On Sat, 20 Nov 2010 21:18:30 -0500, Wayne.B
wrote:

It's definitely not perpetual motion but it does depend on a number of
assumptions that may not hold true in the real world. Extracting
energy from spinning props is definitely possible under the right
conditions but my calculations show that the maximum power available
is relatively low, quite possibly enough to supply house loads, but
very unlikely enough to recharge a large battery bank.

Wayne, you hit the nail on the head as they say.

As most of you know I spent about 6 years living on solar and wind
power, and built electric cars, trikes and motorcycles. So I'm all for
renewable energy and have put my money where my mouth is. But, and a
very big BUT, there are very severe limitations as to what it can do.

You just can't get vast amounts of power from wind, sun and water
unless you take up vast amounts of space and spend vast amounts of
money. One thing that comes to mind. Recently the town here dedicated
a Windspire wind generator at the library. The wind gen and everything
else was donated by various companies in town. It's a 30 foot high
thing that maxes at 1,200 Watts. Of course, like most that figure is
with a 65 MPH wind. More like 100 Watts at the normal 15 MPH winds
here. Anyway, they stated that it should save the libray about $200 a
year in electric costs. Their normal electric bill is about $5,000 a
year!

I just had a thought that might illustrate the problem, at least as
far as propulsion goes. Instead of looking at Amps or Watts directly,
lets look at Horsepower.

1 HP is 746 Watts. Now, to charge a battery there are losses, and to
get that 1 HP there are also motor losses. So to make it easy let's
say 1 HP out is 1,000 Watts, or 1 KW, in. That's 74.6% efficientcy.

Let's further say 10 HP is what we need to power the boat at a decent
speed. In the real world, probably 5 KTs for a 30 footer. Our 30
footer with 10 HP motor has 16 golf cart batteries in series for 96
Volts and 21.6 KWH. This will give us 2 hours and 10 minutes of run
time to a flat battery bank.

Most of what I've read about using an alternator on the prop state the
output is about 10 Amps, so at 13.6 Volts this is 136 Watt Hours, or
..136 KWH. Traveling 10 hours a day gives us 1.36 KWH per day. So a 10
hour sail would let us motor for 8 minutes and 10 seconds.

One could hang a big prop down there, optimised to drive an alternator
and maybe get 50 Amps with a large reduction in speed. That would give
us about 40 minutes run time.

How about solar? If we cover that entire 30 footer in solar panels, we
should be able to get about 3,000 watts. At a cost of $15,000 plus
controller at the current $5 per watt. This will give us between 9 KWH
and 18 KWH per day, depending on latitude and season.

Sooo... We can run under power from 54 mintes to 1 hour and 48 minutes
per sunny day.

We all know air conditioners are power hogs. A 15,000 BTU marine AC
takes about 1.5 HP. So imagine generating enough power to run 6-1/2 of
your AC's. That what you need to power a 30 footer.

Rick