[MHml] diesel-electric drive

[Evan Gatehouse Diane Selkirk]

--- Matt Lawrence <matt at technoronin.com> wrote:

> My take on the issue is: Are you in a motorboat or a
> sailboat?  If you are
> serious about sailing, you'll probably only run the
> motor for 15 minutes
> going in and out of a marina.  With that kind of
> usage, I'd probably just
> recharge off of shore power.

Not necessarily.  I'm serious about sailing and will
keep sailing as long as there is wind and I don't have
to get somewhere BUT:

1. What if there is no wind but lots of tidal current?
 Lots of deep channels in B.C. & Alaska qualify with
these conditions.  Hard to anchor and wait for wind...

2. What if you're in a nice anchorage on Sunday
morning and want to get back 30 miles to your marina
so you can get to work on Monday and there's no wind?

3.  What if you need to make that entrance into the
anchorage before the sun gets low enough to hide the
coral (many places in the tropics)?


All these might require a lot more than 15 minutes of
motoring time.

> The advantages I see of electric:
> 
>   Probably lighter (although adding a genset may
> negate that)

Almost never, unless you need power for 15 minutes and
only have a small boat, batteries and an electric
trolling motor, not a genset + batteries.  

Even small capacity battery banks will weigh an awful
lot.  Figure you take out your 18 HP diesel and put in
a 5 HP electric motor.  (you do get higher torque with
the electric motor and you don't think you need to go
as fast).  The power required to run your motor is 5
HP x 746 Watts/HP / 0.90 efficiency = 4144 Watts.  

Say you have a 12 Volt battery bank because most of us
have a good knowledge of that size battery.  4144
Watts/12 Volts (nominal) = 345 A.  Say you only want
to run for 1 hour.  You will need a 345 A.hour 12 Volt
battery.  

However, (a) batteries A.hour ratings are usually for
lower current draw over 20 hours. and (b) you don't
want to totally drain your batteries to 0%, so you
need extra capacity in both cases.  Figure at least x2
(I'm being generous for the electric proponents) for
(a) and maybe x1.3 for (b).

Now you need 345 A.hour x 2 x 1.3 = 897 A.hour.  Now a
an 8-D battery has about 220 A.hour capacity over 20
hours.  So you will need 897 / 220 A.hour = 4.1
batteries.  Let's call that 4 batteries.  An 8-D
battery weighs about 120#.  So 4 x 120# = 480#.

An 18 HP Yanmar 2GM motor weighs 251#.  Add 25# for
exhaust, cooling water, etc. etc.  Add #35 for a fuel
tank.  Add 4# for 1/2 gallon of diesel to motor for 1
hour.  Total weight = 315#.

The energy density (available power/weight) of any
conventional battery is so low it's hard to imagine. 
I need 480# of batteries to provide less power than 4#
of diesel.  That's why electric boats will seldom be
practical except for those with small boats or very
short range requirements.

As Dave Culp says, if you're silly enough to add one
more step to the method of propulsion (generator ->
electricity -> motor) with the resulting losses in
each step, then you deserve what you get!

Evan Gatehouse

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