Space passed through by screw due to the number of revolutions, 65,685 feet.
The average rate of vessel being to that of screw therefore as 1 to 1·2913.
In the performances of the ‘Great Western,’ upon an average of 20 voyages the ratio has been as 1 to 1·2997; but, separating from these 20 such voyages as were unusually short or long, and taking only such as, occupying 14, 15, or 16 days, may be considered as giving a fair average of the speed of the ship when not adversely affected by the wind or heavy seas, the average of these 13 voyages give 1 to 1·283; and leaving out again those of 16 days, and taking only 8 voyages of 14 and 15 days, the average gives a ratio of 1 to 1·27187.
| Of these, 5 voyages of 15 days give 1 to 1·29077, |
| and 3 " 14 " 1 to 1·23901. |
The last three, however, were short passages and homeward, when the currents and winds have been in favour, and consequently we may safely say that the ratio must be above 1 to 1·239; and after making every allowance for the effect of swell and other impediments (the experiments upon the ‘Archimedes’ being made in smooth water), the average of the 8 (5 of which were homeward voyages with favourable current and wind and the vessel in good trim), giving a ratio of 1 to 1·27, may be taken as a fair average.
The comparison between the ‘Archimedes’ and the ‘Great Western’ will therefore stand thus—
| Area of Propelling Surface, the Midship Section being 1·0. | Difference of Speed of Vessel and Propelling Surface, or amount of Slip, the ratio of Vessel being 1·0. | |
| ‘Archimedes,’ screw | 0·203 | 0·2913 |
| ‘Great Western,’ paddle | 0·391 | 0·2708 |
Showing an amount of slip in the ‘Great Western’ very nearly equal to that of the ‘Archimedes,’ while the ratio of the propelling surface to the midship section in the case of the screw is little more than half that of the paddle-boards in the ‘Great Western.’
In taking the average of the eight voyages of the ‘Great Western’ with favourable winds as I have done, I believe I have made full allowance for the different circumstances of smooth water and sea; but there is ample room in the above comparison to make even greater allowance for these circumstances, and still to leave a result which would prove that with similar areas the screw would meet with at least equal, if not a greater resistance, and consequently will slip as little or less than the ordinary paddle-board.
I subjoin a table also, taken from a well-known work on the steam-engine (Tredgold’s), of the slip of a number of vessels, of which in every case the surface of paddle immersed is far greater in proportion to the midship section than that of the screw in the ‘Archimedes.’