MR. RENNIE’S SPIRAL PROPELLER. 
51 
In one experiment this vessel ran 9 miles in 35 minutes, with the tide, which 
would be equal to between 11 and 12 miles per hour in still water. 
The absence of data by which to compare the power of the engines to the 
midship section of the vessel, precludes us from forming any opinion of its merits 
as compared to that of the Archimedes. The arrangement has since been tried in 
America in a ship of large tonnage, called the ‘ Clarion,’ but of its efficacy we 
have no positive testimony. It appears to have excited much attention there, and 
unquestionably deserves notice. 
The object of the arrangement appears to be to apply most of the power at a 
distance from the axis, and avoid thereby some of the diagonal loss created by 
communicating circular motion to the water; but it will be seen in the first place 
that the diagonal loss is of small amount, and will not be so great as the direct 
resistance which the arms (forming the connexion between the cylinder and axis), 
as well as the cylinder itself and the propelling blades, have to encounter. 
mr. rennie’s spiral propeller. 
The Archimedes’ screw is a helix; that is, it consists of an inclined plane wound 
round a cylinder. When such a screw has communicated a retrograde motion 
to the water equal to its own recession, the further continuance of the thread 
will not only be useless, but will occasion a friction by its unnecessary surface. 
Mr. Rennie proposes to make the screw spiral instead of helical; that is to say, 
the thread of his propeller would be generated by winding an inclined plane 
round a logarithmic cone or spire. The object of this is to give a gradually 
increasing pitch to the thread, so that, when the water has acquired all the motion 
which the leading part can give, it will continue to receive a fresh impulse, and 
thus the thread may be continued until it becomes almost straight. 
Further, if the leading edges of the propeller be radial, it is obvious that the 
resistance of the cutting edge will be greater than if it form a diagonal line to the 
direction of its motion, and this Mr. Rennie proposes to effect by making the 
thread of an increasing radius. His propellers are represented by figs. 39, 40, 41, 
and 42, of which 39 and 40 are side elevations, and 41 and 42 end views. It will 
be seen that the radius constantly increases from the points c c, figs. 39 and 41, 
and from the centre in figs. 40 and 42, by which the direct resistance of a radial 
leading edge is avoided; the curved lines thus acting upon the water like the 
bow of a vessel, and with a similar decrease of inefficient resistance. There is one 
other feature in the arrangement which requires to be noticed: this is, that 
between the points cc, and the axis (in figs. 39 and 41), the length of the propeller 
is somewhat decreased, differing in this respect from the screw of the Archimedes,