Swe` Pea`s Electric conversion

Swe` Pea`s Electric conversion
Boats with Volts:
The Shocking True Story of
Swe‟ Pea‟s Electric Conversion
We recently showed our newly electrified boat, Swe‟ Pea, to the public at the 33rd Annual Wooden Boat
Festival in Port Townsend, Washington. Electric boats are still unusual, and one successfully converted from a
gas outboard is even more so. Literally hundreds of people that weekend were eager to see how it was
accomplished so this article describes the process.
The benefits of electric power are obvious: it‟s safer; no fumes, noise or pollution; lower fuel costs; greater
reliability. But we‟ve discovered that the real enjoyment of electric boating comes from its flexibility to give you
what you need for each particular outing. Whether it‟s a social afternoon lake cruise where the propulsion
source disappears in the background as you visit with your guests, or a weekend long expedition where tides
and long distances must be constantly in mind. Electric boating is not just strapping on a source of power to
be exploited, rather it enables you to experience nature similar to the way paddling does, while engaging your
mind as much as sailing. Get ready for a great ride.
We hope that many other small boat owners can construct their own conversions using our experience. But a
few cautions: this is a challenging project, taking longer than a weekend. There are hazardous elements
involved: working with electricity should only be done mindful of personal and property protection. Some tasks
may require professional help if you haven‟t done them before. While most „donor motors‟ share the same
basic architecture there are many makes and types out there, and you may run into differences we didn‟t cover
in this brief article. In my experience, though, most potential difficulties can be overcome with a little
confidence and experimentation. Finally, we included a section on suitability of both your boat and your
expectations for this conversion. As a leading edge electric boater, expect that there will be learning along the
way including experiencing what low-power boating feels like. Powering by electric is a unique experience with
unique rewards but it‟s not for everyone. If you‟re like us, you will enjoy electric boating for a long time to
Swe‟ Pea History
Swe‟ Pea is a 13.5 foot displacement runabout. She began life as a racing sailboat known as a Jet 14. I
acquired her in the 1980‟s and sailed her quite a bit, but the transom and adjoining hot molded structure had
rotted and previous restorations had changed her shape. Sadly her racing days were over. We continued to
sail her but it wasn‟t leisurely sailing.
When we needed a family friendly boat in 2003 I cut the rot away and she got a new transom, more freeboard,
seats, skeg, and a windshield. She also got a 5 hp Honda outboard steered by the original tiller, which was
moved up front in the dash. Her speed was up to 7 knots but we would typically cruise at 4-5 knots. Since then
she‟s been a constant summer companion to the family. Whether on a day‟s expedition from Seattle out to Port
Townsend, salmon fishing, crabbing, cruising mountain lakes, or exploring the San Juan Islands, she‟s proven
the inverse ratio of fun to size with leisurely small boats.
Even so, we tired of the sound and smell of the gasoline engine – so close on a 14 footer. In 2008 we decided
to convert to electric.
The Electric Experience
In planning our conversion, we expected to take advantage of the easily-driven hull shape of a sailboat
knowing that our speed aspirations were not beyond the “displacement-mode” speed of the hull. Establishing
our personal speed and range expectations using the natural needs of a displacement hull eased decisions in
selecting power levels and battery sizes. The new electric drive system is 110 lb heavier than the 4-stroke
gasoline setup, but the vessel trim is much improved and max speed is still acceptable at 5 knots with typical
cruising at 4.
Now with one full season under our belt, and 90 plus hours of the rain, sun and blown salt spray, we‟ve had no
problems with the electric conversion, once we got the components right.
One nice surprise with electric is that running out of juice is less of a worry than you might think. I forgot to
charge the batteries recently and found that we were nearly empty of charge within seconds of launching. We
used the opportunity to test a theory and headed across the large lake at ½ throttle anyway (you don‟t waste a
sunny day in the Pacific Northwest). Every time the battery dropped to the minimum pack level of 30 volts we
lowered the throttle a little more and voltage recovered. By doing this periodically we traveled over 5 miles
starting at 4 knots and ended at almost 3 knots back at the dock. So we found that electric powering gets you
home safely and has a predictable warning behavior that is easy to manage unlike a gasoline motor that quits
altogether. More interesting to me- this illustrates how flexible the range of an electric boat can be and brings
in an element of fun to an otherwise simple propulsion source.
