Naval architect here. As other commenters have noted the size of the pumps isn’t really the issue here - you can have as many as you want it really depends on the time you have available.

For me, the cool things about this are the size of the installed power and the stability problem in having one submerged object with multiple half empty ballast tanks lift up another object with multiple tanks.

The BOKA Vanguard has an installed power of 27000kW, split between two main diesels and two auxiliaries. This is slightly more than a Los Angeles class nuclear submarine, which goes much faster (perhaps 2x according to wiki) underwater - very much a different resistance problem. There are two 35 tonne and 65 tonne generators/engines. These are not small pieces of equipment. That’s what gets me about these ships - they are on another level of huge.

The stability problem is a significant challenge. You can see in the video that they did it on a flat day. Being able to coordinate the pumping of multiple ballast tanks to make sure that both ships remain upright is another cool thing here. Look up the “free surface effect” if you want to learn about that particular challenge.

Sources; various Wikipedia pages and Wärtsilä technical sheets.

> “free surface effect”

I remember this from the sinking of MS Herald of Free Enterprise, a ferry which set off with its doors open: https://en.wikipedia.org/wiki/MS_Herald_of_Free_Enterprise

A fairly small amount of water washing about the car deck was enough to capsize the ship.

Is this something that's similar to what you're describing? https://www.youtube.com/watch?v=7np9rhQmNTU

It's something a little bit bigger, but it gives you a good idea of what's used to propel ships.

Why do marine engines weigh so much for that power? An airplane uses ~100MW (depending on how you want to calculate it) but obviously its engines don't have a combined weight of 200 tonne.

Aircraft engines are designed to be light, and sacrifice low cost and the ability to burn cheap diesel fuel for that.

Ship engines are the way round: their weight doesn't matter much, but it matters that they use the cheapest fuel possible.

Exactly. In fact, for some applications GE are using modified aircraft turbines on ships: https://fas.org/man/dod-101/sys/ship/eng/lm2500.htm

Those are for military use, where speed matters but money is basically unlimited.

Money is unlimited, but fuel efficiency isn't just about money for the military. More fuel usage means more supply lines. That's less of an issue for navies than armies but still important.

I see your point but aircraft engines can burn diesel, and many small power plants and backup power stations use aircraft engines in stationary services with diesel fuel.

> Aircraft engines are designed to be light, and sacrifice low cost and the ability to burn cheap diesel fuel for that.

Small correction: aircraft engines generally use kerosene-based fuels (Jet A-1, Jet B). Also cost-wise, aircraft-grade kerosene costs more relative to other uses of kerosene since those applications can tolerate kerosene of lower quality than aircraft-grade kero.

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

That's what I meant by "sacrifice ... the ability to burn cheap diesel fuel". Probably worded ambiguously :(

Your sentence was fine, and I don't know if anyone thinks jet engines burn cheap diesel fuel, do they?

The entirety of the drive train is huge. You have to turn two propellers which have to propel a ship weighing thousands of tonnes. To produce the necessary torque you need to have some big components. The supporting infrastructure you need to heat and circulate the fuel, then to supply it to the engines is another thing too. Gas turbines are much smaller but would be unsuitable for this particular application as they have good efficiency only at high loads. The range of loads this is used for would mean they are unsuitable.

A number of very large cruise ships now have the drive entirely decoupled from the fuel burning engines. Same idea as a diesel electric locomotive. Big engine burns heavy fuel oil, turns generator, power goes to electric azipods.

Smaller new vessels do this too.

Interesting! Thanks. The stability problem does seem extreme, which is what I assumed all the tugs are around for. One of the other thoughts I had was ... what possesses someone to build something like this? It obviously has more uses than dealing with disabled cruise ships, but not many more right? What’s the ROI on something like this?

These are also often used to reposition or completely move entire oil rigs, which I think is probably their biggest use case.

Oil and gas.

As I recall, the captain of the docking ship (the smaller ship) relinquishes control when the bow crosses the dry dock's threshold. Does that apply for these open ocean vessels as well?

I think it's going to be as soon as you lose your ability to maneuver - i.e. as soon as you moor to another vessel in this case.

The pumps are probably pretty large just so it doesn’t take that long. It’s more of a large volume of water problem than a push it forcefully problem if that makes any sense. There’s something called hydraulic head or pump head that determines how high a pump can pump. It’s sort of a trade off between height it can pump and flow rate. The pumps used are more than likely centrifugal pumps rather than positive displacement. As an aside on pumps, on the ship I work on, we have two fire pumps that are run off 11kV motors, they have to pump a large volume of water potentially pretty high for a deluge/sprinkler system. Our normal ballast pumps are 480v and when they come on the lights flicker but we don’t do much ballasting of the same type as the heavy lifts, it’s more for trim and heel. There are also considerations that a partially filled tank can lead to instability (free surface effect) and they have a definite sequence that they must pump out the tanks to prevent excess stress on both the vessels (on ships the bending is called hogging and sagging). Some ships if they are loaded/ballasted in a certain configuration can actually potentially break in half.

If time is no object, you could do it yourself with a straw. The pumps only needs to be powerful to drain the ballast tanks quickly.

Not strong at all theoretically speaking. All you have to do is have enough headroom to pump higher than water level for a given diameter of pipe and have an air intake to match which is higher than water level. You could take forever with a strong pond pump and achieve the same effect.

I designed full-building hydroponics systems. You get similar issues with things like vacuum pressure in tanks causing your pumps to not work properly.

Keep in mind that those tanks aren't under vacuum or have any special physics at play - you could have a guy sitting in the tank bailing them out with a wooden bucket.

He would have to run up a lot of stairs though. But with prefect absence of leakage (a precondition for the bucket-weilder's success was well), proper rain protection and good ventilation it might be just as fast for all practical considerations (not fast at all) to just stand back and allow evaporation to do the job.

What I find even more impressive than the engineering dimension is that a market for those things exist, and how easily the the basic financial craftsmanship of playing through price/demand curves and ROI apparently scales. Because the business side looks so amazingly calm from the outside, entirely not like some Isambard Kingdom Brunel cable laying adventure or a 21st century version thereof (seriously, make a movie about that guy and get Musk to play him)