ON STEAM VESSELS ADAPTED TO LONG VOYAGES. 



71 



TABLE XI. Giving the Speed and Consumption of Coals of nine of the most 

 recently constructed of Her Majesty's Steam Vessels. 



It will here be seen, that the speed both of the Medea and the Dee exceeds that obtained 

 from the smaller vessels ; which clearly shows that the advantages of increased tonnage are 

 even greater than the theory gives it : the consumption of coals per horse power is also less 

 in these vessels ; which shows that there are several practical advantages in their favour, 

 which are not embraced in the theoretical view of the case. One of which is, that the 

 diameter of the wheel increases in a greater proportion than the variation of immersion of the 

 vessel, and is consequently proportionally less buried in the water when the vessel is laden, 

 which is a cause of great loss of the power of the engine, as we shall explain more parti- 

 cularly in another part of this article. 



There is another advantage in a large engine, from its increased momentum, which causes 

 it to act as a fly wheel, and is, I am satisfied, of more importance than is generally supposed. 

 One often hears of the motion of the vessel acting as a fly wheel to the engine ; which is quite 

 an erroneous idea, as the action of a fly wheel is that of a reservoir of power, receiving it at 

 one time from the engine, and exerting it at another on the machinery to be put in motion : 

 now as the paddle wheel is always exerting a force, although a variable one, on the water, 

 it cannot possibly receive any assistance from the motion of the boat, which therefore cannot 

 act as a fly wheel to it. It certainly so far assists it, as by its velocity through the water to 

 allow the engine to make a greater number of strokes, and increases the momentum produced 

 by its weight; but this is all it does, and this effect is greatly increased by giving more 

 weight to the paddle wheels. 



The above observations are given merely to show, that the advantages of the increased 

 tonnage of steam vessels, which have been calculated theoretically, are fully borne out in 

 practice. I shall now proceed to calculate with the data, which the performances of some of our 

 sea-going vessels afford, what may be accomplished by others, whose power and tonnage 

 are better adapted to making a long voyage. 



The Medea, the largest of Her Majesty's steamers, is 835 tons' burden, and has engines of 

 inches cylinder, and 220 nominal horse power. The average speed at sea is 7-8 nautical 



