Craig
For a motor – AND a given voltage it will reach maximum efficiency at about 80-90% max rpm on the same voltage at a fixed amp draw
Electric motors are swings and roundabouts, so as the voltage increases the amp draw at which max efficiency occurs also increases, the tradeoff is often reduced efficiency at lower amp draws
to making the prop spinfaster
a/ using the same volts you make the gearing taller and as the same torque will be required to turn the prop at 2000 rpm the motor will use twice the amps for the same rpm
b/ keep the reduction and raise the volts and you will use no more amps and however above 2000 rpm you will consume more power
simple message keep the gears, adjust them properly, because you motor doesn’t make enough torque per amp to do away with them
Yours,
Andrew Gilchrist
fastelectrics.com
Australia
0419 429 201
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of Craig Carmichael
Sent: Monday, 15 October 2012 11:30 AM
To: electricboats@yahoogroups.com
Subject: [Electric Boats] Re: O/B Gear Ratios? + 12V Accessories
Hi,
I converted a Honda 7.5 at one point with an early version of my
"Electric Hubcap" motors (Honda is now sitting with no motor in it).
Short clips for anyone interested:
http://www.youtube.com/watch?v=X1MkdixNLcc - inside motor
http://www.youtube.com/watch?v=n0JCRY8yGV8 - running in bucket
http://www.youtube.com/watch?v=yUfqSSnEh5c - launch
http://www.youtube.com/watch?v=R2f33577ADo - onboard
(Later examination revealed the gear noise proved to be because the
vertical shaft was pressing down quite hard on the gear at the foot.)
It has about 2.8 to 1 reduction at the foot. This proved frustrating
with a 2000 RPM motor, as it wouldn't get the prop going fast enough
at full speed to get the boat making much wake, still less up on a
plane.
For a runabout, it seemed the prop should be going about the same
speed as the motor (if not faster), not reduced. But all the gas
outboards seem to have a considerable reduction at the foot, and
seemingly no way to change it.
Another person used an induction motor and a variable frequency drive
(in a Johnson 10 or 15 HP?), which they set to 0-120 Hz insead of
0-60 Hz to get more speed. Of course, that's double as fast as the
motor was designed to run.
My latest idea has been to make an outboard from scratch, using two
socket set drive or similar U-joints at the foot (running in oil) to
do the 90 degree bend, 45 degrees each, with (obviously) a 1 to 1
ratio. I still think this should work well - it seems to turn with
low friction by hand (assuming the shafts are fixed in the desired
positions with bearings, not flopping around) - but I've been onto
other projects all summer. (So many things, so little time!)
=====
Since I'm writing... I did make or am making some 12 volt things that
might be interesting to some electric boat users:
- a 12 volt, 5 cubic foot, superinsulated (3" foam), Peltier cooled,
shallow chest fridge, which freezes ice during the day while the
solar energy is there, then lets the ice melt to keep the fridge cold
overnight without needing any battery storage.
- A 3D plastic printer (RepRap Mendel) from a kit. This helps produce
some more things:
- A new standard 12 volt plug and socket: Connector based on AT fuses
(CAT plugs). They're the same as ATO/ATC DC fuses except one pin, the
negative, is turned sideways to ensure polarity. So far I have plugs
for cords, and sockets: for cords, a duplex panel mount face, and a
duplex face for "1110 electrical box" (the ones for house lights and
plugs).
- I plan to wire my house with 12 VDC, with these sockets and plugs.
The power will come from solar collectors now on my roof (charging
NiMH batteries) when available, or from a 12 volt battery charger if
the batteries get too low. I'm designing a 12 volt solar main
distribution box with a number of features and circuits as well as
main and branch breakers.
- I've been doing LED 6" plastic globe lights that are 75 or 100
lumens per watt (where "120 VAC LED bulbs" are usually 67 or less).
I'm just now doing circuits to make lights that can accept
unregulated battery voltage instead of 12.0 volts only. The present
circuit still dims somewhat with falling voltage... which at least
helps conserve low batteries.
Craig
Victoria BC
=====
>Hi,
>
>I am hoping to convert a gas outboard to electric and was wondering
>which gas outboards might be better to start with.
>
>One thing I would particularly like is as low as possible (high
>numerical reduction) of a gear ratio so the prop turns slower.
>
>Does anyone know what the standard gear ratios of common small
>outboards are? Is there possibly a list that I might be able to see?
>
>Thanks!
>
>Bruce
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