# Fast submarines - Closed question

Fastest Underwater Speed of a 2 Person Privately Built Submersible?

The problem with this question is perhaps that it has a reality-check tag making people think they can easily answer it. In practice it's a candidate for a hard-science tag.

Movement resistance for a vessel of this type can be calculated, power required to move it at those speeds can also be calculated, as can the required energy density of the power system.

It is some fairly heavy duty naval engineering rather than a simple reality check, but it can be answered.

The problem is that OP did not give a tech level.

For calculating resistance, supercavitation makes it really, really hard. Next to impossible without actual testing. And before you say this speed is impossible for submarines: it was attempted in real world in 2009 and it still looked promising in 2016. Is tech supposed to be limited to 1986 Akula-class tech, as Akula is mentioned in the question?

2 person requirement is imprecise as we do not know how long such submarine should support them - and without tech level we have two big unknowns about required size of a life-support system. Privately built requirement suggest some limitations to availability of stuff to private citizens, but we don't know what it is.

Last line of the question asks for maximum depth, not speed, so it is unclear which one OP is really asking about.

Last but not least, it is quite possible that OP is not even building a fictional world, but only curious about limits in our world. That would make his question off-topic.

• We're talking way below the speeds where supercavitation has an effect. That can be a problem for propellors at these speeds but not the hulls themselves. Turbulent flow is as far as it goes. OP did give some examples for the size he wanted, around 6.5m x 4.5m 4000kg. We don't need to worry about tech, because the calculations come out with an energy density rather than anything else. The fact that energy density is going to be way over anything currently available is merely the answer to the question. – Separatrix Oct 12 '18 at 11:07
• @Separatrix "We're talking way below the speeds where supercavitation has an effect" - why? OP wants fastest possible, and there are torpedoes that use such effect, and there were plans to use this for a submarine: popsci.com/military-aviation-amp-space/article/2009-07/… - if there is supposed to be a hard limit, it should be in original post, edited in by original poster. – Mołot Oct 12 '18 at 11:10
• He's only actually asking reach speeds of close to what a Russian Akula class can do- around 35 knots. it's less unreasonable. – Separatrix Oct 12 '18 at 11:12
• @Separatrix if title says "fastest" and content does not want fastest, question should stay closed as unclear until this is fixed. Also, actual question in question body is "maximum depth?" which is incoherent with question title. One more reason to keep closed as unclear until clarified. – Mołot Oct 12 '18 at 11:13