# Could whale-sized creatures propel themselves through the sea by expelling water like a cephalopod?

It's incredible that 80-120 ton animals like blue whales can even swim, though water physics helps them in that regard. This got me wondering if a squids method of propelling itself through water would work when scaled up that much. I imagine that much of the creatures anatomy should be remodelled to allow for enough force in its water jet(s). Cephalopods like squids and octopuses can propel themselves by drawing water into their mantle, though the gills and spit it out through their siphon. This is great design because they breathe as they swim. I thought I could take inspiration from this and make a filter feeding giant that draws water for feeding and expels it through tubes on its back for propulsion. I could make it breathe water as well to get three birds with one stone, but I wonder if it has to be a warmblooded mammal to get that big. Anyway...

Is this method of propulsion equally or more viable than regular swimming? How much water pressure would a 80-120 ton animal need to move itself against water currents?

• In the real world, there are quite a few very large all-steel creatures which use pump-jet propulsion. Plus of course the famous, albeit fictional, Red October... Aug 30 at 11:56
• Blue whales can get a bit more massive than you state. Or did before almost allof them were killed by whalers. "The highest recorded weight for the species is 199 tonnes (196 long tons; 219 short tons).[16]" en.wikipedia.org/wiki/Blue_whale#Size Aug 30 at 18:30
• @M. A. Golding You're right, I didn't pick the highest recorded weight but the estimated average. Aug 30 at 18:45

Is this method of propulsion equally or more viable than regular swimming? How much water pressure would a 80-120 ton animal need to move itself against water currents?

Whales are about as dense as the water around them (otheerwise they would simply float or sink).

A quick search in Google gives estimates to the gulp volume of blue whales between 50,000 and 80,000 liters. That volume of water has a big fraction of the whale's own mass, since a thousand liters of seawater weights a bit over a metric ton. It would be like a 60 kg human filling in their mouth woth 40 liters of water.

Of course the blue whale can't swallow it all... They actually filter most of that out, swallowing just a thousand liters at a time. If a 120 ton whale could funnel 80,000 liters of excess water and push it backward at 10 m/s (which is quite a lot underwater), the whale would be pushed forward with a speed of 6.6 m/s. However, in order to get that much water in their mouth, the whale must first do a tail-powered burst of about 13.8 m/s.

That, and the amount of anatomical changes required to purge filtered waters backward, make it seem like the blue whale is better off using just its tail for propulsion.

• “ If a 120 ton whale could funnel 80,000 liters of excess water and push it backward at 10 m/s (which is quite a lot underwater), the whale would be pushed forward with a speed of 6.6 m/s.” That’s with regular whale anatomy right? The result might be different if 100% or slightly less of its muscle mass was allocated to its water jet. Aug 30 at 12:58
• @LiveInAmbeR that is actually an ideal scenario in which water would have no drag nor turbulence just like a vacuum, so in any real scenario, even with proper anatomy, you wouldn't get that much acceleration. Aug 30 at 17:00

Colossal squids do already use that type of propulsion.

The largest known specimen was around 500 kg, which would be less than 1% of the mass of your creature.

Considering that this sort of propulsion would obey the conservation of momentum equation

$$m_1\Delta v_1=m_2\Delta v_2$$

If your creature wants to get a $$\Delta v_1$$ of 10 $$m/s$$, it would need to have a $$m_2\Delta v_2= 120\cdot 10=1200 \ kg\cdot m/s$$.

They can achieve this for example by pushing out 10 liters of water at 120 m/s (highly unlikely) or 1200 liters of water at 1 m/s.

• It is worth noticing that the specimen referenced here had a total length of 10m, on par with some smaller whales. The giant squid is usually less massive, but also up to 30% longer, which brings it even more into whale-size territory. Aug 30 at 11:56