An electric boat is any boat primarily propelled by an electric motor. However, there is a broad range of capabilities within that definition.
The Role of Shape
The performance of electric boats, like all boats, is determined by the type and shape of their hulls more than anything else. For a given displacement, shape is primary in determining speed, stability, seaworthiness, efficiency and required power.
Shape determines whether an electric boat is limited to small electric only lakes, or whether it can hold its own on general waters: Large lakes, rivers, the intracoastal and even off shore. These waters are most taxing to the duration of battery power, so only the most efficient hulls qualify, as you may have surmised.
Pleasure boats can be classified into two broad categories: Displacement and planing. Displacement boats float in the water and depend on streamlining to move through the water efficiently. Examples are: sailboats, launches, canoes and rowboats. Planing boats have the power and design to rise up out of the water and escape the impossible: the ability to push the water aside fast enough to attain their speeds if they remained down in the water. Any boat that sustains speeds above 12 knots (K), with very rare exceptions, is a planing boat. A planing boat is characterized by a flat bottom which provides the surface on which it planes.
The empirical formula for the fastest practical speed of a displacement boat is
S = 1.34 L
where L is the length of the boat at the waterline. S is typically called the hull speed. The formula is applicable to monohull boats but not to low width to length ratios found in multihulls. Above hull speed even tripling the applied power will typically result in less than 20% increase in speed, the rest of the additional power going into increased wake.
To exceed hull speed a boat must not only be designed for planing but have sufficient power, power in the realm of twenty times the power required for a displacement boat of equivalent size. However, it is typical for a planing boat to use 100 times the power of a displacement boat. Speeds however are not 100 times as great but typically 30 to 50 MPH compared to 5 to 7 MPH. In other words you get only 4 to 10 times as much speed by increasing power by 100 times. Chalk up one for displacement boats.
Performance Data Proves Electric Power is Adequate for Efficient Displacement Boats
The most efficient displacement boats can attain hull speed with power between 1 and 2 HP per ton. These are the most desirable boats for electric power. At these powers enough batteries can be carried to run at or near hull speed for 8 to 12 hours.
|An example of an electric boat with these characteristics is the Electric Explorer. Another is the 26 ft. Electrocat. Both boats are powered with one Ray electric outboard.|
|26 ft. Electrocat w/solar canopy||
Electrocat hull designed for efficiency
|Performance is shown in Figure 1. Included is the performance of a 22' pontoon boat with 23"diameter aluminum pontoons for comparison.|| Figure #1
| Figure #2
||Battery performance, published by Exide Corporation, is shown in Figure 2. These types of graphs are no longer published. Too bad, for they are just what is needed for performance predictions. Graphs can be drawn by using such data that is published and Peukert's formula.|
Of interest is speed, running time (RT) and power per ton to confirm figures above, and range.
Explorer cruises at 25 amps, 48 volts
See Figure 1
Approx. prop shaft HP = (25 X 48 X .72) / 746= 1.16 HP
...where .72 is an estimated overall electrical and mechanical efficiency factor. (746 watts = 1 HP)
Displacement = 1800 lbs. = .9 tons
HP per ton = 1.16 / .9= 1.3 HP
Calculated hull speed = 1.34 sqrt. 19 = 5.8K. Cruise speed is slightly lower at 25 amps = 5.4K or 6.2 MPH.
From Figure 2, RT = 8 hrs. Range = 6.2 X 8 = 50 miles. (Eight batteries, battery current = motor current.)
With optional 12 batteries and 36 volt operation: at 1.16 HP motor current = 33.3 amps. In this case, with two 36V banks in parallel, battery current = 1/2 motor current = 16.6 amps. From Figure 2, RT = 12.5 hrs. Range = 6.2 X 12.5 = 77.5 miles.
The Electrocat cruises most efficiently at either 25 amps or 55 amps achieving 4.8 K and 6.1K respectively. Sixteen batteries are standard. Battery current = 1/2 motor current.
At 25 amps, HP = 1.16 as above.
Displacement = 3600 lbs. = 1.8 tons
HP per ton = 1.16 / 1.8 =64 HP
Battery current = 12.5
Range = 18 hrs. X 5.5 MPH = 99 miles
At 55 amps HP = (55 X 48 X .72) / 746= 2.5 HP
HP per ton = 2.5 / 1.8 = 1.39
Battery current = 27.5
Range = 7 hrs. X 7 MPH = 49 miles
The Other End of the Spectrum: Inefficient Electric Boats
Inefficient electric boats, boats that don't want to "go" in the displacement mode, either have hulls intended for planing or displacement hulls of inefficient design.
