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osirissail · 09-10-2009, 08:50

I can only conjecture that as with electric motors that I commonly use, the start up current is a large spike then it settles back down to a steady state current. That is why some motors have the "capacitors" attached to them - to provide that large spike of current to get the thing rotating. It also might be that the conversion from AC to DC is not "pure" but contains ripples in the wave form. Batteries provide zero ripple steady voltage. In this case of using electric motors in a boat, the batteries may also be used as a "smoothing" factor to supply the power to the motors besides being used as a source of transient high current to get the motor going. Without the batteries in line it might just be that starting, shifting -fwd/reverse - could overload the genset.
- - I wonder how the giant cruise ships handle their electrics. The new ships have very large electric motors in pods supplied by their generator plants onboard. The old 3 foot thick prop shafts between the motors and propellers are gone. And some of the pods installations are steerable eliminating also the need for rudders.

Hydra · 09-10-2009, 13:17

All-electric ships continuously monitor the power available from the gensets and prevent the electric motors from drawing too much and overloading the generators. Then, the maximum possible speed depends on the number of gensets running at the moment.

Almost all pods are steerable. Some very fast ships such as Queen Mary II have 4 pods, only 2 of them steerable. Steering with pods results in impressive manoeuverability as long as the propellers are turning. When coasting (just before anchoring, for example), you lose steering ability, just like with outboard engines.

Alain

osirissail · 09-10-2009, 13:27

I wouldn't see where they would lose any steering ability as two fixed pods would be put into reverse to slow the boat and the other two pods used for steering. We do the same thing in multi-jet engine aircraft. They could do the same even with only two pods if they were steerable. Plus they all have massive bow thrusters so they can actually end up pushing the ship sideways into the dock. I have seen them do it and it is impressive to see a mega-ton ship stop parallel to the wharf and then move sideways to the wharf.

Hydra · 09-10-2009, 13:40

Osirissail, I agree with you: with reversing the thrust on pods, it's easy to steer even at zero speed. With a bow thruster, it's easier again. I just mentioned that they have to keep the propellers turning. I have seen a young officer miserably fail an approach for anchoring because he forgot: he tried to operate the same way as with rudders and propellers on straight shafts.

Alain

LtBrett · 09-10-2009, 13:46

I have driven a ship with pods and a controllable pitch screw. You can indeed balance the thrust to hold the vessel perfectly still or crab in any direction.

Brett

LtBrett · 09-10-2009, 14:18

To answer Osiris's original question, large ships use synchronous motors where the speed of the motor is adjusted by varying the frequency of the electricity fed in. They are also likely to be three phase. There is no huge surge in current at the start like on single phase induction motors.

Brett

fastcat435 · 10-10-2009, 02:39

Quote:

Originally Posted by osirissail

Hypdrv -
- - - - - -
- - Fastcat435 -One concept question - Does your system allow the motors to operate directly from the Genset? Or does the genset power a battery charger that supplies the batteries that feed the motors? In other words can the batteries be switched out of the system and the motors driven solely by the genset?

'

hallo Orissail
Our system is designed so that you can drive your motors direct from the batteries or from the generator, any energy not used by the motors will flow into the batteries, if the batteries are full and the genset is switched to automatic, the gen set will cycle on and of when needed.
The minimum point where the gen sets kicks in is 40 % battery power and it switches off again at 90 % power. the switching points can be changed. besides charging the propulsion batteries the house units are also charged if needed with a separate 3 kw alternator on the gen set.

Gideon

creakyboy · 10-10-2009, 03:10

May be a bit off topic but has anyone with a small cruising boat which would normally only have about a 8 hp out board tried those small electric outboards that are marketed as 'trolling motors' aimed at recreational fisherman in their tinnies?

**GordMay** · 10-10-2009, 03:34

Quote:

Originally Posted by creakyboy

May be a bit off topic but has anyone with a small cruising boat which would normally only have about a 8 hp out board tried those small electric outboards that are marketed as 'trolling motors' aimed at recreational fisherman in their tinnies?

Yes.

See the discussion (& my references) at:
http://www.cruisersforum.com/forums/...eed-21946.html

creakyboy · 10-10-2009, 03:50

cheers Gord

osirissail · 10-10-2009, 05:52

Fastcat 435 - - Let me modify my question a little - If due to collision or any other reason the drive battery bank had to be physically disconnected from the motors so that the only electrical connection to the motors came from the genset - would the motors operate? Or are the batteries needed for power dampening or whatever and must always be in parallel with the genset or in series between the genset and the motors?
- - One other question raised by the large cruise ship electrics - why did you choose DC electric motors over AC motors?
Thanks, Jim

fastcat435 · 10-10-2009, 06:01

Hallo Jim we use AC motors and yes we can run the motors directly of the generator.
gideon

osirissail · 10-10-2009, 07:38

Ah! One more real plus in my opinion - The "other" boats use a DC motor system which means batteries must be on-line/available for the motors to operate. So far, half the boat weight and AC motors to boot. Getting better and better.
- How do you reverse the propeller direction using AC motors? Or can the whole motor pod rotate 180 degrees for "reverse thrust?"

ctsbillc · 10-10-2009, 08:01

I had the pleasure of a tour of a rather nice electrically driven boat a while back, A submarine.

The whole of the bottom was batteries, 600V of lead-acids. Drive motor was huge.

Electric drive is the future, it will come, to boats as well as cars.

NiMH and Lithium batteries take space, but are relatively light. Charging can be by the props when under sail, in addition to solar and wind. Taking power from the props cuts your speed down a bit of course, but it is free - when you have paid the high price of installing all the motors and batteries. An auxilliary engine to charge the bats is probably part of the equasion as well - It is just a generator.

Cost is the thing. And be careful about maintenance, a 300V battery pack in a marine environment is not a thing to be messing with,