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.

Thanks

A big question is
whether I should use a conventional gasoline/oil based motor or go with
electric.

Easy to calculate running times for any given trolling motor and battery
combination. Now calculate or observe your hull speed. Now you know your
best possible range. Derate that for possible problems.

My bet is a battery does not store enough energy to do what you want. The
energy density of a lead storage battery vs gasoline is ludicrous.

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.

wrote:
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.

Why not get a canoe & paddle? Or a boat which can be rowed?
Likely to be almost as fast as a small outboard motor, or
electric motor, much more reliable, good healthy exercise.


... 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?

Sure, you could just connect the batteries to your car's
start battery with jumper cables. It might take a couple
hours of running at RPM above idle to recharge though.


... Is this a realistic approach anyway? Solar
power(likely, unrealistic)/other alternatives?

That depends on what the math of battery life vs recharge
times, and the speed/range of your chosen motor with the
numbers you get.

Years ago, I went fishing and frog gigging regularly with a
neighbor of mine who used an electric trolling motor on a
small (10' or so) aluminum jon boat. We went ten miles or so
on a plain old car battery, but then we could take a few
days to get it charged again vie household 120VAC current.
And went noticably slower at the end of the trip than at the
beginning.

... 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.


Richard J Kinch wrote:
Easy to calculate running times for any given trolling motor and battery
combination. Now calculate or observe your hull speed. Now you know your
best possible range. Derate that for possible problems.


Also remember you don't want to run the motor at over 3/4
throttle for best range. I don't think hull speed will be an
issue.


My bet is a battery does not store enough energy to do what you want. The
energy density of a lead storage battery vs gasoline is ludicrous.


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.

Fresh Breezes- Doug King

Thanks everybody for the replies

I guess the best decision would be to go with a gasoline outboard.
Since there is going to be no easy access to AC, recharging will be
available through the alternator only. Meaning the following flow of
energy: fuel - alternator - battery - motion with multiple
transition of energy back and forth from/to chemical (2 types),
electric (2 times), and motion (2 times) type forms. This cycle just
does not seem as efficient as a simple fuel - motion. Plus, the time
required to recharge---I don't think I'll have it. I'll start looking
for some 4-stroke outboard like Honda now.


Why not get a canoe & paddle?

This is a good idea. But, for a person who spends all most of the time
at his job just sitting on a chair this might too much of an exercise.
Plus, I want to have some energy left to build a camp/cook a descent
(hot) meal.

wrote:

Thanks everybody for the replies

I guess the best decision would be to go with a gasoline outboard.
Since there is going to be no easy access to AC, recharging will be
available through the alternator only. Meaning the following flow of
energy: fuel - alternator - battery - motion with multiple
transition of energy back and forth from/to chemical (2 types),
electric (2 times), and motion (2 times) type forms. This cycle just
does not seem as efficient as a simple fuel - motion. Plus, the time
required to recharge---I don't think I'll have it. I'll start looking
for some 4-stroke outboard like Honda now.



Why not get a canoe & paddle?


This is a good idea. But, for a person who spends all most of the time
at his job just sitting on a chair this might too much of an exercise.
Plus, I want to have some energy left to build a camp/cook a descent
(hot) meal.


The outboards on small day sailers seldom have alternators, which means
no way to charge batteries.
Best of the best seems to be the air cooled 5hp Hondas - but no electrics.

I've wondered about powering an electric motor directly from a small generator?
I saw a Honda that makes both 12vdc and 120vac - but (at the store) couldn't
find any info on actually usable output currents.

Retail is often so frustrating...
Outboards are rated in horsepower. No thrust rating?
Trolling motors are rated in pounds thrust. No power rating?

I'm thinking a 60-100 pound thrust motor on a 1500 pound displacement hull
isn't going to win any drag races, but should (eventually?) move out.

The limiting factor is battery charge (as noted this thread).

But with a gasoline powered "battery"?

Somebody want to knock some sense into this idea?


Richard

To complex for my poor brain..........

Borrow a canoe and two paddles from your neighbor.
Fast, quiet, cheep, light, easy.

Why are you set on a rubber raft?
Bob

wrote:
.... Meaning the following flow of
energy: fuel - alternator - battery - motion with multiple
transition of energy back and forth from/to chemical (2 types),
electric (2 times), and motion (2 times) type forms. This cycle just
does not seem as efficient as a simple fuel - motion.

No, it isn't as efficient but it can have other benefits.
Simplicity, reliability, instant on, quiet... but you pay
for those.


.... Plus, the time
required to recharge---I don't think I'll have it. I'll start looking
for some 4-stroke outboard like Honda now.




cavelamb wrote:
The outboards on small day sailers seldom have alternators, which means
no way to charge batteries.

I know that with a 5hp Honda, a 4 amp alternator is an
option. We had one. But 4 amps is not very much power! It is
enough to power running lights and maybe recharge a small
battery over the course of a long day motoring.





Retail is often so frustrating...
Outboards are rated in horsepower. No thrust rating?
Trolling motors are rated in pounds thrust. No power rating?

You can convert both to watts if you like. it's not going to
be a head-to-head match up because the engines are measured
at very different RPMs.



I'm thinking a 60-100 pound thrust motor on a 1500 pound displacement hull
isn't going to win any drag races, but should (eventually?) move out.


Sure. I know of many people using trolling motors to move
small sailboats when the wind dies. Works like a champ. Of
course, you ain't gonna pull no water skier with a set-up
like that anyway.

Fresh Breezes- Doug King

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.

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

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.