Range is highly dependent on speed with Electric. With a full charge, at top speed Swe‟ Pea‟s full power range
is only 8 nautical miles (about 1.5 hours of use). But if you slow a little from 5 to 4 knots, her range more than
doubles to 18 nautical miles and at 3 knots, it‟s 34 nautical miles. Charging time from a full discharge is 8 hours
using dockside electricity.
Chart of Swe‟Pea‟s range/speed tradeoff.
While some would find this performance inadequate, the reality is that speeds much higher than what some
refer to as “hull speed” are problematic using mainstream electric propulsion. While more powerful motors are
readily available, they discharge the battery pack faster, requiring larger battery banks for a reasonable range.
In turn, extra battery weight would require even more power and so-on. Eventually we‟d exceed the safe
loading of the boat and it would be too heavy to go much faster anyway. The design spirals out of control
unless you use clear criteria for limiting power, and one clear criterion is hull speed.
Is a conversion in your future?
Based on my experience, one simple guideline for suitability is to ask if your boat can be driven at an
acceptable speed for you using a 2-3 hp gasoline outboard. That‟s about what we get from our conversion.
The following section illustrates other factors to consider.
So what is an acceptable speed? Hull speed is a traditional and useful place to start. Swe‟ Pea‟s hull speed
(the speed where the “speed-to-length” ratio is 1.37) is 4.8 knots (= 5.5 mph). Driving a boat harder than that
just makes more waves and reduces the efficiency and range considerably while adding very little meaningful
The hull-speed for a boat is largely determined by the length of the hull in the water (called Load Waterline
Length, LWL or just waterline). Boats of different lengths have different hull speeds so your boat may go faster
than Swe‟ Pea if it‟s longer than 13.5 ft and if it‟s the same weight and similar hull shape.
Waterline length impact on maximum displacement speed.
Weight greatly affects speed. Swe‟ Pea weighs around 650 lb with me aboard and goes up to 5 knots. If
we add another 300 lb in passengers the max speed falls to 4.5 knots.
Another consideration is hull shape. Swe‟ Pea has a rounded sailboat hull designed to be driven easily.
Most sailing hulls of appropriate displacement and waterline length are also appropriate for this conversion.
Fewer power boat hulls will be unless they are light and have less than an inch or so of immersion at the
transom at rest. You‟ll probably have poorer performance with a deep-V planning hull because their
deeper transoms create lots of drag.
Trim of the boat has an impact on how effective your hull shape is. If we move the passenger weight in
Swepea from the front to the back seats, the transom immerses and we find that speed goes down by ½
knot. This is the main reason for having remote controls for our conversion- it moves me forward to better
trim the hull. Batteries are also placed forward under the helm seats to better balance the boat.
For a very clear description of hull shapes, I highly recommend Dave Gerr‟s outstanding book, The Nature
of Boats.
Where you go boating, strength of currents and tides, will affect your range and speed. Sailboat masts and
standing rigging provide additional “windage”, or resistance to the wind, so consider this in your decision to
convert using a low power solution like ours. If you‟re often pushing against a Santa Ana wind to get back
to the dock, expect slower progress and reserve battery capacity for a long, full-power slog.
There are many traditional small boats with inboard motor wells such as the Caledonia Yawl, traditional
dories, and some small catboats. Conversion seems ideal for this kind of boat.
The final requirement, again, is more about the boater than the boat; what are your speed expectations?
Our family feels that we‟re at our destination when we‟re simply aboard our boat. So it does not much
matter how fast we go - within reason. But that does not suit everybody. Some might want to think about a
more powerful option so we‟ve compiled a partial list of higher power electric motors (See „Ready-built‟
electric motors‟ chart).
The Elements of the conversion
Propeller: The propeller is the most inefficient part of any electric drive system. While it‟s relatively complicated
to match a propeller to the boat and to the motor we can keep things simple in this case by sticking to a
propeller pitch between 6 and 8 inches per revolution and between 7.5 and 10 inches in diameter. Stick to the
larger diameters if you can, because diameter can have a large influence on the overall system efficiency.
