ELECTRON, ELECTRIC CUDDY BOAT, 18' X 5',
900 POUNDS EMPTY
Electron was an interesting project done
for a man who had done his homework on electric boats. In
particular he had studied Doug Little's great book ELECTRIC
BOATS. He wanted a cuddy cabin boat capable of some cruising,
the ability to go about 30 miles on batteries when desired.
We started with my old **Fatcat2**
design, stretched it to 18' and narrowed it to 5', which
made it visually a lot sleeker. He told me right off the
bat that his figures showed he would need six trolling motor
batteries to get the desired range while using a 24 volt,
2hp Minnkota trolling motor at half throttle.
To guess at performance you first guess
at the total weight. The batteries will weigh about 400
pounds total and the motor about 60 pounds. With wires
and such the power system should go about 500 pounds.
The hull structure is guessed at 400 pounds, so the total
empty weight might be 900 pounds. Let's add 400 pounds
for the skipper and guest and some gear. The total comes
to 1300 pounds in this set up.
Next we guess at the waterline length.
Let's say 17' for this 18' boat with plumb stem and stern.
Now we go up to the performance chart
to guess at the speed of the boat. At full throttle we might
have a lb/hp ratio of 1300/2 = 650. The chart predicts a
speed ratio of about 1.15. The square root of the water
line length is about 4.12. So the chart predicts a top speed
of 1.15x4.12=4.7 knots which is 5.4 mph. Not bad.
Cruising at 1 hp gives a lb/hp ratio of
1300. The chart predicts a speed ratio of about .97. So
cruise speed should be about .97x4.12=4knots which is 4.6
mph. Not bad (and it shows how boats can cut their power
a lot and just slow down a little).
Now
let's guess at the endurance. The 24 volt motor running
at 1 hp should use 750/24 = 31 amps per hour. The six batteries
in the plan are like the 12v, 100 amp-hour battery shown
in the above chart (power-wise, they are 12x100=1200 watt-hours
each for a total of 7200 watt-hours for the six). They must
be first wired two in series to produce three 24 volt sources.
So in use they will look like three 24 volt batteries, 100
amp-hours each.
If each battery pair pulls
its share of the load, about 31/3=10 amps per hour, then
the chart estimates an endurance of about 8 hours. keeping
the battery draw to 70% maximum would imply an endurance
of 8x.7=5.6 hours. At cruise that means 5.6 x 4.6=26 miles.
At full 2hp (1500 watts per
hour) the endurance would be about .7 x 3hours = 2.1 hours
for a range of 2.1 x 5.4 = 11 miles.
Another check to make is
Little's rule of not drawing more than 20% of the power
per hour. So a 2 hp motor (1500 watts) needs at least 7500
watt-hours total and the battery bank shown is about minimum.
One might ask about putting
a gas motor on Electron. I'm quite sure it will not plane
under control in the usual safe sense. So there is little
point in using over about 4 hp. A modern 4 cycle engine
of that power will run for ever on a little bit of gas,
and even a junker 2 cycle that size will burn about a gallon
every three hours or so. A tiller extension is mandatory
for proper trim, very easily done. For that matter the electric
will need a tiller extension or remote steering for proper
trim.
I think this hull would be
a good starting point for a sail boat. The lines are right.
You will have to rethink the cockpit, trying to make it
more watertight. There is a great temptation to use the
batteries for ballast but I'm not so sure this boat is a
good candidate for a self-righting design because it's side
and cabin are low. There might be a compromise where maybe
two batteries are used, keeping the battery compartment
small and yet with enough endurance for 8 miles or so in
calm conditions. The weight of the two batteries might steady
the boat but it would not be self righting. The hull would
easily take a 100 square foot balanced lug sail and leeboard.
Electron is built from eight
sheets of 3/8" plywood and three sheets of 1/2"
plywood, using taped seams and jigless construction requiring
no lofting. |