Energy requirements for robot lost at sea

Discussion in 'Boat Design' started by T X, Aug 28, 2023.

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T XJunior Member

Background
I'm writing a hard, near-future sci-fi novel wherein a robot's 20' shipping bin is jettisoned from a container ship during an unexpected severe tropical storm. The robot leaves its bin and attempts to swim to shore.

Design
In the novel, the body is streamlined like a typical submarine and is roughly 2.3 m x 0.5 m 0.8 m (l x w x h). Air-filled wheels provide neutral buoyancy and have paddle blades. The robot will also have four arms. Here's a rough diagram; a standard exterior door on the left provides context for size:

Batteries
In 2064, a battery that weighs 267 g yields 585 Wℎ of energy. The robot's body contains 3920 batteries for propulsion, which is enough to power its 2587 Wℎ motor for 8 hours when on land. The body is skinned with solar panels, which convert sunlight at ~61% efficiency. My likely incorrect math suggests the panels yield a trickle-charge of about 348.23 Wh over 8 hours of sunshine (once the storm clears).

On land, the robot can cover roughly 640 km, at 80 km/hour, before its motor batteries are empty.

Problem
The robot needs to find an electrical outlet before its batteries fully deplete. In particular, its separate "brain" battery has 336 hours (14 days) of charge. My guess is that the robot is woefully under-powered to get to land that's about 1,000 km away (30 hours at 18 knots) within 14 days, even with trickle-charging.

Question
How would you estimate the maximum distance the robot could swim?

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BlueknarrSenior Member

The ocean current will propell the vessel faster than it can swim. It only ends to swim enough to stay in favorable currents.

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portacruiseSenior Member

Since it is future science fiction, can't just about anything can be made up as to to what capabilities are possible, for batteries, propulsion speed /range, and solar power?

If you are looking for realistic calculations or actual number possibilities for the future, maybe studying the theoretical limits of these systems would provide some guidance? For example the maximum theoretical limit of solar energy on the exposed surface area can be calculated, if powered by some perfect system other than one based on current solar cells is discovered in the future.

Range and speed would be minimal as has been pointed out, but a sophisticated computer system might be able to take advantage of wind, wave, solar, and thermal variations while in the water to produce more power. Nuclear energy could do it and more, but then a plug wouldn't be needed..

Last edited: Aug 28, 2023
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T XJunior Member

Thanks for taking time to reply.

My understanding is that ocean currents are sluggish, with the Southern Pacific Gyre being slower than the Northwestern Pacific (less than 2 knots). Also, the Southern Pacific Gyre pushes in the wrong direction; the robot is headed to Ecuador from San Francisco. The fastest humans can swim at 4.6 knots, and specially designed racing pedal boats can reach 17 knots. I'm not certain, but I'd imagine the robot would travel closer to 17 knots, being more like a pedal boat than a human. Have I've misunderstood something? Does the ocean current's velocity dwarf that of the robot's propulsion?

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T XJunior Member

It's hard, near-future sci-fi, which is a sub-genre of sci-fi that precludes things like Iron Man or making up infinite energy power sources. The battery energy density, for example, is extrapolated based on trends in energy density over time. That's a big aside because the novel already has the lead-up written as to the how why and wherefore of the situation. Happy to share, but it's 200 pages to explain how the robot got to this point.

Great point. The 61% efficiency I noted comes out of Wikipedia as to the theoretical maximum, with a few percentage points docked for good measure.

Thanks! The robot doesn't come equipped with wave-, thermal-, fusion-, or fission-power. A make-shift sail would be possible, so that could provide a little more oomph. I'm not sure if it'd be enough to make a huge difference.

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BlueBell. . . _ _ _ . . . _ _ _

You can't, there are too many variables.
You can make up whatever you want.
In the ocean, truth can be stranger than fiction.

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DogCavalrySoy Soylent Green: I can't believe it's not people

You've got plenty of information available. All you really need is a real world analog for your swimming robot, and a decent chart. Your analog boat will tell you what your robot's power requirement is to swim at a given speed. The hull form would be clearly working in displacement mode, not planing, and given that it doesn't need to breathe, submarine mode where drag is much reduced.

Place your robot on the marine chart, and note ocean currents, and desired landfall. Plot a route that takes as much advantage of currents as possible. Since this is presumably a paper and pencil calculation, break the route up into discrete legs between waypoints. For example:
1st leg - 400 km with 2 knot current. Without any work, the robot will travel this distance in 108 hours. The energy demand to double that speed is pretty small below the surface. Small enough to justify using up its finite brain reserve? You won't know until all legs are calculated and the robot sets out.

Presumably you can handle the vector addition of swimming with, against, or across the current.

Do all the calculations on the page. Anyone reading a story about a robot swimming will enjoy the insight into the robot's thought processes.

As a possibility, it may realize the trip is impossible, but that it could swim into a shipping lane, and lurk at the surface until a suitable vessel comes buy that it can catch a convenient tow from.

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DogCavalrySoy Soylent Green: I can't believe it's not people

Judging from human kayak records, your robot can probably cut that time by ~60%, at an energy demand of 400 watts. At that rate your robot can go 238 days before recharging, so the only question is whether the brain can draw from propulsion batteries.

Edit:
I got impatient. Going uphill the entire way, it can swim there in six days, using a tiny fraction of inboard power.

