an engine, it spins at thousands of rpm and the field current is negligible compared to the total output. When the boat is sailing, however, and the total output is only a few amps, the field current becomes significant. In fact, at lower boat speeds, the field current will be higher than the amperage produced by the alternator.
Note: To keep things simple, unless I’m referring specifically to one of the devices, I will use the generic term “generator” to refer to any of the three types of alternators/generators.
Prop Shaft Speed
When Nine of Cups is sailing, the prop shaft ranges from zero rpm at 3 knots to around 200 rpm on those rare occasions when she is doing 8 knots through the water. When we’re motoring, taking the speed reduction ratio of the transmission into account, the shaft rotates at between 400 and 1400 rpm. The generator speed will be some multiple of the shaft speed, depending on the ratio of the two pulleys. Ideally, I wanted the generator to produce at least a few amps at the low RPM of the shaft while sailing, yet be able to withstand the much higher RPM while motoring.
The faster the generator spins, the more current it will generate. The ratio of the two pulley sizes determines how fast the generator will spin, which in turn determines how much current will be generated. To generate the maximum current, we want a very large pulley on the shaft and a very small pulley on the generator. On the other hand, when the engine is cranked on and we are motoring, the shaft will turn much faster than when we are sailing, potentially destroying the generator if it spins too fast. To determine the largest pulley ratio that can be used, divide the maximum allowable speed of the generator by the maximum prop shaft speed. For example, Nine of Cups has a maximum shaft speed of 1400 rpm. If I chose an alternator with a maximum speed of 10,000 rpm, the largest pulley I could safely use would have a diameter of 10,000/1400 = 7.1 times the diameter of the alternator pulley.
The three different types of generators each have different electrical requirements. The necessary electrical connections, how the output is controlled and/or rectified, and how the output is handled when motoring differs between the three.
Brush-type DC Motor – From an electrical point of view, the brush-type DC motor is the easiest of the three types. In the simplest implementation, the output is connected directly to a battery and when the generator shaft is turned, the battery gets charged. It’s a little more complicated than this, but not much. First, there must be a diode in the circuit between the battery and the generator. Otherwise, when the prop shaft is not turning, the generator would become a battery-powered motor, and the generator would try to turn the prop shaft rather than the other way around. Also, if the batteries are charged, the generator output current must be disconnected from the batteries to prevent them from being overcharged. One way to do this is to use a regulator that diverts the current into a dummy load when the batteries reach their charged state. Many companies that sell wind generators and solar panels also provide these charge controllers. We use the water heater as the dummy load. Finally, when motoring, the output should be disconnected, allowing the generator to spin freely with no load. The circuit sketch below shows a typical electrical circuit for this type generator. The output first passes through the contacts of a relay that is controlled by the engine ignition switch. When the engine is switched on, the relay opens the circuit, disconnecting the output of the generator. The notes at the end of this article include part numbers and sources for the components.
Brushless DC Generator – The output of the brushless DC generator is somewhat more complicated. Its output is a three-phase AC voltage which must be converted to DC in order to charge the batteries. This requires six rectifier diodes connected as shown in the circuit sketch below. An additional complication is that when this type generator is open circuited, the output voltage can reach 100 volts or more. To protect the diodes, three relays are needed, one for each phase of the output. As before, these relays are controlled by the ignition switch.
Alternator – The output of an alternator depends on the rpm and field current. A standard automotive or marine voltage regulator would work fine, except that at low speeds, the field current would exceed the output of the alternator and sailing at anything less than 3 to 4 knots would put a drain on the battery. A simple switch would do the trick as long as it’s turned off whenever the boat’s speed is below 4 knots and turned back on when the speed increases. A better solution would be a circuit that sensed the alternator speed and turned the regulator on or off accordingly. The circuit sketch below illustrates a possible circuit for an alternator-based prop shaft generator.
Mounting the Generator
Just like the alternator on an engine, the mounting bracket should allow the generator to be rotated toward the shaft so the belt can be installed/removed and then tensioned. I made some sketches and worked with a local machinist to fabricate the bracket.
