Diesel/Electric vs. Diesel/Hydraulic?

[GumbyTheBorg]

Hi all,

Since a lot of you are probably more knowledgeable (or know someone who is) on the subject, I thought I would bounce my idea on this forum.

I have been looking into the feasibility of a diesel/electric or diesel/hydraulic propulsion system for ships 50-180 ft.

What would be more efficient? A diesel/electric system or diesel/hydraulic system whereupon one high powered diesel powerplant would drive two podded drives (that's right, no rudder!!!).

Basic Ideas
========

- Single high powered diesel powerplant that drives an electric generator or a hydraulic pump. In either case, the electric generator or hydraulic pump would power both screws.
- Two podded drives that are electrically or hydraulically powered/actuated.
- In the case of a diesel/electric system, we would still have backup/supplementary power generators (i.e. northern lights, etc...) but when the primary powerplant is running, none of those would be required since the single powerplant should be designed to generate enough power for both the podded drives and all electrical systems aboard the ship.
- In the case of a diesel/hydraulic system, we would require the usual power generator configuration.

Single High Powered Diesel Powerplant
============================

- 8 cylinder turbo-charged engine based on the cool new Revetec design. Look it up on www.revetec.com. From what I concluded, a 8 cylinder revetec engine could generate enough torque to drive a 4000 Hp electric generator or hydraulic pump.
- Can shut off 2,4 or 6 of the cylinders when either idling or cruising at low speeds in ports/waterways with restricted speeds.
- Modified to use Coates spherical valves (I am surprised that these valves are not in every production engine). Look it up on www.coatesengine.com
- Modified to use ceramic pistons/cylinder sleeves. The Revetec engines are uniquely positioned to enable ceramic applications for ceramic pistons and cylinder sleeves.
- Although interesting, the use of ceramics is obviously not a must have since no production engine that I know of has ever had ceramic pistons and cylinder sleeves. The ultimate goals is to attempt to make an engine that requires no lubrication and no cooling. Lofty goals :-) !!!!!

Electric Generators coupled with Primary Powerplant
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High Temperature Superconducting electric generator rated at anywhere between 1000-5000 Hp (depending on how much power is needed to power both podded drives). American Superconductor (www.amsuper.com) makes these generators but I am not sure how cost effective they would be in terms of cost vs. efficiency gains. Anyone heard anything about those?

or

Hydraulic Pump coupled with Primary Powerplant
====================================
A hydraulic pump that would be capable of powering two hydraulic motors (i.e. two podded drives).

In the case of a diesel/electric system, a power buffer (i.e. battery/capacitors) would be used to store unused electric power for future use. This could be interesting since ideas such as charging batteries overnight (or when in port for an extended period of time) using a wind driven generator would allow for basically "free energy". The question would be, what is better, rechargeable batteries or large capacitors? The diesel/electric system would also benefit from the fact that, if the primary powerplant should fail, the backup/supplementary power generators (i.e. northern lights, etc...) could be used to power the podded drives and limp to port.

The big difference I see between electric and hydraulic is:
- Excess electrical power can be stored for future use. As such, the primary powerplant could be rigged to run at a set rpm (i.e. the most optimal RPM) regardless of how fast the ship is going. It also allows for the energy storage mechanism to be slowly recharged overnight or during port stays using wind power (i.e. free power).
- Hydraulic pumps/motors are probably more efficient than electric generators/motors. It would also weight a lot less and be less complicated (i.e. less maintenance/chance to fail).

Any feedback, ideas. Anyone ever dabbled with any of this.

cheers,
Luis

[cyclops]

Hydraulics do not like water. Hydraulics waste loads of power due to compression and system friction. They leak. They are large space eaters. They cost a fortune, now and later.-----------------Electric when allowed is the way.

[marshmat]

I would say electric is the way to go.

I have no experience with American Superconducter's equipment.... based on their spec sheets, though, I think you can expect a fair price premium over conventional electrics for a much more compact and efficient system.

A good electric system can be designed to have only about a 5-6% efficiency loss versus direct drive. Try getting that with hydraulics! Overall, I think for equivalent power you can expect more weight and less efficiency with the hydraulics option. Have a hydraulic system, of course, for operating hatches, rams, etc. but it's really not suitable for main power. Go electric- there's a reason that all the good podded drives are electric, and almost all the cruise ships do it this way.

[cyclops]

Certain AC classes of motors and many DC classes have a ability to operate as either a constant HP, a constant torque or as any combination of both. They can do 10 to 20 X rated powers for short periods. Docking collision avoidance. Response time is limited by design. 0 to 100% in 5-10 seconds is real on a QE 2 ship in crash mode. She could do it if asked. Tugs become less important in normal seas.

[marshmat]

A bit more on American Superconductor- their stuff isn't in full production yet, but their 5 MW motor prototype has been found to be well over 96% efficient. Not bad at all.... factor in losses in generator and controller, and you're still in the 90% range- not much different from the losses in a conventional gearbox.
Cyclops is certainly right about the versatility of many motors. Variable gap types especially can give massive torque over very short time, a handy feature in harbours etc.

[robbieroneill]

I have had a diesel-hydraulic drive in a Sailcraft catamaran. The vavle body and the H. motor are very loud and the vibration is bad. This is a very comon drive on small (-15m) canal boats in the UK and Europe. Volvo makes the drives and valve bodies. Check them out.

[gonzo]

Hydraulics usually have about 35% loss. There are high efficiency, and much more expensive systems that can attain less than 15% loss. Electric is still more efficient, lighter and cleaner. The advantages of hydraulics is that one pump can run several motors and that an overload only pops the bypass pressure valve.

[StianM]

Hydraulics produce heat instead off propulsion power.

And by the way, that revetech engine is a snake oil engine.

If you put a 3:1 reduction gear on a normal engine you will get the same result, the same hp and 3 times the torque. If you want to runn a 4000hp pump you nead 4000hp and not some strange torque stuff.

On the revetech site you find propeganda equal to Adolf Hitler and moust comunetys on the internet seam to take the bait and the hock. The formula 1 atlas forum however has slaughtered it.

Personaly I would not want a revetech even if it was dipped in honey and rolled in marzipane