PROPWASH
8
October 2011
rated capacity. I used around 2800 mah during each heat of last weekend’s P limited
racing.
Presently, the electric rules only control one aspect of power production, the
voltage. The current is limited by several factors. If cost is not a factor, the best
batteries are burst rated at 900 amps for a 10,000 mah battery combination and 1,080
amps for a 12,000 mah combination (40 seconds maximum run time in both cases).
The highest current commercially available model speed controls are rated at is 440
amps. The most powerful motors are rated at around 380 amps. That results in almost
17 hp from 37 (T) volts. A combination that could use the available battery current for
a brief period (SAW runs) might have 40 hp at 37 volts. Limiting the currents to 100
amps reduces the cost a lot.
The P limited classes do this by restricting the motors. The AquaCraft UL-1 motor
is rated at 50 amps continuous with 80 amps for a 5 second burst. Since the speed
controls are not restricted, the motors are usually run at a little less than 100 amps
average current with current spikes to as much as 150 amps. This results in limited
motor life due to insulation deterioration. Water cooling helps prevent this. However,
the $60 motor replacement cost is very reasonable compared to the cost of motors that
can handle over 200 amps. The P limited motors also work well on many of the 3.5
size hulls available.
The P limited rules also help beginners get started in electric power plants. There
are selections of motor Kv’s available that give choices of 30,000 rpm, 26,600 rpm,
and 22,200 rpm. The usual choice is the highest rpm, but propeller selection can make
lower rpm motors work well. The stock speed controls have limited current handling
capacity and a higher internal resistance than some aftermarket controllers. However,
aftermarket speed controls such as the Turnigy Marine 120, the Castle Hydra 120,
Castle Phoenix ICE 100 with added water cooling, and other similar speed controls
work well. Aircraft speed controls like the ICE 100 will need a Pistix adapter to run
with pistol style controllers. You need at least 5.5 mm bullet connectors on the motors
and batteries. Otherwise the heating from the connector resistance will soften the
solder connection. Since batteries only need to supply around 150 amps maximum,
40C, 4000 mah batteries (160 amp continuous rating) should work. However, in the
really low cost batteries, 40C or better, 5000 mah batteries (200 amp continuous
rating) are a more durable choice. The advertised ratings of low cost batteries may
overstate the actual capability of some of the batteries. Even so, the battery, motor, and
speed control can cost under $200.
Memories of the 2011 NAMBA Nats
By Alan Hobbs
NAMBA Past President
There are few places as good as Legg Lake to hold a big boat race. And the 2011
NAMBA Nats was a big boat race. I was told that there were more than 700 entries
this year. There are not many boat races bigger than this one was.
With so many friends coming in from Australia, the Pacific Northwest, Texas,
Colorado, Northern California, Arizona and Southern California, there was no way I
was going to miss this Nats. As it turned out, other old friends showed up as well. I
had a pleasant conversation with Richard Hazelwood and Bobby Tom in Joe
Monohan’s motorhome. Steve Muck dropped by to say hello and watch the races.
Those of you that were around twenty years ago will remember how much these
people influenced NAMBA and model boat racing. It was truly my pleasure to see
these friends one more time. I hadn’t seen Ron Russell in several years. He sure hadn’t
forgotten how to drive and won A Hydro. Let’s hear it for the grandparents that are
still racing model boats. And winning for that matter.
For me the most important part of any Nats is seeing friends, finding out how they
are doing, watching their children grow up, and of course, running my boats fits in
there as well. I remember at the New Mexico Nats when Alan and Brenda Terry’s
children were running around the pit area. I told them to stop running in the pits. This
summer I got to see Allie and Ryan again. Ryan is now a Marine and a daddy. Who
Electric Power Basics
(Continued from page 7)
will stand quite a bit more than this for a
short time. However, that makes it hard
to decide when you are over propping
the motor. Note the fuzzy area on the
graph. Over time, too much heat will
cause insulation break down resulting in
a short circuit that will destroy the
motor and often the speed control.
Another electric advantage is much
more freedom to pick propeller rpm.
The motor in the example was chosen so
we could run the same propellers as the
nitro tunnels. The increased torque made
the setup very different, even with the
same prop. Should you run a motor with
a higher Kv and a smaller prop? It
depends, and I don’t know a clear
answer.
As another example, the P limited
outboard tunnels often run a 42 mm
prop with a motor Kv of 2030. That
means the nominal rpm is around
30,000, higher than a 3.5 nitro engine
can turn the same prop. To avoid
overheating the motor, the prop can be
back cut and have the blade area
reduced. Then the cup can be increased.
Further cup increases might require
slight diameter reduction. Finding the
right combination is what electric racing
is all about.
Run time is calculated from battery
capacity, stated as millampere hours or
mah. Dividing the mah by 1000 gives
the amount of current in amperes a new,
fully charged battery will deliver for an
hour. Multiplying that by 60 gives the
current for a one minute run time. A
typical race is over in less than two
minutes. That means a motor drawing
100 amps average will use 200 ampere
minutes or 3333 mah. That’s well within
the capacity of good, new 4000 mah
packs, but 5000 mah packs will be safer.
The table shows times at 80% of the