I learned about what planing hulls do in the displacement mode as a boy growing up on the Ohio in western Kentucky. Many of the locals, including my grandfather, owned wooden jon boats, 16 ft. fairly narrow with flat bottom turned up at both ends. The "turn up" at the stern was called "rake". They rowed quite easily. However, when more powerful outboards came along it was found that these boats would rare up if too much power was applied and increase in speed would not follow increase in throttle. To correct this problem, local builders began extending the bottom straight back to the transom eliminating the rake. This modification allowed the boats to plane and speeds went up.
I haven't forgotten the first time I tried to row one of those straight bottom boats although it has been some 58 years. Every time I pulled forward the boat seemed to pull back with no glide at all. There seemed to be a suction behind the transom holding the boat back. Planing hulls hold electric motors back in the same fashion.
Recently Ray Electric Outboards, Inc. had the opportunity to do some quantitative tests on a 16 ft. Carolina Skiff with and without rake. The main purpose was to develop a reduced draft outboard boat but we took the opportunity to test for reduced drag also. To produce the rake we cut 9" off the bottom of the 20" transom (up to the waterline) and curved the bottom up to meet the transom starting at a point about 36" forward of the transom (four times the height removed from the transom). Our tests showed the modified boat ran at almost all speeds with only 1/2 the power formerly required. Thus we have Ray's Rule of Rake: Rake reduces the power required to produce displacement speeds by one half.
Quantifying Performance Spread
Rake may be considered the first degree of streamlining. However, the best, no holds barred designs, require not 1/2 but only 1/3 or less of the power of a planing hull used as an electric boat. The Explorer, built on the Sea Pearl 21 sailboat hull, a "stretched" Herreshoff design, does 6.2 MPH at 25 amps and 48 volts while standard aluminum pontoon boats and aluminum fishing jons of equal displacement attain 6 MPH only full out at 75 to 80 amps if at all.
At 25 amps these boats attain less than 4K (4.6 MPH). Note in Figure 1 that the Electrocat with twice the displacement of the aluminum pontoon boat requires only 1/2 the power to achieve 5K.
The Significance of the Difference
Standard aluminum pontoon boats are OK for electric only lakes and the cocktail circuit but unsatisfactory for general waters. Likewise, the fishermen who fish Baltimore's heavily stocked reservoirs, some seven miles long, can get by with Ray outboards on aluminum jons but they don't often venture into the rivers or off shore with these boats.
I have no qualms about using or selling inefficient electric boats in applications where they are efficient enough, but these applications are too few to allow for much expansion in the use of electric boats. We must produce electric boats for use on general waters, and this means we must produce efficient boats. The best must be tweaked for efficiency in all areas not only in the hull design but in the electrical and propulsion system as well.
How Do the Best Stack Up?
If an electric boat can achieve 6 MPH for eight hours, this seems to be approaching acceptability for general waters. After all, you find many sailors putting up and down the intracoastal at this speed, but they go all day. Duration is critical for acceptance of general water electric boats. If you are fishing or cruising it is customary to go all day and have no fear of getting home or into a marina before nightfall.
At Ray Electric Outboards, Inc. we have set 6 MPH for eight hours as the minimum which general waters electric boats must meet and we provide 6 MPH for 11 to 12 hours as standard or optional whenever possible.
A Free Boost From the Sun
Solar power can enhance performance and reliability of general water boats. We provide lightweight integrated solar canopies as options wherein the panels are installed without frames onto a canopy constructed of square aluminum tubing with the panels forming the roof. These canopies provide backup propulsion and increased range. They propel the boat from 3 to 4 knots without drawing from the batteries.
You select what you do with the solar amps with the throttle. You have an ammeter to measure motor current, another to measure solar charging current. If motor is stopped solar current charges the batteries. If you throttle up motor current to exceed solar current the difference comes out of the batteries. To run on solar alone, as you might want to, to keep from over discharging your batteries, just set motor current equal to solar current. There is no switching required.
You Need a Juice Gauge
If you have an E-Meter it will keep track of the net battery amp-hours used. It will display amp-hours left or running time left at any motor current. It will tell you when your batteries are fully charged or to what capacity they are charged. It is like a fuel gauge and should dispel any concerns about running batteries down prematurely.
We hope other manufacturers will take a look at our efforts to produce general water electric boats and produce boats that meet or exceed our criteria and let the public know. I'd like to see some performance graphs from other manufacturers.
I don't believe the public knows the true capability of electric boats. Some news stories have shown what was intended to be typical performance but was significantly below actual capabilities. One would-be-expert actually stated that electric boats are limited to short day trips on protected waters. Unfortunately this may be close to what most people believe. I'd rather they think something like: "Electric boats can cruise the Bahamas safely and as long as you want", which they certainly can be built to do. In fact I have sketched a 40 ft. cat which would be great for the Bahamas.
Availability of Explorer and Electrocat
The Explorer and Electrocat were built as prototypes to demonstrate the capabilities of electric boats. Three Explorers, including the prototype, have been sold, the original ElectroCat has also been sold.
The good news is, both these boats plus three other performance boats are now avaiable as shown on this website.