Comment by alright2565 | Hacker News Reader

Comment by alright2565 | Hacker News Reader

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Let me elaborate. Let's say we have largest ship in the world, a Triple E-class container ship[1]. 366m long, 49m wide, 15m draft.

Let's also assume this ship has decided they will position themselves in the most violent position possible on "rough" seas[2]. These crew member & cargo are getting thrown up and down 3 meters every 10s[3] -- think of the bridge of the Star Trek USS Enterprise.

Let's assume the weight is 30m wide, 10m tall, 30m long, and full of water. That's 900000kg. Running the math... and that gives us 2.6MW of power generated. That's assuming perfect efficiency, assuming everything is in the best case scenario (aka worst case for safety).

At the same time, the engines on the same ship are rated at 60MW total. 4% fuel savings.

Assuming the crew is not suicidal, they will try and face the ship into the waves as much as they can. I can't be bothered to calculate 366m ship and 80m wavelength, but let's say half a meter of up and down motion? Comes out to 440kW, or 0.7% of rated power.

Will this complex system ever be able to pay itself off with that kind of savings?

[1]: https://en.wikipedia.org/wiki/Triple_E-class_container_ship [2]: https://en.wikipedia.org/wiki/Sea_state [3]: https://en.wikipedia.org/wiki/Wind_wave

> Let's assume the weight is 30m wide, 10m tall, 30m long, and full of water. That's 900000kg.

I think that's missing one order of magnitude: 30m * 30m * 10m = 9,000 m^3; at a density of 1t/m^3 that's 9,000,000 kg.

you're right--thank you for catching that.

I think the idea is still sound though. there is so much loss (4m waves ≠ 3m height change; rocking motion is not up-down motion) that these wattage numbers would in practice reduced by orders of magnitude.

This is the energy machine bottleneck, what kinds of machines are net-energy positive? Probably only hydrocarbon extraction/burning, nuclear, hydroelectric dams and maybe, maybe solar PV (barely, not convinced we can continue building solar panels without hydrocarbon or another source).

Some food processing plants are net energy positive - they burn waste products and produce more then they need. But in general yes, only machines designed to produce energy, produce energy.

solar PVs are definitely producing more than put in. It takes years, as opposed to months or days for fossil fuels and nuclear, but it does pay back. look up EROI https://en.wikipedia.org/wiki/Energy_return_on_investment

And the EROI on solar is constantly improving. Every year we are using less material per cell to make more efficient cells.

Exactly the point, EROI of PV is questionable. It relies on a lot of industry (mining for example) that is/was only possible from oil/"fossil"

And the fact this system is damping the natural motion of the boat means the boat is going harder into the waves - thereby most certainly nullifying any energy savings.

> At the same time, the engines on the same ship are rated at 60MW total. 4% fuel savings.

Would a ship keep its engines constantly running at max output?

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