Marine Electric Propulsion

Michael Hackleman

©1993 Michael Hackleman

/first learned about the Good Ship Esther Project when Richard Orawiec approached me following one of my workshops at the Midwest Renewable Energy Fair (June 1993). Richard had a story to tell me, pausing now and then to hand me a videotape, letters from government officials, and photos of the Esther in various states of restoration.

Richard gave me a copy of Launches and Yachts by William Swanson. For the most part, this is a reprint of the original 1902 catalog for ELCO (the Electric Launch Company). I wrote to Swanson, asking permission to reproduce some of its drawings in this article. I took this opportunity to engage Swanson in a technical discussion of refitting the Esther with today's technology. The sidebar below, arranged by topic, is a summation of his detailed reply.

Bill Swanson has supplied many useful "rules of thumb" here that should help evolve the updated propulsion system for Esther, or any other electric boat, for that matter. About most of these matters, I find myself in complete agreement with him. I have these reservations about these topics: voltage and safety, 6 V vs. 12 V batteries, volts vs. capacity, controller type, and solar-charging.

Voltage, Battery Capacity

The "below 50 Volts" rule undoubtedly stems from safety issues. High voltage and water don't mix. Low voltage reduces the shock hazard.

The original specifications sheet says that the ELCO 36 foot launch had 44 cells in its battery pack. The

Refitting the Esther with Today's Technology wouldn't spend two minutes trying to duplicate the original specs of Esther. Everything has changed — motor efficiencies, battery capacities, new technologies (mainly chopper pulse regulator). Start from scratch.

Propeller Size and Pitch

Generally in a displacement boat like Esther, you want the largest possible propeller that will fit in the aperture (cutout). So, measure the maximum possible clearance, and subtract about two inches. That will be the maximum diameter of the prop. Consult a naval architecture manual and cross reference prop diameter and engine rpm on the nomograph to see what the prop pitch should be. Then, go to a prop manufacturer and tell them you want a prop with those dimensions. For example, an 18/15 prop is 18 inches in diameter and has a pitch of 15 inches.

The rpm will be in the hundreds, not thousands. For a big prop, the lower the rpm, the better. Low rpm means less "slip" (loss of efficiency), less cavitation (air bubbles), and generally less wear-and-tear.

A three-blade prop is the safe, sane choice. A two-blade would be too little in surface area, and a four-

blade is good only if you've got the dollars. It doesn't have to be anything fancy or "high tech". Get quotes from Michigan Wheel, Federal, and maybe one or two other prop manufacturers for making the prop .

The prop must be bronze. Esther will have too many electrolysis problems with an aluminum prop. A thrust bearing must be installed. Consult a thrust bearing manufacturer for size and installation.

Displacement Hulls, powerplants, controls

I'd say an electric motor in the 5-7 horsepower range is about right. Unlike cars, displacement boats have a very low "maximum" speed, so that even if you put, say, a 100 hp motor in Esther, you're still only going to get the same performance as a 10 hp motor. It's a different story for planing powerboat hulls. Expect to pay as high as $100 per horsepower over the counter.

All powerplant calculations should be at 75-80% of maximum speed. Use this formula: 1.2 x square root of waterline length (in feet) = maximum speed of Esther. Now, multiply this number by 0.8 (80%) to get your "cruising" speed. "Cruise" is the speed you'll use in ANY calculations involving motor rpm, battery capacity, etc.

Esther should use a "chopper" (motor control) system. It'll deliver 20-50% more operating time.

Battery Voltage and Capacity

I believe there is a U.S. Coast Guard boatbuilding

Above: solar powered tools help with the reconstruction of Esther.

Scientific American article says that 66 cells were used in the launches at the Exposition, at 150 A-h capacity each. The article said that the speed controller arranged them in three packs of 22 or two packs of 33. This implies system voltages ranging 40-80 Volts, with battery capacities at 300-450 A-h.

I recommend the use of 6 V batteries instead of 12 V batteries for Esthers pack. It is true that 12 V batteries are the fastest, most space-conserving means of reaching high voltages. However, a 12 V battery has proportionately higher internal resistance, delivering fewer watts per pound of battery, than a 6 V one. That is, the 12 V battery regulation somewhere that differentiates systems above 50 Volts and those below. If I were you, I'd do whatever was necessary to keep Esther's system at a maximum of 48 Volts. Above 50 Volts, you have a ton of specification requirements. Below 50, you're golden.

