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I am trying to design the fastest pelagic fish that is mechanically possible. The fastest real life fish can swim up to 80 mph. What body design would allow a fish to swim significantly faster than 80 mph? How large will it be? What kind of fish? What would the fins look like?

I would like the fish to be able to swim as fast as mechanically possible, preferably at least 150 mph.

IT CAN NOT BE A SUPERCAVITATING MISSILE WITH A FISH PAINTED ON IT.

All evolutionary and metabolic constraints will be taken care of.

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    $\begingroup$ For reference, it takes a supercavitating torpedo to go 230mph and these are basically flying through an air bubble in the water. That works out to be a pressure of 991psi for a bullet shaped object an inch in diameter. So it would almost certainly need a nose even sharper and narrower than a swordfish's head or an airfoil-like head. And if it hits something, both things are obliterated. Then there is debris in the water. Not sure how it would navigate since it would need to see so far ahead to steer, if it could even steer at all. $\endgroup$
    – DKNguyen
    Commented Jul 18, 2020 at 2:22
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    $\begingroup$ The current top seed for fish is a hard limit from nature. Tuna and blue merlin get seriously hurt when they reach that speed, and cavitation prevents them from going faster. $\endgroup$
    – The Square-Cube Law
    Commented Jul 18, 2020 at 2:28
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    $\begingroup$ The fastest non-cavitating torpedo is 86mph. $\endgroup$
    – DKNguyen
    Commented Jul 18, 2020 at 2:56
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    $\begingroup$ Nature is pretty efficient if they design something. Engineers are often looking at nature and not the other way around. The fish you've seen are probably already nearing the max in speed. I think all you can do is make them stronger in scales and muscles near magically (carbon nanotubing maybe?) and present that as the fastest. $\endgroup$
    – Trioxidane
    Commented Jul 18, 2020 at 5:17
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    $\begingroup$ This question as it now is ill phrased. If you remove all evolutionary and metabolic constraints then you are left with the only hard limit on velocity in nature: c. $\endgroup$
    – L.Dutch ♦
    Commented Jul 18, 2020 at 5:47

1 Answer 1

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Supercavitating Missile with Fish Painted on the Side.

enter image description here

Body design: Sleek metal cylinder with solid fuel rocket engine at the back and gas pump at the front. The pump ejects a bubble of gas through which the fish travels. This prevents cavitation damage.

Fins: Two stabilising fins at the back.

Top Speed: 230 mph

All evolutionary and metabolic constraints: Taken care of

Sarcasm aside, it's not obvious what type of answer you are looking for. There are several comments that point out how objects moving quickly though water suffer cavitation damage:

enter image description here

Cavitation is when the fluid gets churned up so small low-pressure bubbles are formed. The bubbles then collapse and damage whatever is nearby. There is a well known predator that hunts this way. See also the sexy version.

If cavitation can make holes in a sleek metal propeller imagine what it can do to a living animal!

The question is tagged "hard science" so these comments are entirely appropriate. Ignoring problems like this makes the question no longer "hard science" and it's unclear where you draw the line for ignoring problems. In my answer I have ignored the problem "is actually a missile and not a fish".

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  • $\begingroup$ Isn't it weird how the speed of sound acts as a speed barrier for aircraft. So you might assume that since water has a much faster speed of sound, you could go much faster before encountering such a barrier. But I guess due to the incompressible nature of water, cavitation pops up which limits your speed to be even less than what it was in air. Of course, that's ignoring the greater forces involved due to the higher density which is yet another limitation. $\endgroup$
    – DKNguyen
    Commented Jul 18, 2020 at 17:34
  • $\begingroup$ @DKNguyen I think there's some extra condition in there, like "speed caused by pressure differences cannot exceed the speed of sound". I remember that fact from a question about a monster throwing parts of itself into space. Does that ring any bells to you? $\endgroup$
    – Daron
    Commented Jul 18, 2020 at 18:01
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    $\begingroup$ My completely unsupported, handwavey, reason why cavitation exists in water but not air is that water can evaporate while air cannot. So if you're talking about that gaseous water that might be where the speed of sound comes into play. But I'm not a mechanical engineer. I don't know about the "speed caused by pressure differences" thing. Kind of reminds me of the rocket equation when I hear that. $\endgroup$
    – DKNguyen
    Commented Jul 18, 2020 at 18:04
  • $\begingroup$ Sounds like a good reason for why air cavitation doesn't happen. The pressure differences of course does happen because that's how aircraft work, but they don't produce tiny little pressure difference bubbles. $\endgroup$
    – Daron
    Commented Jul 22, 2020 at 14:37
  • $\begingroup$ @DKNguyen I mean there are certainly supersonic aircraft so the speed of sound isn't a hard barrier. Though it might be a hard barrier for travel without XX type of engine. $\endgroup$
    – Daron
    Commented Jul 22, 2020 at 14:38

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