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This challenges the validity of the standard use of the large reservoir approximation in relating jet velocity and intramantle pressure in the Bernoulli equation. The simplification assumes that flow velocity in the mantle is negligible compared to jet flow velocity.

Volume flow and jet aperture data were used to determine the time varying jet velocity over entire jet events. Jet velocity was used to calculate propulsive efficiencies of approximately 65%. As a result of this detailed investigation, the use of Froude efficiency to calculate propulsive efficiency in squid has been called into question and the equation for rocket motors is proposed as the appropriate equation. Jet velocity, jet aperture and volume flow were, also used to calculate intramantle pressures and thrust forces using the unsteady forms of the Bernoulli and momentum equations respectively. These results were compared to the same data treated by the quasi-steady analysis commonly employed in the study of squid locomotion. The comparison strongly suggests that unsteady effects may have a significant impact on functional morphology and should not be ignored. Peak jet thrust calculated by unsteady hydrodynamics corresponded better to peak squid acceleration than did quasi-steady peak thrust. Predicted negative thrust forces due to unsteady effects corresponded to decelerations observed to occur while the jet was still ejecting fluid.

In summary, the standard methods used (1) to calculate propulsive efficiency and (2) to link jet velocity, jet thrust and intramantle pressure in a simplified and quasi-steady sense in squid locomotion are deemed unsatisfactory. They may also prevent a proper understanding of functional morphology in these and other jet-propelled organisms.

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