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another circumstance which has long been remarked on the Waikato. When 

 a stern wheel boat enters shallow water, the engines move slowly and appear to 

 drag with difficulty ; while if a side paddle vessel of like draft of water enters 

 the same part of the river, the engines get away very much faster. The speed 

 of the vessel being in both cases somewhat lessened, in proportion to the 

 shallowness of the water. The explanation of this, no doubt, lies in the same 

 path as that of the comparative useful effect of the stern wheel above noted, 

 and also with that of the phenomenon of negative slip of screw propellers. 



In the case of screw steamers, the paradoxical result of the vessel being 

 propelled through the water faster than the advance of the propeller, has often 

 claimed serious thought ; and in all single-screw steamers the positive slip is 

 very small, as compared with paddles. Many ingenious theories have been 

 propounded in explanation, but the one now generally held to be the true one, 

 applies, I believe, equally to the explanation of the case in point. When a 

 vessel moves at some speed through the water, skin friction must necessaiily 

 impart motion to a sheet of water under and around the hull. The water in 

 contact with the hull having nearly the same velocity, and lessening in 

 proportion to its distance from it. There must then be a sheet of water having 

 a mean thickness, and an average velocity, which cannot be at once brought 

 to rest, but must follow up the vessel, and gather in a column of moving water 

 having some ratio of speed to that of the vessel. In this column of moving 

 water, the screw works, and as the velocity of this may in some instances be 

 greater than the positive slip of the screw, the result is apparent negative slip.. 

 In the case of a flat vessel like the " Rangiriri," the stern wheel moves in a 

 moving sheet of water, which has the same result of giving back part of the 

 power expended in putting it in motion ; which power is entirely lost to side 

 paddle vessels, and also, in a great extent, to twin screws, hence the great 

 observed slip of those, as compared with a single screw. The effect of shallow 

 water above referred to, is, no doubt, traceable in great part to the same 

 source. 



I have thus endeavoured to make clear the observed results of the two 

 systems of applying power in paddle vessels, but in doing so, I must not be 

 understood as advocating for river steamers, stern wheels as preferable to the 

 ordinary method, unless in very exceptional circumstances. There are 

 constructional disadvantages attending the design of such vessels, which are 

 serious, while the uncertainty of steering in shallow water, the impossibility of 

 using disconnected paddles, and thus steering without way on the vessel in 

 shallow water, make, even for a tortuous narrow river, a stern wheel vessel, 

 unadvisable. Neither must the effective results of the " Blue Nose " be taken 

 as an instance of those of side paddles, as with her general form, and small 

 immersed sectional area, the constant of efficiency ought to be nearly equal to 

 that of the " Rangiriri," which would then, with everything considered, still 

 leave a balance in favour of the latter. 



