In article ,
says...
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.


As a device to get you from your anchored yacht to the shore, this might
be an attractive alternative. In most NW anchorages, you are seldom
more than a few hundred yards from the beach, so rowing isn't a big
problem---unless you have to cope with a few kids and a dog and an
inflatable that rows with all the directional stability of a soup
bowl! At times like that, a 1/2hp OB with an hour's endurance
might be useful.

With resonable efficiency 1/2Hp for one hour would need
about 1200Watt hours to recharge. That's about 100 Amp
hours at 12V---or about 4 hours at 25 Amps. Not an unreasonable
load for a husky alternator on a diesel trawler running 4 to 5 hours
between anchorages. (Friday night--11:30PM, good wine with dinner---
check my math!)

Mark Borgerson