Installing the shaft pulley and belt
The prop shaft will have to be disconnected from the transmission in order to slide the pulley and belt in place. On Nine of Cups, I was able to accomplish this while in the water, but on some boats, this may necessitate being hauled out. Since it is difficult to replace a worn or broken belt, I slid a spare belt over the shaft while it was disconnected. I used a cup hook to suspend the spare over the shaft until it is needed. The spare belt is visible in the photos of the installed generators.
Decision and implementation
So which type generator did I choose? On my first attempt, I used the brush-type DC generator that was a spare for our wind generator. It had a maximum speed of 5200 rpm and I selected a pulley ratio that insured I would be operating within its range. On our passage from Durban to East London, it performed quite well, generating a consistent 2 to 4 amps throughout. As we motored the last few hours getting into the harbor, however, I smelled the odor of melting insulation. I discovered that the generator had overheated and I removed the drive belt. Based on our engine speed, we had been running it at around 3000 rpm, well below the specified maximum speed. Apparently it wasn’t happy running at that speed continuously.
On my second attempt, I found a generator that was a hybrid of sorts. WindBlue Power, a company in the U.S. that makes components for wind turbines, buys standard automotive alternators and morphs them into brushless DC motors. They rewind the windings with finer wire so the output is higher at low rpm; they replace the field coil with a permanent magnet, eliminating the need for the 3 to 5 amps of field current; and they remove the internal rectifying diodes. The resulting generator overcomes most of the shortcomings of a standard brushless DC motor for this application. I modified the mounting bracket to fit the new generator and built the rectifier circuit of circuit sketch 2. The new generator had a maximum speed of 10,000 rpm, much higher than the brush-type DC motor. Unfortunately, I wasn’t able to find a larger diameter pulley to replace the existing one while in Cape Town.
Measured output – I completed the project in Cape Town and it was in use during our Atlantic crossing. At 3.8 knots through the water, the generator had a 1-amp output, which increased to 8 amps at 7 knots. On a typical 125 nm day, it produced about 80 amp-hours. Except on those calm days when we averaged less than 4 knots, we did not have to start the engine. The output would have been 50 to 75 percent greater had I been able to increase the size of the prop shaft pulley. The photo of our alternative energy monitor shows the amps being generated and the total amp-hours generated over the past 24 hours on a typical day during our crossing.
A side effect is that both the spinning prop shaft and the generator produce noise when we are sailing. After a few hours, however, the hum of the prop shaft and the whine of the generator soon became part of all the other background sounds — creaks, groans, squeaks, chirps — as Cups sailed along, noticeable only when something changed. So far, that has been the only negative, and I regret not doing the project years ago.
Another option – If the generator is disconnected from the shaft when motoring, a much larger pulley ratio could be used and a higher output could be achieved. One method of doing this would be to remove the drive belt whenever we started motoring and put it back when we started sailing again. This was not a reasonable option for me. I would quite likely forget to remove the belt as we started motoring into a harbor entrance or not have time to remove it if I needed to start the engine in an emergency.
Another method of disconnecting the generator would be to fit it with some sort of clutch. There are several different types of clutches — centrifugal, hydraulic, and electric — that might work for this application and would be worth researching.
Is it for you?
Nine of Cups is a 45-foot, heavy-displacement boat with a 23-inch fixed-blade prop. Once she gets going, the prop shaft has a lot of torque, more than enough to drive a much bigger generator. A smaller boat, or a boat with a smaller prop will produce less torque, and may not be able to drive a generator. If in doubt, get the advice of a marine engineer.
Some hydraulic transmissions may be damaged if allowed to turn for extended periods of time when the engine is not running. Nine of Cups has a Borg Warner Velvet Drive transmission, as does Eric Forsyth's boat Fiona. We now have more than 6000 nm on our shaft generator, which pales in comparison to Eric's 300,000+ nm on his. While neither of us has had any sign of problems with our hydraulic transmissions, I have not researched other types and models of hydraulic transmissions. If your boat has a hydraulic transmission, it would be prudent to investigate whether your particular transmission can be allowed to freewheel before making the decision to add a prop shaft generator.
This article was originally published in the May/June, 2016 issue of Good Old Boat Magazine