Last edited: Aug 28, 2023
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T XJunior Member

I found a global map of ocean conditions and sketched the routes. White shows the container ship. Red shows the jettisoned robot. It looks like the robot can be carried with the currents, so presumably a small gain of 2 knots, which is better than a loss of 2 knots.

The robot has three battery packs to power the brain, accessories (arms, light, mico-drones, etc.), and motor. Each can draw from one another.

Thank you, all the way from Coquitlam.

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portacruiseSenior Member

The container ship company might want to charge a fee for the tow? Maybe the tow could also be used to recharge the robot's batteries- if it could deploy a tow behind generator while also catching a ride?

Huge number of advances are on the horizon when comparing EVs to ICs. We don't know how long the lithium battery era will last, there are other systems with possibly better potential, but I don't know if you can incorporate them in your story. Another different possibility is the use of room temperature super conductivity in Motors, SC just recently had a false start, but it may eventually get a breakthrough.

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KJL38Senior Member

TX, the dimensions you gave would give a box of 0.92 cubic meters and your battery would weigh 1.047 tonnes so it won't float. Also a box would be an inefficient hull shape and being only 2.3m long would have a maximum displacement speed of around 3.7 knots.

If you made it a submersible that was as efficient as a tuna you should have enough energy and could travel at over 40 knots, although this may pose problems on land unless the robot can change shape.

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RumarsSenior Member

Your math is off because your grasp on physics is poor, wich is unfortunate when you write hard sf.

585Wh x 3920 = 2293200Wh = 2293.2kWh = 2.293MWh. The drive motor uses 2587W = 2.587kW = 3.46hp wich gives the robot an autonomy of 886.4h = 36.9 days. The propulsion motors power isn't very realistic, the robots propulsion battery already weighs 1t, so let's say the entire robot weighs 2t. Even with best aerodynamics using 2.6kW to reach and sustain 80kmh is suspect, your robot is going to accelerate like a snail.
On the other hand if the robot has an autonomy of 8h, then the motor actually uses 286.6kW = 384.3hp wich is way to much for 80kmh, even if it's made out of depleted uranium.

One square meter of solar cells with 61% efficiency means 610Wp. Your robot has around 4 sqm of panels, so a theoretical 2.5kWp, but actual
production differs by geographical area, panel orientation, etc. A reasonable expectation in California would be maybe 7kWh per day.

As for how far the robot can swim, as mentioned its hull speed is around 3.7kn = 6.8kmh. At this speed 1000km need 147h, that's 6.1days. With the 2.5kW motor the robot has 36.9 days autonomy, six times more then needed to reach shore, even ignoring solar production. That's theory of course, because that's not enough power to go against waves and current, so with this motor the robot will happily drift across the pacific.

If it can plane the question becomes: can the thing reach 67.5kn (125kmh) with 384hp? I suggest your robot grows foils.

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Robert BieglerSenior Member

That is a rather important point.

The wheels would have to be quite large to reduce immersion to the point where the paddle blades are shoveling water mostly backwards, rather than around. Deeply immersed wheels with modest blade area are not efficient as propulsion in water, as you can see here:

If the robot is supposed to stay on the surface, inflatable sponsons could be a solution, and could also generate a more streamlined shape. If they are filled with water instead of air, that could help the robot get the streamlined shape assumed in the novel. Depending on how deep it can dive, it may be able to exploit subsurface currents that run in directions other than surface currents, analogous to how balloons can be steered by selecting the altitude that offers a suitable wind. If any cavities between components can either tolerate seawater or are filled with some less corrosive incompressible liquid, then there may not be a limit to how deep it can dive because it would not need a pressure hull.

I suppose the paddle wheel would have paddle blades that fold out from the wheels, because otherwise propulsive efficiency would be abysmal. If those blades can have an additional degree of freedom, a wheel can turn into a Voith-Schneider propeller or cyclorotor: Cyclorotor - Wikipedia https://en.wikipedia.org/wiki/Cyclorotor

The robot could be an underwater version of this:

To find out how much power is needed, look at remotely operated vehicles of similar size. I am not aware of any fundamental reason for the efficiency of Voith-Schneider propellers to differ from that of conventional propellers, but one of the naval architects here should be able to offer a far more reliable opinion than mine.

If the robot is to stay on the surface, let it turn on its side and use whichever wheel is forward when travelling through water as the propeller.

Further, if the robot stays on the surface, it could unfurl from some of its arms membranes that function as both sails and solar cells. If that increases the total solar cell area beyond what it needs to provide enough power for the engineering equivalent of basic metabolism, endurance is unlimited.

Good catch, but a shift in decimal points can happen to people who understand the concepts, and is easier to fix than not understanding the concepts at all. The people who crashed a Mars lander because they forgot to convert between metric and imperial measures certainly had a grasp on the physics, it was not checking the units that was the problem.

Last edited: Aug 29, 2023
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portacruiseSenior Member

Solar appears to be part of the story?

If it runs as a submarine, would significant solar cell generation still be possible, assuming water absorbs or blocks sunlight depending on depth ?

Is there significant solar cell generation possible if they are attached to or part of a sail, considering the mostly unfavorable overhead sun angle of incidence and shading?

Last edited: Aug 29, 2023

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fallguySenior Member

Another idea is that the robot could 'tread' water or sail. The solar panels could pop up and be the sails; unfurl in favorable winds, and also charge the batteries.

...like the Volitan

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