Electric Boats 1
MY FIRST EXPERIENCES WITH ELECTRIC BOATS...
... were never very good. They were all with sailboats or rowing boats
fitted with trolling motors and car sized batteries. Typically what
would happen is that the boater lugs his boat, oars, motor and battery
down to the lake and fits it all together. He has to row out far enough
to lower the motor. He has the motor, battery and himself sitting in
the stern and the boat is badly out of trim. He turns on the motor and
the boat starts to zip along at a good pace, but hardly faster than the
boat could row. Within ten minutes the boat slows noticably and within
fifteen minutes is crawling so slowly that he has to return to his
oars. In no case did the boat ever go more than a mile under electric
power. The boater always commented that his battery was needing a full
charge.
THEN I GOT THE BIBLE...
...about electric boats. It is ELECTRIC BOATS by Douglas Little,
published by International Marine in 1994. I don't know if it is still
in print but if you have any interest in electric boats you need to
locate a copy as a starting point. It renewed my enthusiasm to the
point where I decided to try for myself.
MY NEXT EXPERIENCE WITH ELECTRIC BOATS...
...consisted of trying a borrowed trolling motor and "marine" battery
in my Piccup Squared boat. Little's book made a point of properly
fitting a pirogue type craft for proper trim by making controls remote
so that the skipper sat mid ships, with the battery directly behind him
and the motor in the stern. In doing so he would cut the control head
off the top of the trolling motor and relocate it by the skipper's
seat. I couldn't cut apart my borrowed motor, so I rigged Piccup
Squared like this:
It was a very simple mod. The trolling motor was a small 12 volt unit,
probably about 30 pounds thrust. The mount lock that keeps the motor
down (remember trolling motors are usually mounted on the side or bow
of a boat to provide thrust in all directions and thus must be locked
down) was disabled with a piece of wire so that the motor could flip up
if it hit anything. A tiller arm, about 18" long, was fashioned out of
wood that clamped to the motor tube. That arm was connected to a
push/pull tiller stick that ran the full length of the boat's cockpit
such that I could steer from anywhere in the boat. The push/pull stick
was actually a lovely "found" cane fishing pole with screw together
ferrules. The stick was fastened to the tiller arm with a loop of
linettied through each and then the looseness of the joint taken up by
wrapping the joint several times with small diameter shock cord. The
speed of the motor was set on the control head of the motor with its
twist grip before starting. Starting and stopping the motor was done
simply by connecting and disconnecting the battery alligator clips
which were right behind the seat. It worked perfectly. Docking was
effected by pulsing the motor quickly with the alligator clip.
I was able to charge the battery completely to avoid the complaints of
the first trials. But although the battery was a "marine" battery it
was not a trolling motor battery. In fact it was probably about half
the size and weight of a true trolling motor battery.
With a full charge and full throttle, the rig pushed Piccup Square and
me at about 4 mph, certainly as fast as I could row. But after a mile
it would slow down considerably. The return trip to the dock would be a
crawl and I don't recall making it all the way back on the electric.
One day I gave it a full charge and vowed to operate it at half
throttle and I recall getting two miles out of it, but at reduced
speed. I could row as fast.
I thought a different prop would help since these motors are supposed
to hold a heavy boat stationary in a wind, or propped for very slow
speed. I thought increased pitch was in order to get speed out of a
light boat. I bought a different plastic prop ($12 new) which was
called a "power" prop. But it looked and acted exactly like the
original. I found that by heating it in the toaster oven when my wife
was out to just the right temperature, the plastic would get like putty
and could be reformed. So I did that and increased the pitch of the
blades. But then I thought, "Hey! Wait and minute! I can increase the
pitch at the tips this way but the root pitch can't be changed. The two
parts of the prop will be fighting each other." But I tried it anyway
since lots of great discoveries happen by accident. The repitched prop
really went no faster than the original and this was not a great
discovery.
By this time it was clear to me that this battery setup was never going
to be any improvement over rowing. You lug all this stuff, maybe 50
pounds of battery and motor, around and you spend all night charging a
battery. You go a mile in electric luxery -it is totally quiet and
there is nothing like it - and then you spend the rest of the day
boating with oars. Then you have to lug it all back home. After a few
times you learn to leave the electric at home and stick with the oars.
Luckily, Little's book explained all of what was happening and gave
guidance toward designing an electric system that was for real. Almost
everything I'm going to say is distilled from Little's book.
THE REAL BASIC BASICS...
