| Hybrid Propulsion LLC achieves a new high water mark in clean, environmentally friendly electric boat propulsion |
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Story by Bill Southworth, Portsmouth NH, March 8, 2010
In 2008, I started a refit of our 57' sloop the S/V Barbara Ann. For a number of reasons I started looking into electric power to replace our 100hp Yanmar diesel and 5kW generator. The project started by contacting the numerous companies claiming electric propulsion systems. I even placed a n order with one company and it turned out to be a hoax with a great web site.
I found that folklore and "conventional wisdom" were rampant in this nascent industry. If you read the brochures you'll quickly become convinced that these self-proclaimed wizards had finally discovered the secret to perpetual motion. It's taken me a while to separate the myths from fact.
This is one of the most prevalent myths and has been quoted and re-quoted so much that it's considered gospel. The claimed reason is the electric motor's "flat torque curve". It's true that an electric motor can provide much better acceleration and deceleration because all the power available can be delivered to the motor instantly. When you are running your prop at quarter speed from a directly coupled diesel engine, you are operating at an inefficient point on the diesel's torque curve. Of course, you could correct this with a multi-speed transmission, the way a car does.
There is no free lunch and you can't break the laws of physics. For a given continuous speed, it takes a certain amount of power to the propellor shaft to move the boat. It doesn't make any difference whether this is coming from an electric motor and batteries, a diesel engine, of a wheel with gerbils. A displacement boat, one which like most cruising sailboats and trawlers doesn't lift out of the water, is limited by the physics of displaced water and hull form. Generally, this is around 1.3 times the square root of the waterline length. For our 57' sailboat, the "hull speed" is 9.2 knots. Getting close to and past the hull speed is like breaking the sound barrier. More and more energy is expended for less and less gain.
Detractors say that electric power doesn't make sense because of the losses converting the mechanical power to electricity and the back. That would probably be true if you were always powering at the optimal efficiency of the diesel engine. The truth is that the electric motors let you run the diesel optimally while generating power and run the motor optimally while propelling the boat. It's certainly close enough in efficiency that diesel electric propulsion of the megawatt variety has become pretty standard in navy ships and large passenger ships.
This one is a flat lie. The physics just don't support the facts. When you are motoring, you are pushing the water at a much greater speed than the boat is moving through the water. When you are sailing, the effective water speed over the prop is somewhat less than the speed of the boat. Realistically, you can probably generate up to 25% as much power while sailing at hull speed as you'd expend powering the boat. That said, it's still pretty good news. In the case of the Barbara Ann, it takes around 35kW to motor the boat at a good clip. If we generate six or seven kilowatts under sail, that could be all the power we need for motoring in and out of harbors and for running all the systems. Of course, the power generation is not without a price. When you convert some of the forward speed into battery charging, you are also introducing drag in the prop that slows the boat a bit. However, in a real blow, the wind power may be more than the boat can use efficiently, so this speed loss may be negligible.
Secondly, these boat systems are generative not "regenerative". When you put your foot on the brakes in a hybrid car, you can reclaim some of the energy of the vehicle's forward motion, sending some power back to the battery. In a hybrid boat application, you "generate" electric power by siphoning off some of the power of the wind moving the sails pushing the boat. This is actually a much bigger gain than you get from a hybrid automobile's braking.
There is one interesting side benefit to this. If you are moored or anchored in an area with significant tidal or river currents, you may be able to use these currents to generate enough power for your house loads and keeping your batteries charged.
One truth is that hybrid electric power is still much more expensive than an equivalent diesel engine and diesel generator. However, the benefits are substantial. A properly designed system can provide much more power for house needs, giving you the flexibility to implement lighter and simpler boat systems. There's also something pretty wonderful about being able to have instant acceleration and instant braking without having to start an engine. It's also pretty nice to motor silently into and out of a harbor and have your batteries topped off while you are hanging on a hook.
There's one case where hybrid electric power is a clear win. That's for motor-sailing. Let's say you have five or six knots of wind and you'd like to make a passage at eight knots on a close reach. Unfortunately, the wind is coming straight from the direction you are headed. This seems to always be the case on the New England coast, regardless of which way you are heading. Using the motor for that extra few knots gives you a higher apparent wind as well as moving you along quicker. It also doesn't take a whole lot of power if you can use your stored power efficiently. If you are sailing through some slop, you can actually generate some power while sliding down a wave and use it while climbing the next.
To implement the hybrid propulsion system on the Barbara Ann, we found no off-the-shelf systems that really exist. I ended up building a great engineering team with decades of experience in electronic design, power systems, software and mechanical engineering. The team has developed crucial relationships with vendors who have helped us developed the specialized componentry for the systems. It's been an interesting and pretty enjoyable task. I've learned a hell of a lot and I don't mind sharing some of our experiences with others who are thinking of pursuing a hybrid project. We've developed some components available nowhere else. We're setting up a capability to build these to order so that the next guy doesn't have to go through the long engineering cycle that we've had.
Our "products" include a custom 3:1 planetary gearbox for the UQM motor, programmable logic controllers and custom interface electronics for the drive motor, safety circuitry for the high voltage DC, and a kit for converting a Westerbeke AC generator to DC operation. Please contact us about your hybrid project to see if we have a fit.
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