DSK writes:

Think of that 50-pound trolling battery as a gas tank that holds (the
equivalent energy of) a pint of fuel, takes all day to "fill"
(recharge), costs $$$, and wears out in a few years.

And the power equivalent is much more than
a pint of gasoline, especially if you factor in the woeful
inefficiency of internal combustion engines.

Nope. Here's the analysis:

My Group-27 deep-cycle trolling battery weighs 53 lbs and provides 115
Ah x 12 volts = 1.4 KWh. Divide by 746 watts/hp and multiply by 80
percent trolling motor efficiency, you get about 1.5 hp-hours at the
prop, from a full charge to full discharge.

How much gasoline is 1.5 hp-hours?

My 25 hp Tohatsu burns about 2 gal/hour. So 2 gals for 25 hp-hour, or
12.5 hp-hour/gal, or 0.08 gal/hp-hour, times 1.5 hp-hours, is 0.12 gal,
which is to say,

1 GROUP 27 TROLLING BATTERY = 1 PINT GASOLINE

Gasoline has over 50 TIMES the energy density of lead storage batteries.

A plain old lead-acid battery
can easily run thousands of charge-discharge cycles if it's
treated properly.

Huh? They're good for about 200 cycles, assuming you can log that many
before 3 or 4 years of aging works its harm. Then they degrade rapidly,
holding less charge, and self-discharging faster. Think of it as a gas
tank that starts shrinking and leaking after a few hundred fills, or a
few years of just getting old.

Unless you mean by "treated properly" that you don't fully discharge, in
which case, your realized energy density is even less favorable.

8' ... have you ever heard of ... oars?

Richard J Kinch wrote:
My Group-27 deep-cycle trolling battery weighs 53 lbs and provides 115
Ah x 12 volts = 1.4 KWh. Divide by 746 watts/hp and multiply by 80
percent trolling motor efficiency, you get about 1.5 hp-hours at the
prop, from a full charge to full discharge.

How much gasoline is 1.5 hp-hours?

My 25 hp Tohatsu burns about 2 gal/hour. So 2 gals for 25 hp-hour, or
12.5 hp-hour/gal, or 0.08 gal/hp-hour, times 1.5 hp-hours, is 0.12 gal,
which is to say,

1 GROUP 27 TROLLING BATTERY = 1 PINT GASOLINE

Gasoline has over 50 TIMES the energy density of lead storage batteries.


I still think that's a large exaggeration. And your math is
skewed. Why not multiply your gas outboard's power output by
it's theoretical efficiency of about 40% or less? FWIW I
agree that gasoline is very much more bang for the buck than
any other type of stored energy, that's why it's so hard to
find "alternative fuels."

A better way to compare would be to measure how many
ton/miles each power plant could move at a given speed in a
given hull form. For moving a slow efficient hull, the
trolling motor would be better than you think. And for
moving a light load fast, it won't do anything.



A plain old lead-acid battery
can easily run thousands of charge-discharge cycles if it's
treated properly.


Huh? They're good for about 200 cycles

Baloney. Even the advanced metals batteries go for over
1,000 ycles if properly cared for, and old fashioned
lead-acid batteries are limited only by how they're treated.
Their life is theoretically infinite.

If you're only getting 200 cycles out of your deep-cycle
battery, then you're doing something wrong. Maybe you need a
proper 3-stage charger with a desulphating function? They're
not expensive these days.

I know of many people getting 8+ years of heavy (practically
continuous) use out of traction or golf-cart type batteries.


Unless you mean by "treated properly" that you don't fully discharge, in
which case, your realized energy density is even less favorable.

Sure. But if you discharge it 100% and get to use it less
than 200 times, vs discharging it 50% and using it over
2,000 times, what does that do to the energy density
averaged over the life span?

Fresh Breezes- Doug King

wrote:
Why are you set on a rubber raft?

Easy to transport, easy to store (does not take much space, and I don't
have a house), light weight, fits into a backpack (a heavy backpack,
but could easily carry it a couple of kilometers). Does this sound
convincing?

Hi:

Not if you are lugging around two Trojan 105s
16 square feet of solor pannles
or a gas motor
or generator
or 35 pounds of gasoline.

As soon as you say raft I think the problems and weight have just
increased exponentially.
Why not a canoe?
Cheep, light, fast?

On Wed, 12 Apr 2006 20:08:25 -0700, dmitri.maslov wrote:

Hi,

I want to put a motor on an 8' inflatable dinghy. A big question is
whether I should use a conventional gasoline/oil based motor or go with
electric. The situation is as follows: I plan to take the boat out to a
complete wilderness (in a car/SUV) and use it there for, say, a week to
explore lakes and streams. There will be no AC outlets out there. This
creates a problem---how do I recharge the battery/batteries if I decide
to go with electric motor? Is it possible to do it through the running
car in some way? Is this a realistic approach anyway? Solar
power(likely, unrealistic)/other alternatives? I would not have asked
unless electric motor was not so nicely quiet and environmentally
friendly and thus appeal to me. Using electric motor has its value, but
it is not clear if it holds it in the above set up.