Here is a sketch of a lead-acid battery. Lead-acid batteries are still
the best for this type of work. They are actually light and compact
compared to most other batteries, and they are fairly cheap and
available. Properly cared for they can be recharged hundreds of times.
What we have is two lead alloy plates dipped in sulfuric acid. If the
plates are connected to a charger or another battery such that one
plate gets a positive charge (the anode) and the other plate gets the
negative charge (the cathode) the lead plates undergo chemical changes
such that when the charger is removed, an electric potential exists
between the plates.
My ancient handbook says the basic chemistry of the lead-acid battery
is like this: Pb2O2 + Pb
+2H2SO4 = 2PbSO4 + 2H2O.
The reaction goes back and forth with charging/discharging. The old
handbook also adds, "Between the extremes of complete charge and
discharge, complex combinations of lead and sulphate are formed. After
complete discharge a hard insoluble sulphate forms slowly on the
plates, and this is reducible only by slow charging. This sulphation is
objectionable and should be avoided." So a promp recharge is in order.
A lead-acid cell has a natural full charge voltage of about 2.1 volts
measured across the anode and cathode. If you gang three of the cells
in series you will get what is called a 6 volt battery. Gang six of the
cells together and you have a 12 volt battery. Gang two twelve volt
batteries in series and you get the equal of a 24 volt battery (which
is actually more like 25 volts). And so forth. But remember that the
number of electrons stored in the thing is actually determined by the
raw size of the cell and area of the plates, not the number of cells
that determines the voltage. So a really big heavy 6 volt battery (like
a golf cart battery) can carry around a lot more stored energy than a
little 12 volt motorcycle battery.
Now if you connect the anode and cathode of a charge battery with an
electric gadget, like a trolling motor, the electrons flow through the
gadget at a rate proportional to the resistance of the gadget. An open
circuit, which has almost infinite resistance, results in no flow and
the battery stays charged .A short circuit, which has almost no
resistance, results is a drastically rapid flow that can melt things.
As the elctrons flow from anode to cathode the chemicals in the battery
return to their uncharged state. If you double the voltage by using two
cells you would double the flow of electrons assuming that the gadget's
resistance is unchanged.
AMPS...
Amperage is a measure of how many electrons are flowing through the
wire. The flow is often called "current" and I recall being told in the
missile factory by an electrical engineer that it can be also called
"juice" although he added that "juice" was a technical term that he
didn't have time to explain.
AMP-HOURS...
If your battery has capacity such that it can deliver say 5 amps for 20
hours it would be called a "100 amp-hour" battery.
POWER...
It has been ordained that volts times amps is called "Watts" and is a
measure of power, just like horsepower. In fact one horsepower is about
750 watts, easy to remember. So if you could put an ammeter and a
voltmeter in the circuit to measure the current and voltage of the the
juice flowing through the gadget powered by the battery, you could know
the power it is taking in.
NEXT...
If you make the comparison to a gas engine, you might say the electric
motor has a certain power and that the battery is like the gas tank. So
the question is how much power is stored in the battery. It's is done
by multiplying the amp-hours times the voltage. For example a 100
amp-hour battery (that delivers 5 amps for 20 hours) that is a 12 volt
battery, is said to store 100 amp-hour x 12 volts = 1200 watt-hours.
That is actually a pretty common size for a trolling motor battery that
you can buy at Wal-mart for maybe $70. It will be about 14" long, 7"
wide, and 9" high and will weigh about 60 pounds. That's about twice
the weight of a typical car battery, even though both are 12 volts. It
has more lead and acid that provides more electrons.
BATTERY REALITIES...
There is a big difference between a battery and a gas tank. A gas tank
will deliver all its fuel until empty, at any rate of delivery provided
the pipes are large enough. The battery won't. If you discharge a
battery rapidly, as you would by running a large electric motor, a lot
of its energy gets lost. Little doesn't say where it gets lost, but
perhaps in heating up the circuits, the motor and the battery itself,
and perhaps the chemistry becomes inefficient at higher discharge
rates. In fact in the US batteries are rated at how many amp hours they
can deliver over a 20 hour period. For example a 100 amp hour battery
can deliver only 5 amps for 20 hours. But in the example Little gives
that 100 amp hour battery will deliver about 17 amps an hour over a 4
hour period, a total of about 70 amps-hours. And only about 43
amp-hours over a single hour. (British batteries are rated over a 10
hour period so their 100 amp hour battery gives 10 amps over 10 hours,
maybe 10% more than the US battery). Little presents a graph of the US
100 amp hour battery that goes like this:
Batteries used to start gas engines have a total different rating
system since they aren't intended for long term operation. They are
intended to deliver lots of amps in a quick jolt. A battery with "100
cold cranking amps" should deliver 100 amps at zero degrees F in thirty
seconds, after which is might be useless. No doubt all the batteries
used in my first electric rides were of this type.
