Fundamental Studies on the Low-resistance of Swimming Fish

It has been confirmed that the body-resistance of a living fish in water is far less than that of any fish-like-formed rigid-body. And according to the hydro-dynamical calculations, between the co-efficient-values of their frictional resistances, there seems to be a difference of 1/10 order. In order to clarify the process of this resistance-reduction，various resistance-tests were carried out, with the use of a lot of models, by the author, basing on the hitherto ascertained view-points, namely: ‘Passive Fish-skin Effect’， ‘Slime Effect’，’Slotting Effect derived from the Gill-cover-holes’，’Active Fish-skin Effect’ and the ‘Caudal-fin Effect’. (I) The Effects of Passive-ski, Slime and Slotting on the resistance-reduction. Having got a hint from Kraemer's Dolphin-model, the author measured the hardness of the bodies of living Scombroid and Frigate mackerel, and made models of silicone rubber having the hardness similar to those of the two fishes. Some experiments were made, in order to make a resistance-comparison between the rigid and the flexible models, at the Kagoshima-University Circulating-water-channel, within the Reynolds number: 0.5×10^5 ~ 9.0×10^5. Consequently it was confirmed that generally there was little difference between the rigidly bodied model and the flexibly bodied one. As to the effect of oil coated on the surface of the fish-model, it was ascertained that the fish-oil which was used usually, for example, a whale-oil, rather increased the resistance than otherwise, with the author’ s failure in confirming the effect of slime-‘Toms effect’ by polymers. And concerning the effects coming from the opened or the closed mouth of the fish, and from the opened or closed gill-cover of the fish, respectively; the expected slotting effect by jet-flow from gill-cover-holes was not recognized; conversely it was ascertained that the resistance was made rather larger by the effect coming from the opened mouth and the opened gill-cover. Basing on the view-point of respiratory movement of fish in sea-water, the calculated velocity of jet-flowing from the gill-cover-holes was found to be quitenegligible. Consequently, the effect of slotting due to the motion of the gill-cover holes on resistance reduction was fixed to be negative. [the rest omitted