Here‟s a chart showing the relationship measured while developing a new ultra-lightweight electric drive
system, the Electric Paddle1. In this test the propellers had varying pitches, but the performance is clearly
driven mostly by diameter:
Propeller Diameter Effect on Thrust,
40 watts input
ts 4.0
ru 3.0
lb 2.0
propeller diameter, inches
Looking at the chart you might think that even larger propellers should be used. That would be true except that
you would need a higher gear ratio to drive the increased friction with the motor still operating in a reasonable
torque range. Normal drive ratios in the lower unit will be between 3 and 4 to 1 which will give reasonable
performance with the specified motor. With standard lower-unit gear ratio limitations in mind, a 10” propeller is
just about the largest swing reasonable for the motor. We use an 8.5” diameter propeller in Swe‟ Pea‟s
conversion with reasonable results though I am thinking of re-installing the motor into a larger donor lower unit
for more prop diameter.
Motor: This conversion uses a 1000 watt brushed DC electric scooter motor. This photo shows the motor,
controller and Hall-effect throttle.
Motors of this type are widely available as are their speed controls. Now-a-days, some people view brushed
motors as outdated, inefficient and prone to wear compared to newer lighter brushless motors2. However, a
brushless motor, while lighter, operates at 10x higher rpm and therefore requires an extra 10:1 gear unit
The Electric Paddle is a portable electric drive system I designed for portable craft, launching in 2010. See
I am a member of the Lasermotive team that just won the 2009 NASA Power Beaming Challenge (the „Space Elevator
Games‟). For that application we wanted lightweight and high performance so we used a brushless motor that can provide
the same continuous power as this brushed motor in about 1/5 the weight and volume. Still, for boating I recommend
brushed motors for their availability, cost, reliability, and RPM suitability.
erasing any weight savings. Furthermore, brushless systems of equivalent power cost at least twice as much
as a brushed system. Finally, I have had no reliability problems thus far after one full season of use of a
brushed motor. In fact, when I disassembled the electric motor to explore this point for the article, I found that
the brushes were hardly worn and the commutator was still like new as can be seen below.
The motor we suggest is face-mounted, making it ideal for mounting to an adapter plate. Brushed motors are
well-made, reliable and cheap and this one comes with a nice heat sink fin arrangement that stays warm to the
touch - never hot.
Battery: Though we use NiMH batteries in the new Electric Paddle product, I‟d recommend avoidance of the
newer high performance battery chemistries like NiMH and Lithium variants in home-built, high-current
applications like this unless you‟re definitely a battery expert. Unless professionally designed and protected,
systems using these battery technologies can be dangerous to life and property. Also the cost is currently at
least 6x times that of plain old lead acid batteries you can get at the local auto parts store.
Our conversion uses three 12-volts, wet-cell lead acid batteries in a common battery box. Our box is a built-in
custom affair but you could use standard approved boxes. Be sure to place the weight on the keel or stringers.
De-rating of lead acid batteries must be understood at least superficially as it affects the range from lead acid
batteries considerably3. Our batteries are listed as having 80 amp-hours capacity but de-rating shows that at 3
knots, capacity is 61 Ah and at 5 knots, capacity is only 41 AH. So the range of your boat will be a function of
speed not only because of hull resistance but also battery capacity.
Derating describes how much faster power capacity will decrease at high current flow when internal temperature
increases and alters the battery‟s chemical reactions.
Gas Gage: You may be tempted to try a volt meter. I‟d not recommend that because voltage recovers and
dips with use meaning that you may think that your pack is fine, when in reality, a little more throttle will drop it
down to dangerously low levels. What you need is a good “integrating” meter.
