T-Pain Is On A Boat: [Electric Boats] Re: O/B Gear Ratios

I've read your post a couple of times. As I understand it you are advising matching motor torque and rpm to the Internal Combustion Engine you are replacing and a system that was already functioning well.

What if I wanted to do something different? What if there were no existing system? The typical ICE based system on a boat is very energy inefficient and it doesn't seem to me to be a very good idea to just swap out an ICE and replace it with a motor spinning at the same RPM and torque.

What if I had a given motor and wanted to find the sweet spot of motor efficiency, propeller efficiency and boat speed speed. How would one go about calculating all the variables? As a displacement hull approaches its theoretical hull speed it takes greater and greater amounts of power to achieve smaller amounts of increase in speed compared to approaching .5 hull speed from slower speeds. Therefore, for the sake of calculations in our example, lets assume we want to achieve 80% of hull speed at 75% of throttle.

If my boat were used as an example: 24' LWL 8,000 lb displacement. 16 inch draft. Prop limited to not more than 13 inches. It should be a two bladed prop so it can hide behind the skeg when the boat is sailing. HP? torque? Blade RPM? Blade diameter and pitch? How to calculate such things for any given boat would be useful to this list. A long time ago I found a web page that would take information about a boat and calculate ideal HP, blade diameter/pitch and RPM. If HP or blade diameter was limited you could vary the inputs to get the best solution for your situation. Recently looked for such a site and could not find one.

Outside the ideal, theoretical world, in the real world I am limited by availability and cost of materials. I'd use this motor http://www.electricmotorsport.com/store/ems_ev_parts_kits_pmac-ssr.php
coupled to a used outboard lower end and figure out how to mount a two bladed prop on it. One prop manufacturer couldn't help me with it, but perhaps a machine shop could press a sleeve into one of their props to decrease the diameter of the bore so the prop would fit on an outboard lower end. However, galvanic corrosion might be a problem. Aluminum or synthetic would be best.

Eric

Someone asked how much HP is needed to drive a 30' sailboat. The typical container ship or tanker uses about 1 HP / ton. Big slow turning props.

Eric

--- In electricboats@yahoogroups.com, "Andrew Gilchrist" <andrew@...> wrote:
>
> Electric motor torque is specified on the makers spec sheet - a kit motor
> should provide the same specs - specific box ratios are unimportant - its
> important to work out what torque you are replacing at a desired rpm or
> range of rpms. Electric power rotates a prop the same rate ICE power does.
> That's why electric motors are used in dynos testing ICEs
>
>
>
> The match able to be obtained will be less than perfect but it must be based
> on the combination of kt, amp draw and drive ratio - assuming the prop is
> the same.. And the inputs required should top out close to the motors
> inputs and load at maximum efficiency
>
>
>
> If the gears are too tall the amp draw will be such as to damage the motor;
> the motor will never get used at an efficiency throttle setting or loading,
> runtimes, will be poor and heating increased
>
>
>
> If the gears are too short the motor runs at too high rpm, the prop rpm is
> low, thrust is low, efficiency poor, heating may be a problem ..
>
>
>
>
>
> Andrew Gilchrist
>
> fastelectrics.com
>
> Australia
>
>
>
>
>
> From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
> On Behalf Of bb386scn
> Sent: Thursday, 18 October 2012 4:37 PM
> To: electricboats@yahoogroups.com
> Subject: [Electric Boats] Re: O/B Gear Ratios? + 12V Accessories
>
>
>
>
>
>
>
> Above a 3:1 gear ratio sounded great so I did a little research. According
> to this Force Outboard Database:
>
> http://proto.piranha.com/motor_specs.php?id=3003
>
> The gear ratio of the 1984 - 1998 Force 9.9HP 2-stroke outboards is 1.57:1.
>
> Perhaps your motor was an earlier (or later) year?
>
> Still on the search for that 3:1 gear ratio...
>
> .
>
> --- In electricboats@yahoogroups.com
> <mailto:electricboats%40yahoogroups.com> , Joe Grez <joe@> wrote:
> >
> > Our Force 9.9 hp 2-stroke had a higher ratio, above 3 for sure. I'd guess
> > that you would want to look into two strokes. They rev higher.
> >
> >
> >
> > On Tue, Oct 16, 2012 at 11:45 AM, bb386scn <bb386@> wrote:
> >
> > > **
> > >
> > >
> > >
> > >
> > > The 2.8 to 1 gear ratio in your Honda outboard is the highest reduction
> > > that I have heard of. Sorry to hear it didn't work out for your
> > > application, but it should work well for a motor with more common RPM
> > > characteristics.
> > >
> > > Has anyone else heard of an outboard with a gear reduction of greater
> than
> > > 2.8 to 1?
> > >
> > > --- In electricboats@yahoogroups.com
> <mailto:electricboats%40yahoogroups.com> , Craig Carmichael <craig@> wrote:
> > > >
> > > > 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
> > > >
> > >
> > >
> > >
> >
> >
> >
> > --
> >
> > Joe Grez, President of PropEle Electric Boat motors Inc.
> > joe@
> > 425-241-9241
> > 10404 428 Ave SE, North Bend Wa, 98045
> >
> > http://www.linkedin.com/in/joegrez
> > www.electricpaddle.com
> >
>