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BrianBM

Podded power - for small hulls?

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A general question on marine architecture.

 

Cruise ships and such seem to be pushed around by electric motors these days, with the prop in a pod underneath the hull. The diesels don't push water, they crank generators.

 

Do "power pods" (? don't know what the proper term is) offer some superior efficiency in terms of energy spent to move a hull through the water? Or, since cruise ships need vast amounts of current to move elevators and power the kitchens and provide light and heat and water and sewage for 5,000+ people on every cruise, is it just easier to design a system in which most energy is spent within the ship and a relatively modest proportion is all that's needed to push the hull around?

 

Are there any partyboats that use such a setup?

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Pod power systems are becoming more common on recreational boats, especially above 35 feet. Volvo, Mercruiser and others make such systems in single and dual configurations. I have handled such a boat with dual Volvo pods and it was remarkable in its maneuverability. Since they operate independent from any strut or rudder, they can each turn in all directions, thus allowing the boat to perform maneuvers that no ruddered boat can possibly accomplish. The 42 footer I was on didn't need a bow thruster. By the way, Mercury now has a joystick option that turns dual outboards independently to accomplish some of the same maneuverability.

 

Best regards,

 

Irv

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Interesting .... thank you.

 

The real price of diesel and gasoline isn't going to come down, at least not enough to matter, and we'll probably see increasingly complex power trains come down the size ladder, as time goes on. I would be surprised to see an electric propeller pod on a 35' sportfisherman soon, but ... at some point it'll happen.

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I'm sure you're correct about new power systems coming on line, but a cautionary note: I recall at the 1964 World's Fair in Flushing Meadow (Yes, I really am that old!) seeing a big sport fisherman with twin Chrysler turbine engines. It was to be the power choice of the future in mid-sized boats. Then gas prices began their rise, and that was that, at least in rec boats.

 

Best regards,

 

Irv

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Also, for your knowledge, MANY of the smaller ones are not electrical, as they become inefficient with low voltage power. However, on cruises, where they actually produce 3-phase power, their efficiency goes up because of the reduction in power consumption of three-phase power. In smaller boats, many of the pods are run through hydraulics. IE You have a motor that runs a hydraulic pump, and you are controling valves that change the flow of fluid through the gear on the impeller. Or at least most of the ones I saw say 10 years ago were run on.

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Chief, I think you're still correct about the hydraulics. Interestingly, the pods are engineered in such a way that they break away if they hit an immovable underwater object, thus saving the hull from being torn away.

 

Best regards,

 

Irv

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I'm pretty sure both Volvo's IPS drives as well as Mercs Zeuss drives are geared, not hydraulic.

 

Besides the maneuverability, the one big advantage is a gain in efficiency as your thrust vector is parallell to your direction of travel, where a conventional inboard's thrust is vectored downwards. Tunneled hulls try to minimize that loss of effeciency, but not completely.

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Irv, I remember the boat. The gas turbines were United Aircraft of Canada PT-6 turbines ... my father worked for UA at the time.

 

I was thinking of the electrically powered pod units, rather than geared units like those of Volvo.

 

What are the smallest hulls that currently are powered with three-phase power? Does that technology scale down to partyboat size yet? I'm still hypothesizing my dream partyboat. If it's a catamaran, then, for operating in shallow water, you'd put the pods BETWEEN the hulls, keeping that nice thrust vector. Admittedly it might limit your steering a bit...

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Tunnel drives. I can immediately see the loss of maneuverability vis-a-vis a pod, but why would they be less efficient, in thrust vector terms, then a pod? Do the tunnels induce energy-wasting turbulence?

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Couple of months back in Power Boating mag. I saw one with a joy stick. Said they were great for fighting Tuna and parking a boat?

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Tunnel drives. I can immediately see the loss of maneuverability vis-a-vis a pod, but why would they be less efficient, in thrust vector terms, then a pod? Do the tunnels induce energy-wasting turbulence?

 

They reduce the down angle in comparison to straight inboards, but the pockets do not completely eliminate down angle.

 

The joystick maneuvaribility is simply sick. Point the joystick in the direction you want the hull to move while twisting the joystick to rotate the hull how you want it at the same time....and it happens. Cheating if you ask me!

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Brian,

From the naval architecture perspective, there are three good reasons why cruise ships use electric motors in podded propulsors.

 

1. You are right on about the biggest reason. That type of ship has a high demand for electricity, known as "hotel load;" lights, air-conditioning, galley, etc. By having all the engines hooked to generators, the engineers can vary the combination of what is online to exactly match the load (propulsion load + hotel load), without wasting fuel by having more power available than needed.

 

2. The second big efficiency gain comes from eliminating drag from all the underwater appendages (no rudder, shafts, struts).

 

3. A third advantage, also mentioned above, is the maneuverability advantage of directing thrust.

 

Another cool example of electric propulsion on a big ship is the new Zumwalt class destroyer (DDG 1000). It has the excess electricity available for the radar, and weapons systems.

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Thank you, Greeneflash. Do the Zumwalts use a pod arrangement too? I didn't know that any warships did so.

 

"Hotel load," now that's an excellent turn of phrase. Do marine architects also use it for warships? The ones that come to mind immediately are LHAs and LPDs and so on that also have huge needs for electric current, which needs are probably very variable over the course of the day too.

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Brian,



Sorry for the delayed reply.  No, DDG-1000 doesn't use pods.  They do have electric main propulsion motors, but since the type of motors they wanted to use are not commercially developed into pods already, it was more cost effective to keep the motors in the hull and use shafting.  Yes, hotel load is a phase used by naval archs.  For example A/C is one of the bigger loads on a ship, just like a house.  



Have a great day.


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