In the early days of Steam power, James Watt invented the term “Horsepower” to sell his steam engines. He
needed to communicate to farmers just what a steam engine could do in terms they already understood and
“horsepower” worked like a charm. Nowadays, we know nothing of horses and the entire world is moving to a
more calculation-friendly unit of power name after James Watt; the Watt. For the record, there are
approximately 746 watts in one “American” hp. Our motor is a 1 kW or 1000 watt motor making it a 1.34 hp
The nicest thing about watts is they can be electrically broken down into voltage and current with a simple
equation that‟s illustrated by the two top and the right hand numbers shown on the watt meter:
Watts = Volts x Amps
101.9 W = 17.73 V x 5.75 A
The lower left corner, currently showing 1.24 KWh scrolls through Ah (Amp hours), KWh and a bunch of other
things too. I use the Ah reading to keep tabs on the battery because I know that total “tank-size is between 41
Ah and 61 Ah depending on how speed-hungry we‟ve been. You could also use the KWh capacity if you
know that
KWh = Ah x V / 1000
Using battery cap between 41 and 61 Ah and V equal to 36 volts, WH capacities calculate out to between 1.5
kWh and 2.2 KWh but there are voltage errors in this method so I still rely on the Ah method of reading the
Controller: The controller is an electronic “black box” that varies the average power sent to the motor giving
you any speed from 0 to 100%. It does this using a method called Pulse Width Modulation or PWM. These
PWM controllers actually turn the motor on and off thousands of times per second (pulses) and each pulse
happens for a certain period of time (width). The longer each pulse is on, the higher the average power to the
motor. Of course all this happens so fast the motor knows nothing but the average time it‟s on. We initially
used an inexpensive scooter controller in our conversion because it was literally 6 times cheaper than a beefy
unit and the specs indicated it could handle the job. It worked out pretty well for a long time but at one point it
blew out. Fortunately, we had a spare controller available and plugged it in and were on our way. Not wishing
to be bothered, we replaced Swe‟ Pea‟s controller with a „bulletproof‟ golf cart controller.
Small gasoline engines never live up to their hp ratings because they never get up to the rated rpm on real boats, so in terms of actual gasoline motor
equivalent power ratings, this is more like a 2+ hp motor.
Throttle: The throttle gives our controller the information it needs to know what speed you want to go. It does
this typically by variably dividing up a voltage input. If you use a scooter controller, you‟d want to use a scooter
throttle. This is nice to use as they‟re cheap and they plug into the controller with minimal fuss. For the golf cart
controller we cannot use the scooter controller because of voltage offset calibration issues. So we made use of
a standard 5k ohm potentiometer.
Wiring: There‟s no way around it; you need beefy wires. At max throttle, Swe‟ Pea draws 30 amps and more
than twice that in the motor circuit due to inductance reactions. That can produce significant losses in long runs
of wire so I recommend at least 6 gage wire in long-run (over 3 ft) motor circuits and 10 g for battery. Tinned
marine wire is the standard and I use it for the Electric Paddle wiring. But for this conversion, cost is very
important. My experience has been that soldered and properly sealed non-tined wire works even aboard an
open boat. Inspect it annually looking for green between the strands. For cost and availability, I found that
heavy jumper cables are locally available at a competitive price. These cables are very nice to work with too
because the wire is fine and flexible. Otherwise, tinned cable is available from reputable marine supply houses.
A cautionary note: keep all cables shaded from the sun. The insulation degrades in sunlight but a wrapping or
cable cover helps avoid this.
Fuses: Three batteries in series can provide a lot of current. In addition to making all connections as robust as
possible you must use appropriately sized fuses for each circuit. I prefer placing fuses as close to the batteries
as possible to maximize protection. I am using a 30 amp fuse in an automotive type carrier and have yet to
blow one out.
Flotation: Finally, and certainly not least importantly, seriously consider increased flotation for your boat to
accommodate the extra weight in lead from the batteries.
General location of components: In Swe‟ Pea, we located all of the electronic components remotely at the
forward helm. But I‟d place controller and reversing controls in the cowling if I were to do this conversion again
because the heaviest wires go from the controller to the motor, not from the battery to the controller. Most
outboard motors will have a great deal of extra space in the cowling once the new electric motor is in place. If
you want a tiller controller version than all these bits would definitely best be placed in the outboard cowling.
Reverse: You can save by using the existing mechanical reverse in your motor or use relays to make reverse
remote using switch-operated relays like we did.
