Systems+Engines

Water power

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My last two columns discussed the high cost of generating electricity with a diesel engine and the relatively short payback period for solar panels on liveaboard cruising boats. The problem with solar is that it requires a lot of surface area to produce significant amounts of power. This is relatively easy to find on catamarans, but not so on monohulls.

Coincidentally, I received an email from longtime cruising sailor and author Don Street extolling the benefits of wind and water generators, especially on monohulls. “Go green!” Don wrote me. “There is no excuse for yachts to run engines or generators for electricity generation when on passage.”

Don has always exemplified the virtues of a simple cruising life, so I think he may be underestimating the electrical loads of modern boats. The continuous load of an autopilot, a laptop-based electronic chart system, and a radar set is around 15 amps at 12 volts (180 watts). Over 24 hours, the cumulative load comes to a shocking 360 amp-hours at 12 volts, or 4.3 kWh, and that’s before anything else has been turned on. Nevertheless, even on such a power-hungry boat, much of the energy demand can be satisfied by a water generator when under sail.

Taffrail generators

Water generators kick in at about 3 knots of boatspeed. Once boatspeed reaches about 6 knots, even a small water generator, such as the Aquair taffrail generator (www.ampair.com), can produce 5 amps at 12 volts (60 watts). At higher speeds, energy production increases proportionally faster than boatspeed.

What’s this worth? Let’s assume a cruising speed of 6 knots and an output of 5 amps. That’s 1.4 kWh a day (60 watts x 24 hours = 1,440 watts = 1.4 kWh). Based on an at-anchor diesel-fueled generating cost of between $2.75 and $4.47 per kWh, our water generator is producing the equivalent of $3.96 to $6.44 of electricity a day. The Aquair costs around $1,000, so the payback period (without taking into account the reduced cycling of the batteries) is 155 to 250 days, which is about the amount of time spent on passage during a circumnavigation. During this time, maintenance chores will be significantly reduced, and you won’t have to listen to the engine for two or three hours a day.

There are more powerful taffrail water generators, such as the Hamilton Ferris Waterpower 200. At 6 knots of boatspeed, this has a rated output of 12 amps at 12 volts. It is more expensive than the Aquair, but it will keep up with more demanding electrical loads with a reduced payback time.

At this point, some readers may have detected a flaw in this argument. The reality is that most sailors motorsail in light air, even on ocean passages, at which point the water generator is superseded by the engine-driven alternator and becomes irrelevant for power generation. Depending on the amount of motoring you do, this may double the payback period.

Shaft Generators

A taffrail generator needs an operator to deploy and retrieve an impeller that spins on the end of a long length of line. For some reason, only the hardiest ocean voyagers have been willing to do this. Another way to use the boat’s motion through the water to generate electricity is to let the boat’s propeller freewheel, then drive an alternator off the spinning propeller shaft. Given the much larger size of a boat’s propeller compared to a towed impeller, much higher power outputs should be possible. For this application, Don Street recommends the Electrodyne low-rpm alternator, which is specially made for vehicles such as delivery vans that spend a lot of time idling their engines. Fischer Panda and others also market shaft alternators.

There are some drawbacks. A shaft-driven alternator requires a custom installation and can’t be used with some gearboxes (notably hydraulic transmissions, which will suffer from a lack of oil circulation). In the past, a shaft-driven alternator has required a fixed-pitch propeller, with all its associated drag under sail.

More recently it has been found that some feathering propellers can be “tricked” into keeping their blades open. Whatever propeller is used, when it is freewheeling there will be a significant amount of shaft rumble.

At 6 knots, a properly sized shaft-driven alternator will be able to feed heavy electrical loads for a total installed cost that is about equivalent to that of the Aquair. At higher boatspeeds, the alternator will not only keep up with the ongoing load, but also replenish the batteries. The payback period will be less than 100 days of passage time under sail.

Outboard leg generators

There’s yet another approach to water generation that does not require deploying an impeller or spinning the propeller shaft. This involves mounting a generator on an outboard leg that can be lowered into the water when needed. Depending on the design (particularly the size of the propeller and the power of the alternator), high power outputs can be achieved. You get the benefits of a shaft alternator without the shaft rumble, and it can fit a folding or feathering propeller on your prop shaft to optimize performance under sail, lifting the generator out of the water when it is not needed.

The DuoGen is a good example of an outboard generator. At 6 knots, it puts out around 8 amps at 12 volts, rising to 11 amps at 7 knots. The principal drawback is a relatively high cost per amp of output. If we work the same amortization numbers for the DuoGen as we did for the Aquair, assuming a boatspeed of 6 knots, we find the payback period (allowing for installation costs) is between 360 and 650 days of ocean sailing. This will be significantly shorter at higher boatspeeds and outputs, assuming the boat can use the power generated.

Hybrid propulsion systems

All of these water-generation systems pale in comparison to emerging hybrid propulsion systems that use “regeneration” to produce large amounts of power off a freewheeling propeller. Florida company Electric Marine Propulsion is ahead of the pack with a recently completed installation in a Lagoon 500 catamaran that has recorded over 9 kW of regenerative power under sail (that’s the equivalent of 750 amps at 12 volts). In fact, because of inadequate voltage regulation, the system fried the boat’s batteries

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