hey guys
thanks for the great replies! thoise 4kw outboards sound like they would be perfect! i would want a couple though which would end up being a bit more than i would like to spend! if i cant figure out a cheaper way of doing it though i may well take that option.
james i had thought about just having the diesel revs controlling the speed of the motors.. would it be possible to have it so that at idle the motor were not turning at all? hadnt thought about the possibility of slightly different speeds would be a good setup with a single electric motor maybe..
in my experience with everything else marine related its all horribly overpriced! i have had experience marinizing diesel engines before would this be a possible option if i found a small enough diesel engine from another source? easy enough to calculate the required size of motor to use etc? 4kw of each side with about an 8kw generator was what i was thinking. is there any loss in efficiency having batteries in the system? or can they be easily bypassed during etended motoring?
as funds to build the boat will just be trickling in i will launch it first just with motors while i save up for a rig and sails etc which could take a while. so i'd like to try and sort out a nice reliable safe system to power it.
thanks for the help!!
adam.
--- In electricboats@yahoogroups.com, James Massey <jcmassey@...> wrote:
>
> At 06:25 AM 25/05/2010, adam wrote:
> >hi everyone, im new to the forum.
>
> G'day Adam, and welcome from Tasmania.
>
> >i live in new zealand and i am about to start building a 35' sailing
> >catamaran.
> ><snip>
> >
> >where i get most confused is it the many possible ways of doing
> >things.. i am on a budget and do not want to spend more $ than i
> >have to. the main reason for having electric drives is just to use a
> >single engine.. i am not worried about having a long battery range
> >or anything just decent efficiency and reliability. i would quite
> >happily just run the motors straight off a generator and start it
> >whenever i want to move.. what would i require for such a setup? is
> >ac or dc the way to go? to be honest i still cant get my head around
> >the difference!! what would be the best speed controller to use?
> >props? as you can probably tell im pretty lost..
>
> Firstly, you're in a good place for help, so just ask. Remember, the
> only stupid question is the one you were stupid enough not to ask,
> the others are all to do with learning. Yes, bucketloads of ways to
> do things, so it's cheap to kick ideas around and look for options,
> and see what you can find that fits the situation.
>
> AC vs DC, the difference is in the complexity and availability of
> parts. Most of the AC stuff readily available is either high voltage
> or a bit specialised, so DC is probably the better way to go, unless
> you are buying everything new. If buying everything new, you have the
> possibility of AC golf cart motors and controllers, which run from
> typically 48 volt DC (so a battery or DC generator).
>
> The most efficient way of generating low voltage from an engine-drive
> is with a DC generator that is already correct for the voltage. If
> you can find a suitable engine, then a permanent magnet DC motor
> being run a little faster than what its' voltage-speed is will
> generate (If the motor is permanent magnet and it says 2000RPM 48V,
> then 2200RPM will have it generating a bit over 48V).
>
> OK, the simplest ICE(diesel)/electric drive is to use large
> permanent-magnet (DC) motors, one as a generator (since if you spin a
> permanent magnet motor, it generates), two identical ones as motors.
> The generator one has to be capable of generating at least twice the
> power as the two motors use (but not necessarily twice as powerful,
> if the ones in the hulls are not going to be easy to cool, you may
> want oversized motors anyway to keep the heat down). Connect the
> motors in parallel, directly off the output of the generator.
>
> As you rev up the engine, the motor-used-as-a-generator increases
> its' output voltage proportional to speed. The two motors follow this
> speed. Reverse the polarity of the connections and you reverse the
> drive direction, so a pair of contactors or "big-as" switches (or two
> pairs if you want to be able to control the motors seperately for
> increased manoeverability.
>
> The down side of this is that they want to run at precisely the same
> speed, all the time, so you if one needs to run a fraction faster
> than the other then that becomes a bit tricky.
>
> Next option, similar setup, but run the diesel at constant speed, so
> the generator is making (for example) 48 volts. Now you add two
> golf-cart style controllers between the 48 volts and the motors,
> giving you full control of each motor individually. You can also add
> a 48 volt battery bank, and charge it from the diesel, allowing hybrid use.
>
> You may be able to start with just the diesel and three motors and
> build it up as you get the funds available and have the need.
>
> Regards
>
> [Technik] James
>
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