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I am deciding how many fins to give a race of aquatic "humanoids" for maximum maneuverability in the water. It is a given that they would have tail and pectoral fins because they have very long hands and feet with webbing that serve this function, but I am wondering if they need anything beyond this. I notice that pinnipeds really just have their tail fins and pectoral fins, like the aquatic people, and pinnipeds are very maneuverable. Dolphins and fish have dorsal fins, too, and many fish have additional fins like pelvic fins and anal fins. Some fish even have two dorsal fins. I've read before that pinnipeds are more maneuverable than dolphins in the water, but perhaps this has to do more with their general body shape or size than their number of fins.

Please note their legs are completely fused, so their fins can go where there would normally be a junction between the two legs, but they are not merfolk: their body shape more closely resembles that of a dolphin, but they can retract any of their fins like sailfish and other fast-swimming fish can for the sake of speed. Also, these creatures did not come about through evolution, so that is not a limitation.

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  • $\begingroup$ Define optimal. $\endgroup$
    – sphennings
    Commented May 19 at 14:49
  • $\begingroup$ @sphennings The maximum number of fins such that adding any more fins yields only a marginal increase in maneuverability. $\endgroup$
    – user1181399
    Commented May 19 at 21:34
  • $\begingroup$ Define maneuverability. $\endgroup$
    – sphennings
    Commented May 19 at 22:03
  • $\begingroup$ @sphennings The ability to turn sharply in the desired direction within a short timeframe. $\endgroup$
    – user1181399
    Commented May 21 at 3:58
  • $\begingroup$ Under what speed regime? $\endgroup$
    – sphennings
    Commented May 21 at 4:18

2 Answers 2

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For a humanoid with two arms, one set of fused legs and a head... the optimal number of fins for maximum maneuvarability would be four:

One vertical fin above and below the head (this might count as two), one fin on each arm, and one horizontal tail-fin on the fused legs.

The arms, legs and head can all rotate and bend to pitch the fin they carry to maximal effect for maneuverability. I would also expect the spine to be more flexible than in land-dwelling humanoids.

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  • $\begingroup$ How come the vertical fins would be located above and below the head and not on the back like whales or fish? And yes, their spines are more flexible than those of humans. $\endgroup$
    – user1181399
    Commented May 19 at 21:25
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All of the fins

If your only concern is manoeuvrability, then the best option is to just keep adding fins until you can't add any more without them ending up in the same spot

Assuming a roughly humanoid/seal bodyplan with webbed hands and a 'tail-fin' for feet, a rough summary of fins would be:

  • Pectoral fins on the side of the head
  • Dorsal fin on the head
  • Pelvic fins on the throat/mandible area
  • Pectoral fins on the back like wings
  • Pectoral fins branching off of the elbows
  • Dorsal fin on the back
  • Pectoral fins on the small of the back
  • Pelvic fins on the breast
  • Pelvic fins on the belly
  • Pelvic fins on the groin
  • Side fins on the back of the pelvis
  • Side fins branching off of the knees
  • Perhaps some more

All of these fins will slow it down, make it hard to move efficiently, and generally make it difficult for them to survive; But, this design will absolutely be the most manoeuvrable a merfolk can get

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    $\begingroup$ This is the correct answer. Though I suspect that any drag these fins might cause could be offset by raw power and by refining the fins geometry. $\endgroup$
    – elemtilas
    Commented May 20 at 22:57
  • $\begingroup$ I have considered this possibility, but do you think that all of those fins are necessary, or are a only few of the fins you listed doing most of the work? $\endgroup$
    – user1181399
    Commented May 21 at 3:53

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