Gardner,

In response to you questions. 4KW is the expected consumption of
8-10KW electric lumbering at less than hull speed for 2.5 hours.
Might be worse than that (6-7KW consumed) but it was an example of
the HUGE disparity between energy stored in a battery and anergy
capability of hydrocarbon fuels. If you are pushing a Visioanarry
at hull speed, you will need at least a total of 10KW. You might
end up a little less than hull speed depending on how "big" a
Visionarry. Takes 20KW to move an African cats Fastcat (14,000lb
dispacement)at 8.5 knots. You should have about half that mass.

That testing was done in flat water. Hull speed is only a flat
water measure, if your design windage is bad then running upwind
you may get 1-2 knots. Rare Bird used (2) 15 HP outboards. 10KW
is only getting you the same "max" thrust as (1) Honda. I don't
find that yet to a handicap, just know I won't be busting 9 knots !
Don't be fooled by advertising a 4KW = 10-15HP Ouboard. It's not
true "except" when the outboard is moving you at 3-4 knots, well
below it's optimum torque curve of the Honda. Speed above that and
15HP outboard is more efficient and has more thrust. Top end thrust
of a 15 HP Honda is more like a 10KW electric. As a side note you
can over-power a electric for brief periods until the motor or
control overheats. So that's when you can get 15HP equivalent out of
4KW motor, just can only run minutes or perhaps seconds at that load.

You can split up 10KW needs anyway you want (2) 4-5KW is fine. Don't
worry electric pods will eventually leak thru the bearing seals.
You won't ruin them if they do leak. In acuality they will have
some water in them due to internal condensation. That little water
gets cooked out as it runs a little. If the POD is stored in a
position out of the water then seals may never have to be replaced.
Otherwise the pod and it's connecting arm with wires is the only
part submerged, all the rest is housed in a separate external box
and generally water tight.

The biggest hassle is how much electrical energy to store, and how
to do it safest and cheapest.