A whole mess of batteries can be wired in either series or parallel. You can come up with any voltage (and capacity) you want. There's no real advantage to 6 Volt batteries when your operational choices are in 12 Volt increments: 12, 24, 36, or 48.

Batteries in Boats

There are five issues when using batteries in a marine environment: type, cost, weight, maintenance, and replacement.

Nicad, sodium-sulphur, lithium, all that future high-tech stuff, gel cells, maintenance-free — it's all hooey. Someday, maybe, one or more of the above may work. But not right now, today. There's only one option here: the bread-and-butter, garden variety, lead-acid deep cycle battery.

In the "real world", you're interested in dollars per Amp-hour ($ per A-h). Pick the cheapest, but reasonably reliable name-brand of battery. At current prices, anything more than about 75-80 cents per A-h of capacity (in a 12 V) is wasted money. There's really not too much to choose from — nor does there need to be.

If a single human being can't reasonably lift the damn battery in and out of its slot in the boat, the battery is too big. If this means you have to design in forty small (group 24 or 27) batteries instead of six monster batteries, so be it.

Batteries will need maintenance. Sealed batteries aren't required. If the boat is going to be upside down (or even 90 degrees over) for any period of time (say, oh, three seconds), you've got much bigger troubles than electrolyte slopping around. What you've got basically is irreversible sinking!

Finally, the batteries need to be easily replaced. If your battery isn't sold at Sears, K-Mart, or your local car battery store, it's not worth whatever other advantages it may have.

Solar Charging

Electric boats use energy at the rate of 50 to 100 Amps per hour. A solar charging system that supplies a small fraction of this is just an expensive toy. A month of sunlight for an hour's running time is not currently practical.


William C. Swanson. Author, Launches and Yachts: the 1902 ELCO Catalog, Swanson Marine Enterprises, 829 Copley Ave, Waldorf, MD 20602 • 301-843-1367

sacrifices energy density for power density. In high rpm motor applications with limited space (like cars), this is justified.

However, Esther's system has different requirements. As Swanson points out, a big prop wants low rpm. Low rpm and low voltage are a good match.

Let's assume Esther is outfitted with a 48 Volt system. Using marine deep-cycle batteries, we get 48 V with eight standard 6 V batteries, each 220 A-h in capacity. At 60 pounds each, eight batteries yield 480 pounds.

This is the minimum size of pack. Esther originally had much more capacity than this. If we double the pack size (described above) to 16 batteries, putting the two packs in parallel with each other, the 48 Volt pack's capacity will increase to 440 A-h .

Speed and Motor Control

Swanson's claim for the efficiency of the electronic controller is generally accepted. It gives infinite speed selection and corrects any mismatch between batteries and motor. However, there's merit in a control technique like Esthers original system: voltage selection. Voltage selection is an inexpensive, low-tech motor control technique that arranges the batteries in various combinations of series and parallel wirings. This is too jerky for use with a road vehicle. In a boat, the prop "slips" much better than do tires on a road surface.

The simplest system would use a DPDT (double pole, double throw) copper knife switch. This is easily wired to supply two speeds — 24 Volts or 48 Volts to the motor — in a single throw. A second DPDT knife switch would add an extra speed — 12 Volts. A third DPDT switch can handle motor reversing.

A hefty knife switch is relatively safe at low voltage and easily handles hundreds of amps of current. The addition of a 12 Volt contactor and a pushbutton (in the shift lever) would enable the operator to momentarily power down the system, change the voltage selection (up or down), and re-engage — eliminating any current arcing at the knife switches. Shifting down provides a braking effect, too.

The primary drawback to this low cost, low-tech control technique is that it does reduce speed control options to a few steps. And, if it's not coupled through linkage to one main control level, it's not safe and probably won't get approved. For good reason: Electricity and water works for eels and hydrogen generators — not humans. Under the right conditions, as little as two Volts or one tenth of an amp can kill you. Do it safe or don't do it.

Finally, it is true that a PV canopy on Esther will generate power at a fraction of that consumed, even at cruise speeds. And that power will come expensively.

Twenty modules represents a $6K investment. The good news is that as little as two days of sun will power Esther more than an hour at cruise speed. As Richard Orawiec has commented, "The plug for this thing is there," pointing toward the sky.


Author: Michael Hackleman, c/o Home Power, POB 520, Ashland, OR 97520

Electric Boat Association of America. Contact: Ken Matthews, POB 11197, Naples, Florida 33941 • 813774-3773 Ä

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