Suitability of a donor outboard
If you‟ve decided to convert, the choice of outboard will typically come down to what you have at hand or can
find inexpensively. But make sure that the lower unit of your donor outboard is in good shape. The most
common problem is a seal leak which you can detect from the milky lower unit oil. If you use a leaking lower
unit, use vegetable oil as a lubricant to preserve our waterways.
The gear ratio should be as close to 4:1 as possible, especially if it has a larger propeller capacity. You can
count the turns of the propeller with each turn of the motor to find out the gear ratio, or refer to the motor‟s spec
sheet or manual. Most are in the 3:1- 4:1 range.
The ubiquitous 2 hp 2-stroke motor is, in most ways, a good candidate. It‟s light, small, and has enough
strength for this kind of power output. An 8” propeller can just be squeezed into the gap below the ventilation
plate but they typically come with a 7-1/4” propeller, which will limit your efficiency through increased propeller
slip. Though 2 hp units are good candidates, I prefer a larger HP donor motor because they come with a larger
propeller diameter, which is important for slow speed craft efficiency. I can fit an 8-1/2” propeller into our 10 hp
donor lower unit and, as I said, the next motor conversion I‟ll do will have a 9-10” clearance. I have a British
Seagull that is calling to me.
Motor conversion:
Converting the motor: Converting the motor is step 1, and for me the most enjoyable part of the conversion. If
you‟re not sure yet about tackling such a job keep this in mind; it involves tearing down something very
complicated (a gasoline engine) without any intention of ever putting the complex bits back together.
Prepare the lower unit by removing the motor cover and propeller, then remove all gasoline from the motor and
clean it well. After unbolting and breaking the engine loose from its gasket it should lift up for disposal.
Important- leave the shaft with the lower unit. Remove the impeller to reduce friction in the system- refer to
your outboard manual for instructions as they all work differently. Once the lower stem is back together, turn
the shaft to assure that it runs smoothly. It should look something like this:
Note- the shaft will not run without friction due to the shaft seals, but that‟s alright- it‟s much easier to rotate
than when the impeller was there. If it‟s not at least smooth, find the source of the problem now.
Your outboard donor has a substantially flat area on the top of the lower unit with a shaft coming up
through this plane that‟s perfect for an adapter plate arrangement. In some cases the shaft will be too short
to protrude. Also in some cases there will be a hollow shaft as can be seen above. In this case, a socket
extension fits this hole perfectly and also provides some degree of flexibility to the motor mounting without
an expensive shaft coupler.
The next step is to make the adapter plate. Use Plexiglas to make a template. Drill a hole that just fits the
driveshaft and lower the template down so it sits on this flat plane with the shaft protruding. Mark the hole
locations for the mounting and trace the outline of your adapter plate. Remove and repeat for the electric
motor. Drill all the holes in the template and cut to shape by following the two outlines- whichever is bigger.
It does not hurt to make the plate even larger than the mounting structure to provide an extra cooling fin. To
ease assembly later, enlarge the motor shaft hole in the template so it clears the shaft-coupling diameter.
This picture shows a Plexiglas template cut out already on Swe‟ Pea‟s donor lower unit.
Making the template is much easier now. Either machine the shape yourself or drop this template off at a
machine shop to have the actual adapter plate made. I used ¼” aluminum plate.
The electric motor comes with a sprocket for a chain that is not used in this conversion. The sprocket can
be removed from the shaft by removing the C-clip, then pulling the sprocket from the shaft stub.
Do not remove the shaft stub bearing race. The motor shaft is too small to allow adaptation without this part.
Lower unit shafts will need to be cut to length. Cut the lower unit shaft with a hacksaw so that the motor
shaft and lower-unit shaft have a gap of 1/16” to ¼” when assembled with the adapter plate.
Install the shaft adapter. In my case, I used a brass machined coupling. You can have a machinist make
such an adapter or purchase a pre-made one. Here, my home-made coupler is shown upside down with
the motor bolted to the other side of the adapter plate.
When making a final assembly, liberally bead the motor shaft where it enters the electric motor with high
temperature grease to prevent rust in the lower motor bearing. Though the bearings are sealed, a little lube
never hurts.