Batteries for trolling motors are heavy and take a long time to
charge. Even if you had a portable generator or oversized alternator on
your car, you'd have to run it for quite awhile. There goes your peace
and quiet.

As others have mentioned, a canoe or kayak with paddles might be the
ultimate solution. Or a small rowboat. Inflatable dinghies don't row
well enough for anything but short distances.

We've been very happy with the 2hp Honda BF2D we use for our dinghy. It's
completely self contained, with no external tank, and weighs only 28lb. It
runs well over an hour on a quart of fuel. It's dead reliable, and not
too loud. I recommend it.

Don't get me wrong, I love electric motors.

Matt O.

oh oh oh. gotta an idea!

Yellow "bannana boats." The are inflatable kayaks.
Very light
Very portable.. just roll them up
reasonably fast
much cheeper than all the other stuff.
Besides trying to compute BTU to KW to BHP, SHP, Jules, moles, etc can
be rather confusing for someone llike myself who still has 1200
flashing on my VCR.
Bob

In article ,
says...
SNIP some good arguments
True but the battery does offer some advantages. It's quiet
and you can place the weight of it anywhere in the boat you
want.

Think of that 50-pound trolling battery as a gas tank that holds (the
equivalent energy of) a pint of fuel, takes all day to "fill" (recharge),
costs $$$, and wears out in a few years.

That seems rather pessimistic. A plain old lead-acid battery
can easily run thousands of charge-discharge cycles if it's
treated properly. And the power equivalent is much more than
a pint of gasoline, especially if you factor in the woeful
inefficiency of internal combustion engines.

If you discharge your lead-acid battery to the 50% or 80% level,
you are unlikely to get thousands of cycles. 400 to 500 might
be a better upper limit.

As for power equivalent: a 100AH,12V battery at a reasonable discharge
level is equivalent to about 1.3HP for one hour. My own experience
with a battery of about that size and a trolling motor is that
you can propel a very light boat for about two hours at perhaps
1-3 knots on one battery. I think I would get about the same speed
with my 4HP Johnson OB at a fast idle. Whether that would take more
than a pint of fuel is a good question.


A big difference between charging a battery and driving an OB
is that the pollution from the motor goes directly into the
water. That can be a problem on some lakes.

http://www.windsun.com/Batteries/Battery_FAQ.htm

Fresh Breezes- Doug King


Mark Borgerson

DSK writes:

old fashioned
lead-acid batteries are limited only by how they're treated.
Their life is theoretically infinite.

You couldn't be more wrong.

Sure. But if you discharge it 100% and get to use it less
than 200 times, vs discharging it 50% and using it over
2,000 times, what does that do to the energy density
averaged over the life span?

Nothing. Learn what "energy density" means.

On Thu, 13 Apr 2006 23:09:15 -0700, Mark Borgerson wrote:

As for power equivalent: a 100AH,12V battery at a reasonable discharge
level is equivalent to about 1.3HP for one hour.

That's a pretty big, heavy battery, not easy to carry around!

My own experience
with a battery of about that size and a trolling motor is that you can
propel a very light boat for about two hours at perhaps 1-3 knots on one
battery.

That's probably about right. How long does it take to charge though? And
how do you charge it? It's one thing if you have shore power or a
heavy duty alternator or genset aboard your yacht, but...

I think I would get about the same speed with my 4HP Johnson OB at a
fast idle. Whether that would take more than a pint of fuel is a good
question.

You don't need that much power. A .5 HP electric motor is probably good
enough. A small dinghy has a really low hull speed anyway, so running
your gas outboard at full throttle isn't much faster than at 1/3 or 1/4.
This translates to a smaller electric motor. IME a pint of fuel an hour
seems about right.

A big difference between charging a battery and driving an OB is that
the pollution from the motor goes directly into the water. That can be
a problem on some lakes.

This is true. In fact electric is all that's allowed on some lakes.

http://www.windsun.com/Batteries/Battery_FAQ.htm

I haven't read this page, but be careful about battery websites. They
contain more old wives' tales, mechanics' folklore, myth, and marketing
drivel than just about anything else.

I'm actually intrigued by small electric outboards. I'd like to have a
self-contained unit with a built-in Li-ion or NiMH battery. It could
weigh no more than a small gas outboard, run for two hours, and charge in
15 minutes off the generator in the "mother ship." Even if it were
very expensive, the quiet and cleanliness would appeal to a lot of people.

Matt O.

Here are some more details. There expected to be 2 people aboard, still
water, the total travel distance of 200-250 km (covered over a period
of few days), and we have to carry all our equipment (including tent,
sleeping bags, etc.) and food supply with us. Would have to carry too
much stuff for a canoe, so it is unlikely that a canoe will offer as
much speed and comfort as you suggest. Plus, I'd hate to find my palms
grind to meat some 50 km from the starting point of the trip.