Electric motor to power a dinghy revisited
 

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

Electric motor to power a dinghy revisited
 

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.

Electric motor to power a dinghy revisited
 

On 12 Apr 2006 20:08:25 -0700, wrote:

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?

Electric power is not going to work for you unless you buy a generator
and a high capacity charger. If you're looking for quiet operation
you will probably be better off just getting a small 4 cycle Honda
outboard. At idle you can hardly hear them.

Recharge times will be too long on solar power or with your vehicle
alternator.

Electric motor to power a dinghy revisited
 

When you move to the wilderness, and disconnect from the artificial
urban environment, you come face-to-face with the key issue: Electric
vehicles require electric generators... somewhere. When you can just
plug your charger into the wall, that nasty generating plant is out of
sight and out of mind. But when YOU have to supply the electricity, you
end up with the same conclusion: burn fossil fuel.

But if you are going to burn fossil fuel, next you need to ask yourself
if burning the fuel in an engine in the generator is an improvement over
burning the fuel in an engine and using the power output directly, thus
eliminating the need to carry (and indirectly, to manufacture) the
battery and the electric motor, and eliminating the inefficiencies of
turning shaft horsepower into electircity, electricity into stored
chemical energy in the battery, stored chemical energy in the battery
into electricity again, and finally converting electricity back into
shaft horsepower.

TAANSTAFL

bob

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.

Thanks

Electric motor to power a dinghy revisited
 

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

Electric motor to power a dinghy revisited
 

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.

Electric motor to power a dinghy revisited
 

Electric motor to power a dinghy revisited
 

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

Electric motor to power a dinghy revisited
 

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

Electric motor to power a dinghy revisited
 

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?