Assemble the adapter plate, motor and shaft to the lower unit and it should look something like this. Here I
show the old 2-stroke engine next to the new motor installed on the lower unit. The size difference is
Re-lube the lower unit, install the propeller and you‟re done. To check, turn the propeller shaft by hand and
you should feel heavy cogging from the motor. This is normal. If it grinds or if cogging is uneven, take apart
and check shaft coupling and everything else until you find the cause. I have had some trouble with warped
couplings and shafts. If this occurs, try a different type of shaft coupling that adapts to misalignments, or
take it to a machinist along with some girl scout cookies to help figure it out.
How to re-install the motor cover is completely determined by the particular motor. You may need to install
new brackets to simulate mounts that were once part of the engine. Ours mounted easily with the white
casting shown below sandwiched in-between the adapter plate and the lower unit.
After being accused of using a “gasoline” motor in an electric lake (no worries; it ended well) I decided to
leave the motor cover off. This photo also shows how we connected power to the motor initially with the
cowling over the motor and before there was a sheath on the wires.
The steering arm tiller can be kept or discarded if you‟re remote controlling. If you‟re using the original
outboard tiller arm, the throttle system will further depend on the controller you‟re using; If you‟re using the
scooter controller option, replace the tiller grip with the new scooter throttle handle. If using the golf cart
controller, you‟ll need to figure out how to couple the potentiometer to the existing twist grip. I‟ve had luck
with rubber hose as a flexible shaft adapter. Just mount the potentiometer in line with the throttle control
shaft, slide on some ¼” tubing that fits both shaft ends and secure with small hose clamps.
For a remote throttle setup I made a remote throttle arm that works coupled with string and pulley that feels
remarkably refined. Alternatively you could use a simple knob on the potentiometer shaft and power up
with a twist of the knob instead.
The electrical connection to our motor is a 7-pin trailer plug. Use three pins for positive and three pins for
negative to make the connector handle the current safely. Use 6 gage wires to assure that power is not lost
unnecessarily. Use contact grease to assure reliable connections.
I used automotive style deep cycle batteries from a local auto parts shop: three Excide nautilus 24 group
wet cell batteries. These are series connected with 10 gauge jumpers as seen in the photo. Note the
marine-grade lug connectors sealing against the wires. The battery box is made of wood and sealed using
epoxy and has a loose-fitting plastic lid. The batteries are set on a neutralizing mat, and foam pads are
placed between cases to prevent chafe, especially when trailering. The batteries are secured using a
wooden beam over all three.
The photo shows our control console. From left to right: the on-off/dead man switch, forward-reverse switch
(hard to see), throttle handle, wattmeter (our new gas gauge), and mains battery switch. I de-activate the
battery bank while at the dock and when on the trailer.
The controller and battery charger are installed in the glove box in the dash, but, again, I suspect the controller
would be happier closer to the motor. That would prevent current losses in the long wiring run.
The schematics for this conversion are relatively simple. I include one for the scooter controller layout, one for
the “axe” golf cart controller and one for the reversing circuit.
Scooter controller: You will need to ask for the wiring color code when you order your controller. The scooter
controller is convenient in that all power connections are via standard lugs and you can use separate lugs into
the connector cases. Unfortunately the lugs are borderline undersized for the application, so make sure they
are clean and female lugs are tight and well connected. Otherwise they may overheat, especially on the motor
side. You cannot connect 6 gage wire directly to standard lugs, instead, connect a short length 10 g wire to a
lug and solder-splice into the 6-gage wire. The other plugs for the key and for the throttle are a smaller plug
type not normally available in stores. You may wish to replace these with standard lugs too. Remember to prep
the spare controller so it‟s ready for use should the first one fail.
Main Contactor
30 amp fuse
Deadman switch
throttle potentiometer
Scooter controller
DC Motor
AXE golf cart controller: This one is very robust which shows in the type of connections offered. The controller
has large high-current tabs protruding from the front of the case that can be bolted to directly with any proper
cable lug. When ordering the controller, ask that it be configured for a resistive throttle and that it be
programmed with a 30-volt cutoff to protect your batteries from over discharge. This controller has a large onrush of current due to the high internal capacitance so it is offered with a 250 ohm 10 watt resistor to be placed
across the main power switch to maintain charge in the caps. Without this resistor you‟d blow fuses every time
you turn the unit on and could damage the controller from the sudden on-rush of current.