MOTORS...
Little also measured the electric consumption of different typical 12V
trolling motors (submerged motors). Here is a general description of
his tests:
Motor type...................Current draw..............Static
thrust................Horsepower
12V.............................. 36 ........................ 33
............................ .55
24V.............................. 22 ........................ 22
............................ .70
24V.............................. 41 ........................ 40
............................ 1.3
Remember that the horsepower shown here is the "electric" horsepower
based on 750 watts per hp. So the first 12 volt motor drawing 36 amps
is using 12 x 36 = 432 watts, which is 432 / 750 = .55 hp. (I don't
think Little says much about this but I would point out these ratings
would be of the power supplied TO the motor, not what the motor
delivers at the prop. Perhaps the efficiency of the system is quite
high. Knowing the output of the motor is important to predicting boat
performance.)
Let's look at the first motor for a while, .55 hp, 36 amps and 12
volts. At first glance you might use that motor with the above 100 amp
hour, 12 volt trolling motor motor battery and think the motor will run
2.7 hours (100/36) at full power on a full charge. Nope! Look at the
above chart and figure that motor will run for about 1.4 hours before
the battery gives out do to the high rate of discharge. But even then
you would be wrong according to Little. Totally discharging a battery
is really tough on the battery and bound to ruin it in short order.
Little would limit the battery's discharge to 70 amp-hours to give it a
good service life. Then you might limit the full throttle operation to
.7 x 1.4 = 1 hour, 37% of the first glance. That endurance can still be
useful in a small boat.
To recap, Little's recommendations are that you shouldn't discharge the
battery more than 20% of its capacity per hour, and never discharge the
battery more than 70% of its total.
NEXT TIME...
I'll dig a lot deeper into the battery chart and size up some battery
requirements for some real boats.
Contents
QT Skiff
QT SKIFF, ROWING SKIFF, 13' X 45", 75 POUNDS EMPTY
Flat iron skiffs are said to be easy to draw but very hard to get
right. QT is my attempt at a flat iron skiff. The prototype was built
by Paul Krayniak of Odessa, NY and a photo of his boat is presented
below. The above photo is of Brad Boerger's QT.
I chose a size that will be easy to cartop and yet ample for two
adults. The bottom is narrow for good rowing but still wide enough to
allow stand-up stability, unlike a dory. The top is wide enough for
good spread of the rowlocks. The result is pronounced flare at the
midsection. But if the flare is carried forward, the result will be
extreme rake at the bow. So I twisted the first four feet of the side
planks to reduce the stem rake.
This is simple nailed together job from three sheets of 1/4" plywood.
No jigs, no lofting.
Plans are $20.
Contents
Prototype News
Some of you may know that in addition to the one buck catalog which
now contains 20 "done" boats, I offer another catalog of 20 unbuilt
prototypes. The buck catalog has on its last page a list and brief
description of the boats currently in the Catalog of Prototypes. That
catalog also contains some articles that I wrote for Messing About In
Boats and Boatbuilder magazines. The Catalog of Prototypes costs $3.
The both together amount to 50 pages for $4, an offer you may have
seen in Woodenboat ads. Payment must be in US funds. The banks here
won't accept anything else. (I've got a little stash of foreign
currency that I can admire but not spend.) I'm way too small for
credit cards.
Here are the prototypes abuilding that I know of:
Caprice: A brave and experienced builder in Texas is making the 25'
Caprice water ballasted sailboat. A big project.Here the builder
plugs away at the aft deck.
Jon Jr has been completed by Joe Leinweber and Dan Ellis and tested
to a certain degree. Click
here to read about the test and the surprize ending! ("The waters
are only safe until next time!")
Mayfly12: A Mayfly12 is now completed and has been sailed. Waiting
for final photos but here is an initial photo.
Contents
AN INDEX OF PAST ISSUES
BACK
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Magazine
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Boatbuilding Community
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Skiff
Bruce
Builds Roar
Dave
Carnell
Rich builds
AF2
Herb builds AF3 (archived copy)
JB Builds
Sportdory
Hullforms Download (archived copy)
Plyboats Demo Download (archived copy)
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