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I'm envisioning submarines just cruising around doing normal things, taking the role of cars, trucks, RVs. I'm trying to estimate how much energy they would need.

https://en.wikipedia.org/wiki/List_of_submarine_classes_in_service leads to a lot of page with specifics on the horsepower and size of various submarines. I'm very surprised by how much energy they use: e.g. the S-80 Plus Class carries 32 people and uses 7400kW (that's 231kW per person!)

Is submarine propulsion (or life-support, or some vital system) inherently really power-hungry? Or is that juice going to weapons-systems etc.?

Food for thought: http://ihpva.org/submersible.htm

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    $\begingroup$ iirc, propulsion is extremely power-hungry because of the sheer volume of water that needs to be displaced. Consider each cubic meter of water weighs upwards of a ton, and then combine that with how fast the thing is moving. Also, submarines are typically very massive, which gives them a lot of inertia that needs to be accelerated. $\endgroup$
    – Dragongeek
    Commented Sep 3, 2021 at 20:30
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    $\begingroup$ Well, making a submarine lighter is difficult because it needs to be about as dense as water in order to reach neutral buoyancy. Eg, a submarine that has a volume of 10 cubic meters and passenger space similar to that of a minivan (cylinder with a two meter diameter that's three meters long) needs to mass around 10 tons. Lighter and it will float, heavier and it will sink. $\endgroup$
    – Dragongeek
    Commented Sep 3, 2021 at 20:37
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    $\begingroup$ Any kind of submarine will necessarily use more power than a surface vessel with the same useful capacity travelling at the same speed. Any kind of submarine will have much less useful capacity than a surface vessel with the same displacement, because submarines must include a very large lot of equipment which surface vessels don't need, and they also need to be of much sturdier build. $\endgroup$
    – AlexP
    Commented Sep 3, 2021 at 20:38
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    $\begingroup$ We did ride horses...with chariots and sometimes carts...why not do the same thing with ehm...water fauna? A submarine pulled by water animals. $\endgroup$
    – user89947
    Commented Sep 3, 2021 at 20:49
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    $\begingroup$ amazingly great idea @Rad140 $\endgroup$
    – Humphrey
    Commented Sep 3, 2021 at 20:54

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There is no strict lower limit to energy use for propulsion: The slower your craft, the more energy efficient it is. If you want to make your sub super efficient, you just have to compromise on speed, and vice versa.

Unfortunately, speed is essential for civilian uses:

  • Passengers want to reach their destination yesterday, and they are generally willing to pay for it.

  • Goods may have a limited shelf life, and if they don't make it to their destination in time, you can deliver them directly to the disposal.

  • Even goods that do not rot are money that's not available for other things. The longer the delay between purchase of a resource and the selling of the product containing it, the lower the annual return of investment for the involved capital.

As such, civilian transportation usually makes use of fast vehicles. Predominantly for passengers and fresh goods, but even the giant container ships today are actually quite fast (30-46 km/h, according to Wikipedia). Slower ships would be much more fuel efficient, but the numbers simply won't please the shareholders enough. And that pretty much spells doom for your efficient sub transportation idea. Sorry for that.

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    $\begingroup$ Average container ship speed in 2021 was about 15 knots (28 km/h, 17 mph), up from about 14 knots (26 km/h, 16 mph) before the current global unpleasantness. (For comparison, that's about twice the average speed of a 19th century sailing clipper.) They can go faster, but at the expense of increased fuel consumption: which means that they will go faster only when customers will be willing to pay significantly higher transport fees. $\endgroup$
    – AlexP
    Commented Sep 3, 2021 at 22:06
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    $\begingroup$ ... And speed is important for the ship owners too. A ship going twice as fast will complete twice as many voyages and earn twice as much income. $\endgroup$
    – AlexP
    Commented Sep 3, 2021 at 22:13
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    $\begingroup$ At the extreme low end of the speed spectrum, you also run into problems when you aren't faster than the surrounding water. A current that slows you down is annoying, but one that actually overwhelms your propulsion means you have very little control over where you're going. $\endgroup$
    – Cadence
    Commented Sep 3, 2021 at 23:11
  • $\begingroup$ @Cadence Indeed, that's the weak spot in my argumentation. And I fully agree that this imposes a practical minimum speed. It's the reason why I added the word ¨strict" to "no strict lower limit": All the reasons for speed that I gave impose some lower limit on the speed below which the ship simply won't earn any money anymore. $\endgroup$ Commented Sep 4, 2021 at 6:47
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The comments already explain that moving on the surface at similar speed will always require less energy. But if not so much extra energy is required and there are other problems to solve it might become plausible.

A strong and constant stream is running along the coast. A couple of islands narrow and long break the stream in such a way that the current turns around them, so you have currents running in opposite directions on the two sides. A city spans over the islands and the land on the coast. The city is an important sea port and the waters around it are really crowded. Some citizens desperately looking for extra space for their mobility started to travel in mini submarines that use the currents as one way lanes.

Odd point to solve. Usually in such places water is dirty and visibility is poor.

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Although it WOULD take more eneegy than a surface or land vehicle, it should be noted that water is not always still.

Take a look at the water during a storm, at how its waves travel and disperse

Take a look at things like rivers and streams, and constant currents in the open waters

One is predictable, one is not. One will fot well with your idea, meaning that whoever uses it will place great value in these conditions, meaning they will pay more to obtain it.

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Not Only Possible, but Been There Done That Got the Tee Shirt!

Personal submarines are already a thing, and they come in an astounding array of sizes, configurations and utility platforms. There are already "partial submarine" underwater scooters, there are serious diving / exploration craft, there are road vehicle / open top submersible combine vehicles, there are ultra cushy submarine yachts, there are aquatic fauna mimicking diving craft and even aquaplanes.

Take a look at this informational video! I'll let you look up the specs yourself!

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    $\begingroup$ Do or don't - what happens with that info when this link will rot in few years $\endgroup$
    – MolbOrg
    Commented Sep 4, 2021 at 7:45
  • $\begingroup$ I'm not sure there are any luxury submarine yachts. I can find concept drawings, but no evidence of them being real. $\endgroup$
    – Humphrey
    Commented Sep 4, 2021 at 10:16
  • $\begingroup$ @Humphrey -- Could be. I'm sure the price tag is prohibitive. In any event you asked specifically about the possibility of submersible "cars, trucks and rvs" being possible. The superyacht excepted, all the other vehicles appear to be real. $\endgroup$
    – elemtilas
    Commented Sep 4, 2021 at 13:55
  • $\begingroup$ The question was about 'low-energy submarines', your response was to say 'personal submarines' exist $\endgroup$
    – wokopa
    Commented Feb 3, 2023 at 12:17
  • $\begingroup$ @wokopa --- Yep. This is what happens when a querent doesn't (appear to) do much initial research. If your complaint is that I didn't specifically say "low energy subs", the OP clarifies that she is "envisioning submarines just cruising around doing normal things, taking the role of cars, trucks, RVs." $\endgroup$
    – elemtilas
    Commented Feb 4, 2023 at 20:42

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