Hi John,
Ive been thinking much along the same lines for my some-day dream boat
(60+foot sailing cat). I'd ultimately like to have redundancy in all
systems with maybe 4 props; one in each hull and twin drop down props in the
deck. While it initally may sound stupid, I'd envisaged having many systems
tied together. I was thinking hi speed hydraulics for reliability, with
many tie-ins. here would be diesel generators (x1 or x2) with battery
banks, and an option to either have the diesels direct drive the hydraulics
or charge batteries, or both. There would then be electric motors to drive
hydraulics on demand also. The two props in the hulls would be hydraulic
drive, while the two in the deck would probably just be some form of
standalone outboard (also diesel).

Further to all this, many other items on the boat would also be hydraulic
driven; compressor for refrigeration, whinches, anchor whicn etc and could
all be driven by either the diesel motor running the hydraulic pumps, or the
electric motors running the same pumps. The props in the water would also
be able to function as generators, and there would be other options for
generating electricity (wind generators, solar etc). this would give
numerous options for drive, and with 2 of everything you would have plenty
of redundancy. The beauty of electric drive is power on demand; no need to
start motors, stalling etc... the power is there straight away. This is a
huge plus in emergency situations, and is just nice in having silent drive
for day sailing.

The downside to all this is efficiency. By running multiple systems like
this your efficiency gets down pretty low. From my point of view though, I
am hoping to run a pretty minimalist scheme of things electrically; no tv,
dvd, minimal refrigeration, minimal nav, LED nav lights, no microwave etc
etc, so the electric is mainly for propulsion. With a boat used mainly as a
sailing boat, I would hope that I can keep propulsion usage low enough that
it could be recovered by means of solar and wind generation meaning that
diesel was only there as a backup. In a cruising mentality, you may be
happy to wait another week rather than burn some diesel. I'm trying to see
the longer view here, but may be completely wrong. While some very large
capacity ships have gone to diesel over electric, most of the stuff
available is still diesel direct drive. For the most part the diesel over
electric stuff is running exotic propulsion systems anyway (not normal
props)

There are some inboard style electric motors available on ebay in australia
in the last year, i think the name is thiele? they advertise up to 44hp,
and sell controllers etc to suit. they have the necessary components to use
the prop to drive the motor as a generator also. a bit of hunting around
should get some useful information. Im far from the buying stage myself,
maybe in 10 years or so

Shaun


"Heikki" wrote in message
...
John C. wrote:

Run it as a generator and use it to charge an oversized bank of
batteries.
With an electric motor you will gain variable speed and direction without
have an engineering nightmare.

I have been speculating about a diesel-electric propulsion for a smallish
sailboat. Does anyone have links to, or experience with, small electric
motors that are suitable for continuous use - most of my googling finds
bow
thrusters and other extra machinery.

I am thinking of a fairly small engine, say 5-10 Hp, to be used mostly in
manouvering in and out of marinas, and occasionally coming home from a
calm
sea. Would it make sense to mount the whole engine on the transom-hung
rudder? That way it could turn with the rudder, and give good steering in
both directions. When not in use, it could be lifted out of the water, so
I
could use a decent size of propeller for it.

Would anyone care to shoot the idea down before I get too attached to it.
The whole project is on a dreaming level, I won't be building anything for
the next many years. But I still want to design it as if I was going to
build it some day soon...

Regards

Heikki
(Copenhagen, Denmark)