G'day mtd (Matt?)
At 08:35 AM 13/03/2010, mtd wrote:
>Hey everyone,
>great to see so many going green on the water, and most importantly
>sharing info as a community.
>I have lived aboard an engineless Morgan 30 for many years now and
>cruised throuhout the Caribbean and Bahamas.
(I envy your lifestyle)
> Alas I am not as young as I once was
As happens.
> <snip> To sum it up, what I have is an encased rear axle - with
> exposed splines on the shaft ends- attached to a reduction housing
> and then to a 2 hp dc 36 volt motor with forward neutral and
> reverse. Also it's variable speed of course.
>Being the real axle - or an axle to be more accurate - I will have
>one end ( where the tire and hub were ) to connect to the prop shaft
>via a universal joint, and one end free where I could attach a large
>pulley, maybe on a PTO, to an alternator.
OK, first things here - what RPMs will the motor do, and what RPMs
will the end of the axle do? Using the axle you have one problem,
being the 'diff' in the middle - you'll need to lock either the diff
action or the inboard end of the axle in order to get drive. If you
want to use the inboard end as a generator PTO it'll need to be
locking the diff action.
However, you may not get reasonable prop speed using the diff (I
don't know what the reduction ratio of a GC diff is, though, so
you'll have to check that). If not, you then have a different problem
- the end of the motor probably has no mounting plate, but you may be
able to chop the 'front' out of the CG axle to provide this.
>AMPS IN: three 110 watt solar panels ( I live in places with almost 100% sun)
> One airx wind generator
> I also have a three foot single blade halyard wind genny (
> that got wet and needs help)
>& The preposed alternator which of course will cause X? Amp draw as
>well as creating anywhere from 25,45,90,110 amps per hr. With a
>supposed? additional amount of amps to spin it respectively as the
>output goes up.
The amount of generating capacity will depend on how much prop drag
you are able to put up with. If you have the space for three or six
more solar panels, that may be appropriate (one panel/pair of panels
per extra 12V battery - more follows)
I'd suggest the way to connect your system will be to get two
heavy-duty change-over contactors with 12V coils, and have your
batteries in 3 x 12V sets, the "lowest" set remains your vessel
systems battery (and may be substantially larger capacity than the
other two), the two others are connected in parralel to the vessel
systems battery by the normally closed contacts of the contactors.
The 12V charging system remains connected to the vessel systems
battery, and charges the other two via the NC contacts of the
contactors. When you switch on to get drive, the contactors turn on
and re-connect the batteries into series (you will not be able to
charge from the 12V source whilst drive is turned on) for 36V to drive.
> Backup: 40 amp output 110v ac to 12v dc converter for gas genny
> - as an emergency application only, not for day to day use.
>THE BOAT: is light for a cruiser. She's about 8000 pounds loaded and
>draws little water with a swing keel/centerboard design. She does
>have an engine room where an atomic gas engine used to sit prior to
>my ownership. It likely had a max of ten or twelve? hp and could
>have been as little as eight depending on the factory at the time.
>Or so I've been told.
Very rough rule of thumb, 1 electric horsepower performs equivalent
to 4 internal combustion horses, so your 2hp electric should perform
approximately as an 8hp internal combustion engine.
>For sake of comparison: a eight horse four stroke Honda with it's
>tiny and inoportunely placed prop, pushes us along at four to five
>knots at mid range to higher Rpms- on the transom
>I have no illusions about motoring for days or even hours. Well
>maybe two hours in a day would be great. But primarily we want to
>utilize the motor for exiting and entering port. We're sailors. I
>hope to not exceed 30 amps/hr to achieve 3-4 knots.
OK, some terminology to get straight here. Amp-hours vs amps per hour
vs power at different voltages. You're looking to 'burn' about 60
Amp-hours each time you use the motor - however, at 36 volts, that's
three times the power as at 12 volts. One kilowatt is about 28 amps
at 36 volts, but equivalent to 84 amps at 12 volts. So if you need to
use the motor at 1kW for 2 hours per day, you'll need to generate 2
kilowatt-hours during the day - 17 amps for 5 hours at 12 volts
(another 225 Watts of solar could do that, under ideal conditions).
>My enduring questions are is it possible to calculate the amp loads
>vs knots achieved to any real approximate? This entails many
>variables of course, but then I'm no engineer either.
Unfortunately it is a 'suck it and see' due to so many variables,
unless you can find a similar boat that has good data, or can borrow
a decent sized electric outboard to try (it's a bit far for me to
loan you my 1.5kW outboard, as I'm in Australia).
>Is 2 hp with it's reduction gear and a huge propeller enough to
>acheive say four knots?
Probably. Huge propellor? Maybe - the prop size will be a
drag/generating trade-off, up to a point you can't have too big a
diameter prop from the propulsion point of view, IF you select the
correct pitch. Then it gets back to pitch/revs/speed etc black art & guesswork.
>Tangentally, is there any way to pull 12 volts from a 36 volt bank
>to feed house system?
As described above, with contactors to switch the batteries around.
If you add extra panels to each battery then you'd still be getting
charge whilst motoring.
>Is there any truth to the arguement that a larger alternator draws
>appreciably more?
A large alternator may need 8 amps for it's field, where a small one
may be half that. You will also need to worry about the RPM/amp curve
of any alternator you choose to for propshaft generation, as many
need higher RPMs to get any appreciable power. You may have to use a
large alternator - it may take a 120 amp alternator to give you an
average of 15 amps at your average speed. Consider the available
space for a very large pulley to drive the alternator (maybe 24" or
more, sometimes found in bigger washing machines) to get the
alternator RPMs up.
>Is there any truth to the arguement that electric hp and combustion
>hp are not equivalent/ elec is greater in ratio?
True, already mentioned above.
>Am I out of my mind?
No more than the rest of us.
>Anyway, I'm sure I've left out some pertinent detail somewhere. Feel
>free to let me know or provide any feedback whatsoever.
>Thanks for your time
You're welcome.
Hope this helps
Regards
[Technik] James
Launceston, Tasmania, Australia
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