STEAM NAVIGATION. 



90. The question of auxiliary steam power to be used occa- 

 sionally, as well for commercial as for war purposes, is one of the 

 highest importance and interest, and one, moreover, which expe- 

 rience has not yet enabled us perfectly to understand and 

 elucidate. For commercial purposes the saving of fuel, when the 

 vessel has favourable winds, and the adaptation of her structure 

 to the conditions necessary for a sailing-vessel, is of the highest 

 importance ; and in naval warfare a propelling power, however 

 inadequate it may be Tor constant propulsion and the maintenance 

 of high speeds in long voyages, may nevertheless be all-sufficient 

 for conducting vessels into action or into hostile ports. 



91. It has been already stated on the authority of Mr. Bourne, 

 and as the result of experiments made on a large scale, that 

 screw-vessels intended to go head to wind and work against 

 head-seas, are not as efficient with the same consumption of fuel 

 as paddle-wheel vessels. Under the combined operation of sails 

 and steam, however, they are generally as efficient, and, when 

 deeply laden, more so. A screw-vessel being divested of paddle- 

 boxes partakes more of the character of a sailing-ship ; neverthe- 

 less, from the experiments made with the " Niger" and " Basilisk," 

 it does not appear that a screw-vessel is more efficient under sails 

 than a paddle-vessel, though such a result may naturally be 

 expected. The advantages, therefore, which attend the use of 

 screw-propelling engines as an auxiliary power, do not result from 

 any superiority of the screw as a propeller, nor from the increased 

 facility which it presents for the application of sails, but are to be 

 ascribed to the late employment in screw-vessels of wind-power 

 which costs nothing, instead of steam-power which costs much, 

 and also to the maintenance of lower rates of speed than are 

 thought necessary in paddle-wheel vessels. The screw is a less 

 cumbrous propeller than the paddle, and since it permits a much 

 higher speed of the engine, a greater engine power may be com- 

 pressed in a smaller compass. 



On the whole, therefore, the screw for all the purposes of 

 auxiliary propulsion is much to be preferred ; nevertheless it must 

 be understood that its superior eligibility is not so much due to 

 its greater efficiency, as to the greater convenience in the applica- 

 tion of auxiliary steam-power which its employment affords. 



92. The horse-power of marine engines is either nominal or real. 

 The nominal power is estimated by assuming a certain average 

 effective pressure of steam, and a certain average linear velocity 



reader is referred for a great mass of important details, for which we 

 cannot here afford space. Still further information on the same subject 

 may be found in Mr. Bourne's work " on the Screw-propeller" already 

 quoted, that gentleman being also the author of the article in Brande. 

 172 



