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Bruce In Bangkok

On Wed, 08 Jul 2009 08:33:05 -0400, wrote:

On Wed, 08 Jul 2009 19:22:34 +0700, Bruce In Bangkok
wrote:

On Tue, 07 Jul 2009 13:02:30 -0500, Rick Morel
wrote:

On Mon, 06 Jul 2009 23:11:05 -0400, Wayne.B
wrote:
Much snipped

I would consider a 4-cycle outboard if the Perkins died, but we have
too much overhang, plus the dinghy/davits would have to go. Here goes
another discussion maybe... If the Perkins dies, I plan on replacing
it with a DC electric motor and diesel generator. The motor would
probably mostly be run from the house bank for short periods and
calmer conditions.

Your preliminary comments are correct :-)

I'm interested in your reasoning for the electric drive as it seems to
present no advantages to have a diesel fueled generator which charge a
set of batteries to provide power to an electric motor to turn a
propeller.

Given that any mechanical or electrical device will have some losses
it would seem that there will be less efficiency then a system that
connects the diesel engine directly to the propeller rather then going
all round Robin Hood's barn, as it were.

Diesel/Electric has been used on trains for a very long time. There
must be some advantages. I guess one would be full power available at
low speeds, and another would be the elimination of need for a
transmission.

Also for quite a few ships, but I believe that for trains and ships
(and some large earth moving equipment) the advantage is the
possibility of all wheel drive on wheeled equipment and the ability to
use multiple, thus smaller, generators paralleled as required, for
more economical fuel use and of course easier maneuvering as the
motors are instantly reversible. Trains too had the advantage that it
was impossible to stall the propulsion motor when starting.

But on a small boat or low power installation - remember the guy is
replacing a 40 HP diesel - I just can't see the advantage of the added
complexity.

Which is why I was asking the question.

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)