Regards,
JT

--- In harryproa@..., "Gardner Pomper" <gardner@...>
wrote:
>
> Hi,
>
> I think you have summed up random findings of mine very well. I
have a
> couple questions. You mention that your monohull would require a 4KW
> electric, but you sized a genset at 10KW continuous. Were you
intending that
> there are 2 4KW electrics on a Visionarry sized boat?
>
> I am on the fence with the outboard vs the diesel genset. I want 2
motors
> for the maneuverability in tight quarters, as well as the
redundancy. I had
> a small cat with dual outboards and they worked well for motoring,
but were
> lousy at charging the batteries. We had solar panels for that, but
the sun
> didn't always shine. I *really* wanted a small generator for battery
> charging as well.
>
> Would you mine resumarrizing your findings to compare a dual
outboard system
> (15Hp each), plus a small genset for battery charging, vs a diesel
genset
> with 2 4KW electrics? I was thinking that those two systems would
be close
> in weight, but unfortunately not in cost.
>
> My main concern about a dual outboard solution is that I don't see
an
> adequate way of getting the leeward outboard far enough away from
the water
> that it will not get wet when hard sailing. Even when flipped up, I
would
> think that there will be a LOT of water intrusion into the head.
Would an
> electric motor in a pod be more waterproof, or easier to shield?
>
> Thanks for the summary. It is often surprising how complicated a
sailboat
> gets once you want it to do something more than just sail.
>
> - Gardner
>
>
> On Fri, Jul 4, 2008 at 11:27 AM, jjtctaylor <jtaylor412@...> wrote:
>
> > In my quest for ideal auxiliary propulsion for our beloved
cruising
> > proas, I have hit a snag. The proverbial weight spiral has begun.
> > Today's technology has left us with gas outboards, hydraulic, and
> > electrical. Outboards have no comparison in weight to thrust
> > ratio. All Harry's and smaller would benefit greatly from that
> > choice. For Visionarry's and larger which will have long motoring
> > requirements the choice is more murky.
> >
> > Outboards can run for extended periods but do need some degree of
> > care maintenance. Have to carry a more volatile fuel and prop can
> > run into cavitation issues in rough seas due to limited leg
length.
> >
> > The most elegant option is electric pod in various installation
> > methods. Now comes the bad news. While the electric pod isn't
> > heavy, all that supporting power source is. As mentioned above for
> > outboards hyrdrocarbon fuel has much more energy/weight ratio than
> > anything else readily available.
> >
> > Case in point, my 36 foot monohull got about 16 miles per gallon
at 6
> > knots. An electric would consume about 4KW. To store 4KW, you
need 8-
> > 10 KW of batteries. That's because you should not discharge
> > batteries beyond 50% capacity or the life cycle rate drops
> > significantly (1500 cycles --> 300 more or less) and batteries
once
> > used do not return to their original 100% capacity due to
sulfation.
> > Sulfation is the coating of the lead plates by sulfur crystals
> > leaching out of the sulfuric acid electrolyte. The crystals
prevent
> > the transfer of charge from the lead plates, thus we are always
left
> > with only 92-98% of the original capacity.
> >
> > So what we have is the difference of 7 pounds for hydrocarbon
fuels
> > versus 600 pounds for 8KW battery capacity. There are many new
> > technologies on the horizon for batteries, ultracapacitors, etc.
but
> > nothing is coming near term for the boating industry.
> >
> > Only technologies that could be options, available for over 20
years
> > are NiMh and Lithium-ion. Both are outrageously expensive,
typically
> > (4) times the cost of lead-acid types. NiMh are available from
> > Nilar, but you are on your own as no one supports or warrants them
> > for marine application. Nimh also has high self-discharge rates
of 1-
> > 3% per day, so would require dedicated solar panels to keep them
> > charged. Lithium-ion technology is reducing their probability to
> > catch fire by adding metal alloys to the lithium to handle high
> > current and each cell has a mini-pc attached to manage charge and
> > discharge rates. See Lifebatt, Valence, and A123 to name a few
> > suppliers all migrating to this new direction for Lithium-ion.
> > Despite the high cost these technologies do double the storage
> > capacity/weight ratio.
> >
> > So for battery technology we have a real boat anchor in terms of
> > energy storage capacity. For a Visionarry sized craft, 8-10KW
motor
> > would be satisfactory for hull speed. Battery capacity would
dictate
> > around 14KW for 40 minutes at full throttle, or 800 pounds. If you
> > have deep cash pockets Lithium-ion batteries are only 400 pounds.
So
> > what happens when battery capacity is gone and no wind to regen.
> > Then we have to rely on a generator.
> >
> > Generator has to be rated to provide some satisfactory cruising
speed
> > AND some reserve to recharge batteries. For a Visionarry that
would
> > be a 10KW. Generators run to full capacity are not intended as
> > propulsion engines and will overheat . Thus they have to be de-
rated
> > to run less than max output. In my case I use a Volvo D1-20
engine,
> > 18HP to the output shaft (13.5KW) and rated down to 10KW
continuous.
> > All that weighs 360 pounds. A whole lot more than a 15HP Honda
> > outboard. Add to that another 80 pounds of AC-DC battery chargers,
> > DC-DC chargers, breaker boxes and cables.
> >
> > The expense of it all is a killer too. $16000 genset, $2400
> > batteries and $8000 electric motor. Buy a few Honda's.
> >
> > As I have mentioned before you can do with less power or less run
> > time, but that compromise at some point becomes unsafe or
impractical
> > to deal with the elements or sea conditions. In many ways the
trade
> > off away from fossil fuels we are getting less for more. In total
my
> > boat looks to weigh out around 8000 lbs light and 10000 loaded,
which
> > is not where I wanted to be. Maybe someone on the forum has a
> > breakthrough we can all reapply. Adding over 1000 pounds to a boat
> > professed to be lighter and easier to build doesn't seem to be the
> > right direction.
> >
> > Regards,
> > JT
> >
> >
> >
>