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Comment by lmilcin | Hacker News Reader

Hi. Skipper here (nothing of the size of the ship in question, obviously:).

Ships of ANY size are hard or impossible to maneuver when they are going slow and have a blackout. The size doesn't matter.

The control surfaces only work when they move with regards to water.

The ship that does not move is uncontrollable with rudder and it needs to use something else to help it maneuver. This something else can be a tugboat that rotates the ship by the force it can generate, or it can be thrusters. Thrusters aren't propulsion method and they are relatively small and can counter only so much wind.

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I was referring to the incident in Hamburg, where there was (according to the article) no blackout, the ship had two local pilots on board and a tugboat attached.

And yet, they were unable to prevent the wind from pushing them into a moored ferry they were passing.

Isn't this why new ships are starting to have multiple 360 degree electrical drivers instead of one big propeller? I think they are called iso-pods.

You mean this? https://en.wikipedia.org/wiki/Azipod

I think the biggest issue is complexity and resulting unreliability. On a ship that has to be in constant use for decades you want things that are simple and reliable.

"The latest design, the Azipod X, incorporates these improvements, with a view to a service interval of five years, and features bearings that can be taken apart and repaired from inside the pod while the ship is harbored normally"

Yeah, that pretty much sums it up. They are working on it:)

Also an important factor on large vessel is fuel efficiency. You don't want anything sticking out unless absolutely necessary, so these would have to be meant for propulsion. But because of complex construction I can expect they are less efficient than just straight through axle and a huge propeller on it.

I expect things like this to be useful on utility vessels of small to medium size where you don't necessarily need so much efficiency but the utility comes from being able to maneuver quickly and in various conditions.

Harbor tugs are more commonly equipped with these: https://en.wikipedia.org/wiki/Voith_Schneider_Propeller

They're quite efficient (although maybe only for lower speeds) and provide extreme maneuverability.

They usually isn't used as the primary means of propulsion on most bigger ships. It's also not uncommon for them to be retractable. If you need a large ship at a very specific position for a week, then you definetly need it, and that is a common use case as it allows for impressively accurate control. Heck, you could avoid moorings altogether and just keep it in place without it if you wanted to.

Can I also ask if it’s true that such a beast like the ever given would have as little as 30% of back maneuver power compare to forward sail?

I have absolutely no first hand knowledge of these beasts, but some common sense here below:

The engine can most likely work both directions equally.

The propeller is optimized to work in forward direction, so it will have worse efficiency going in the other direction.

The rotation would have to be limited when going in reverse because of cavitation (and maybe other structural limitations). That again is a result of the wrong shape of the propeller (when in wrong direction).

The hull will have significantly more drag when in wrong direction.

Now, engine power is defined as whatever it can put out and if the engine works the same way in both directions then power is the same also.

So you can think this way: most likely it has the same power as going forward but it can't use it and whatever it can use will be much less efficiently translated into motion.

For ships that have turbo-electric drive trains rather than big shafts, I wonder if it would be possible to vector the propellers somewhat for better maneuverability, the same way that rockets use gimballing engines? Probably not worth the maintenance though, with sealant and salt water considerations.

This is common on cruise ships. They spend so much of their time going in and out of port the extra manoeuvrability is worth it in requiring fewer tugs.

https://en.wikipedia.org/wiki/Azimuth_thruster

Anything else than straight shaft with a propeller on it is a loss of efficiency and reliability.

When the fuel efficiency and reliability drive your competitiveness that is a huge issue.

Something that people perhaps don't appreciate is that typical large ships like this don't even have a gearbox (which would be perhaps 95% efficient).

They use large two stroke diesel engines which can be stopped and started in reverse. They use an extra valve in the engine head to admit air for starting and have valve gear which controls the direction.

Cruise ships on the other hand have electrical house loads which are almost the same as their propulsion loads. In addition to the manoeuvrability advantages this makes diesel electric drive advantageous.

Megaship engines are so fascinating. Over 100k horse power!

https://www.youtube.com/watch?v=gXeBQrio4Uo&t=680s

I like to think of it as: 5.6 million lb-ft of torque at ~100 rpm means if you could get it up to about 5250 rpm, you'd have 5.6 million hp.

It's not uncommon for ships to have thrusters that can move or rotate the ship in any directions. When doing underwater operations the ship needs to stay in place, and eg. GPS and/or triangulation from land or oil rigs is used to make sure it holds the correct position, even in quite strong winds.

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