Deadman switch
5 amp fuse
throttle potentiometer
Main Contactor
2 3
30 amp fuse
250 ohm 10 watt resistor
DC Motor
Reversing: Both the scooter and AXE controllers can be fitted with an electric reversing feature. It‟s relatively
inexpensive to do, but adds more wiring complications. Nevertheless, it has worked pretty well. Be sure to
leave this system in forward (reverse switch open) when not in use or the batteries will drain through the relay
coils. Three 12v relays are used as the cheapest way to create a 36-volt relay system. The third relay does
nothing other than reduce the current through the other two to keep their coils within spec. It also acts as a
spare should something go wrong with the contacts in the other two.
Parts and suppliers:
Minimum parts cost including batteries, cables, chargers, etc. is $790 assuming you have a free donor motor.
This is what is required for the basic system including a spare controller since it‟s the most likely part to fail.
item description
local auto parts store
80 ah deep cycle wet cell group 24.
unit cost
total cost
battery boxes
local auto parts store
marine battery boxes
36 Volt 1000 Watt Electric Motor (Currie Technologies)
Controller and spare
36 Volt 1000W Universal Speed & Voltage Controller
Scooter throttle
36 volt 4-wire hand throttle with LED meter
HD jumper cables
local auto parts store
heavy duty cables, 6 g. (one or 2 sets depending on length of run )
high power plug
7-Pole RV Blade Trailer End Plug #48505
Pole RV Blade Vehicle End Socket #48485
Soneil 36 Volt 8 Amp
shaft coupling
or machined part- depends on application
aluminum plate
1/4" aluminum plate
disconnect switch
local auto parts store
battery disconnect switch SWT-700
local hardware
wire, switch, connectors, lugs, screws, solder…
For an additional $350 you can have a very strong controller and a very nice Watt meter that helps you keep
accurate tabs on the battery‟s watt-hours and voltage. If opting for this, remember to delete the scooter
controllers and throttle above.
item description
Watt's Up Watt Meter
300 amp controller
Altrax AXE 4834 TH = 0-5 volts, UV =30, 50% output, resistor, diode
300 amp version throttle
5K-Ohm Linear Taper Potentiometers, model: 271-1714
2.99 $
reversing relays
Cole Hersee RC-700112
$ 354.75
Using a converted boat
It’s amazing how easy it is to use an electric propulsion system. I’ll often launch the boat with a friend aboard
and let them tool around for the first time without my help as I park the car. There’s no starting, choke,
warming up, etc. and the throttle is so responsive that newcomers can get an instant feel for the drive system.
You’ll find that your cruising range opens up to include backwaters that kayaks and canoes frequent, and of
course now you can take advantage of electric-only waterways. Be prepared for a more social boating
experience because everyone wants to know about electric boats- even kayakers and canoeists.
Perception of speed: If you’re moving to electric from being a power boater, at first use you may have a
feeling that’s something’s wrong. With power boating, much of the feeling of speed comes from the noise
generated. When power boaters start out using an electric drive, speed should be measured using a GPS, it
being a more reliable measure of speed than the senses. Further, plotting speed against power is a useful
exercise to learn about your boat’s performance. One thing you’ll see right away is the fact that half throttle
gives you most of the ultimate speed attainable. In our case (see below) we go 4.2 knots at half throttle and
doubling power only yields another 0.8 knots. This is much more the nature of the boat than the motor so we
must learn to live with it. We have found that Swe’ Pea is most comfortable cruising at 400 watts. We goose
it to 800 watts when we want to show some wake. It does not take long to realize that the range of your boat
is a variable unlike it’s ever been with liquid fueled engines. Now you have a huge expanse of possibilities for
trading off speed for range.
