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I'm currently in the process of writing a sci-fi underwater monster book, and I need to get specs for a commercially built two person submarine that can reach speeds of close to what a Russian Akula class can do- around 35 knots.

This small submersible needs to be able to fit at the back of a yacht. I've looked at the other personal submarines on the market like the Orcasub and U-Boat Worx Hipersub, but it seems they can only attain a maximum speed of 6 knots underwater. Is it possible to build a private submarine similar to these two commercial models yet can go faster? What kind of engine would you need? Will it be electric, Diesel powered or something else?

If its not electric what would be her maximum depth?

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closed as unclear what you're asking by Mołot, Frostfyre, elemtilas, JBH, JohnWDailey Oct 10 '18 at 22:01

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    $\begingroup$ The problem with small submarines is that they have small batteries, and are rather too cramped to have air independent propulsion systems (not to mention that those who can actually make subs with AIP systems won't sell them to the the public). So it makes no sense to install motors capable of pushing the sub at 35 knots underwater if doing so would deplete the battery in five minutes. And the Akulas are nuclear subs; the state-of-the-art German Type 212 subs do 20 knots tops. $\endgroup$ – AlexP Oct 10 '18 at 16:15
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    $\begingroup$ Requesting the community devise the exact specifications for a vehicle capable of achieving deep-water journeys is too much for the Stack Exchange format; months/years go into development on such things. I would suggest to devise your own specifications, though I'm curious why you think you need to define them. Are you going to be calling them out in your work? Does your audience need to know them? Is there a reason you can't say some company (fictional or not) developed the (fictional) XSub and it does A, B, and C? $\endgroup$ – Frostfyre Oct 10 '18 at 16:21
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    $\begingroup$ en.wikipedia.org/wiki/Supercavitation I believe this is relevant. $\endgroup$ – B.fox Oct 10 '18 at 16:38
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    $\begingroup$ You seem to have three criteria. Fast, small, and commercially cheap. You can have any two, but not all three, in the same design. $\endgroup$ – Justin Thyme Oct 11 '18 at 2:24
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    $\begingroup$ To voters: Please see and possibly participate in meta question about this question before voting reopen or leave closed. To @JTriptych - consider adding clarifications for things I mentioned on meta. $\endgroup$ – Mołot Oct 12 '18 at 12:04
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No

Well, yes, but it'll sprint for only a very short distance.

Speed is a function of:

  1. drag, and
  2. power.

Power is a function of:

  1. volume (how much energy+engine you can stick in the vessel), and
  2. density (how much energy is there in a liter of fuel).

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

Storage material        Energy type       Energy density (MJ/L)
Uranium (in breeder)    Nuclear fission   1,539,842,000 
Diesel                  Chemical          35.8
Lithium-ion battery     Electrochemical   0.9–2.63

As you can plainly see, batteries have a pathetically tiny energy density, and why the Major Powers have invested in nuclear power, and caused the Intermediate Powers to invest in Air-Independent Propulsion (carrying your oxygen with you). But that's really expensive.

Your minisub just doesn't have the volume to store enough usable energy to go very fast for any meaningful distance.

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Probably not much better than you stated ~6 knots.
You've constrained the size and weight to be able to fit in the back of a yacht. In that space, you need to fit in two people, enough air for two people, an engine, fuel or batteries, the structure to support all that, and a hull that has to be able to both bear water pressure and the stress of being lifted out of water by pinions--the latter is worse.

I'll add another constraint, effective submerged time at full capacity.

Your commercial two person submarines are probably close to as good as you will get. Throwing more money at the problem is likely to improve the constraints, but only marginally. The specs for the first hit on Google are on the second page of http://www.seamagine.com/assets/SEAMAGINE-OCEANPEARL-2.pdf.

Maximum Operating Depth 150m–1000m (650ft–3300ft)
Standard Maximum Speed 3 Knots
Propulsion Type Brushless DC Electric 2X 10 HP Thrusters
Mission Time: 6 Hours

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You're talking about a small submarine, so, unless your science fiction universe has Mr. Fusion, it's going to be propelled by electric motors when underwater, and it's going to rely on some sort of battery. An Akula is large enough to have a nuclear power plant, and yours isn't.

The alternative would be to have some sort of snorkel arrangement, so you could run Diesel engines. That would limit your dives to the depth of the snorkel, and you'd lose propulsion completely if you were pulled beneath snorkel depth. There isn't room on your sub for two propulsion systems. Note that, unless the snorkel is well above any waves, it's going to be quite unpleasant to use, since it has to close off if water goes over it, and the Diesels will, for a short time, pull air out of the crew compartment.

If it's science fiction, then you might be able to get away with just having an advanced sub that can do 35 knots underwater, and never ever say anything about how it manages.

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    $\begingroup$ Re there not being room for two propulsion systems, I'd think it'd be pretty simple (and cost effective in terms of reduced R&D) to simply pull the engine, motor, and battery out of a hybrid car like the Chevy Volt, and use that as your propulsion system. $\endgroup$ – jamesqf Oct 10 '18 at 17:31
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You can look at torpedoes, they often use fuel that burns without air (just like in space rockets): https://en.wikipedia.org/wiki/Torpedo#Modern_drive_systems https://www.quora.com/How-are-torpedoes-propelled-under-water-if-internal-combustion-engines-need-air-to-operate

While torpedoes do not go that far, you can have a bigger tank, or a combined approach: air-burning fuel for near-surface patrolling, oxidizer for combat dives, and maybe a small battery for transitions.

Also, torpedo fuel leaks can lead to pretty bad explosions (look up Kursk)

And do have your pilots wear air masks, so they do not have to have air inside the sub, or to carry ballast to offset the buoyancy of that air. Air for the mask comes from main air tank in the sub, and small personal air tank for emergencies.

PS. While torpedoes are military tech, your civilian protagonists can buy (older) torpedo engines from non-Western countries, or design the sub themselves. The only reason it was not done IRL is the cost and time necessary, vs. limited market for recreational subs (and industrial subs do not need that speed).

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    $\begingroup$ Now this is very interesting. I wonder if this mini sub can use a secondary hydrogen peroxide system used by old tech torpedos that can act as a "turbo boost" for a short burst of speed in a few seconds to say, escape an attack. Plausible? $\endgroup$ – JTriptych Oct 10 '18 at 18:51
  • $\begingroup$ The main problem with torpedoes is that they are all propulsion, limited payload, one-shot deals, and are mission specific. $\endgroup$ – Justin Thyme Oct 11 '18 at 2:21
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    $\begingroup$ Not to mention bad at avoiding ships :P $\endgroup$ – nzaman Oct 11 '18 at 16:01

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