Watts in
Test performed Sunday, Oct 12,
Rattlensake lake, WA
Wind was 6 to 10 knotts. Outside
temperature was 50 degrees F
Swe pea had batteries, new, state of
charge unknown. Standard chrysler prop in
poor condition
Below values were with one 220 lb person
amp hours at 4 amp draw
stated capacity
wi nd
condi ti on
i nto
i nto
i nto
i nto
i nto
i nto
i nto
va r
va r
va r
va r
va r
va r
va r
Mea s ured mea s ured mea s ured
vol ta ge,
wa tts
vol ts
s peed, kts
Sailors leverage water and the wind to generate propulsion and manage this interface with their senses. They
know that to reach their destination they need to tack, watch the tides and predict wind shifts. It’s a great
feeling of accomplishment to sail. Similarly, paddlers feel of the movement of their craft in direct response to
their effort, must choose tides and tacks, and must offer themselves in a measured manner to make the
distance. Also a very satisfying and rich experience. Though electric boats have a more “reliable” wind, the
quiet and direct response of the hull to the smooth power of an electric drive is every bit as engaging; As in
sailing, electric boaters must choose the tides carefully and as in paddling, electric boaters must choose how
much to exert the batteries to guarantee enough range to make their destination. This engaging sport is
hidden in the batteries - - but it’s as real as a broad reach or an open water paddle. Electric boating is here,
it’s ready and we’re just beginning to master the possibilities.
Swe‟ Pea‟s owners, Joe and Linda Grez, have been so impressed with the potential of electrically driven boats that, in
their “spare time”, they‟ve developed the Electric Paddle, a new ultra-portable electric outboard just for portable boats
launching in early 2010. See www.electricpaddle.com for more information. Joe, a physicist and electro-magnetic
product designer, brings over 20 years of product development, patenting inventions and project management experience
to bear on the subject as well as a hefty interest in small boats. Both Joe and Linda work for Philips Oral Healthcare,
makers of the Sonicare toothbrush, and are the parents of 5 year old Anna, who loves “her” Swe‟ Pea.
Ready-built electric motors:
When considering the purchase of any electric motor option including trolling motors, be sure to add the cost of
charger, batteries, battery boxes, hold-downs, cables, connectors, etc. Most power units do not come with
these “extras” and they completely transform the cost of the overall solution.
There are essentially two kinds of electric motors available for small boats;
submerged motors like trolling motors and Torqeedo,
more conventional un-submerged motors with a shaft and bevel gear power transmission just
like a gasoline unit.
The advantage of a submerged motor system is that cooling is easier for engineers to work around and it
requires fewer parts generally. However, if the seal fails then a submerged motor will also fail and you‟d lose
your purchase unless it‟s under warranty. If you opt to purchase a submerged motor, never run it out of the
water as seals will fail under these conditions.
Trolling motors are great for heavier slower moving applications like bass boat maneuvering but if you want to
go faster than 2 mph, trolling motors become increasingly inefficient requiring much bigger batteries for a
reasonable range. The submerged architecture has a risk of flooding too so I do not consider them a viable
source of main propulsion for any serious application.
Though it‟s a submerged motor type, the Torqeedo motor is extremely efficient, using brushless motor
technology and contains something like a 40:1 gear box and a large high pitch propeller. The Torqeedo “801
Base” is roughly equivalent to our conversion in terms of power. It costs $1200 plus at least $500 or so for
essentials like batteries large enough for equivalent range, boxes and charger. If you do not wish to convert
your own motor, this is the lowest cost equivalent system I know of, depending on the batteries you install.
Following is a table of more powerful motors I considered before deciding on a conversion. All of these options
were more expensive than my budget even without batteries, chargers etc. And they all have more power than
I felt I could supply with batteries that fit the boat. Remember that to maintain range, a more powerful motor
adds a lot of extra battery weight that will, in turn, slow your boat so added power may not give you the extra
speed you„d expect.
Motor name
whisper electric
$4995 plus battery and charger
$5190 to $5320 plus battery and charger
5000 to 5500 plus battery and charger
Briggs & Stratton
Last time I checked- $3,000- Fresh water
$5450 plus batteries and charger
Torqeedo Cruise
$3200 plus charger and batteries*
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