571. Q.--Whether do you consider paddle or screw vessels to be on the whole the most advantageous?
A.--That is a large question, and can only receive a qualified answer. In some cases the use of paddles is indispensable, as, for example, in the case of river vessels of a limited draught of water, where it would not be possible to get sufficient depth below the water surface to enable a screw of a proper diameter to be got in.
572. Q.--But how does the matter stand in the case of ocean vessels?
A.--In the case of ocean vessels, it is found that paddle vessels fitted with the ordinary radial wheels, and screw vessels fitted with the ordinary screw, are about equally efficient in calms and in fair or beam winds with light and medium immersions. If the vessels are loaded deeply, however, as vessels starting on a long voyage and carrying much coal must almost necessarily be, then the screw has an advantage, since the screw acts in its best manner when deeply immersed, and the paddles in their worst. When a screw and paddle vessel, however, of the same model and power are set to encounter head winds, the paddle vessel it is found has in all cases an advantage, not in speed, but in economy of fuel. For whereas in a paddle vessel, when her progress is resisted, the speed of the engine diminishes nearly in the proportion of the diminished speed of ship, it happens that in a screw vessel this is not so,--at least to an equal extent,--but the engines work with nearly the same rate of speed as if no increase of resistance had been encountered by the ship. It follows from this circumstance, that whereas in paddle vessels the consumption of steam, and therefore of fuel, per hour is materially diminished when head winds occur, in screw vessels a similar diminution in the consumption of steam and fuel does not take place.
573. Q.--But perhaps under such circumstances the speed of the screw vessel will be the greater of the two?
A.--No; the speed of the two vessels will be the same, unless the strength of the head wind be so great as to bring the vessels nearly to a state of rest, and on that supposition the screw vessel will have the advantage. Such cases occur very rarely in practice; and in the case of the ordinary resistances imposed by head winds, the speed of the screw and paddle vessel will be the same, but the screw vessel will consume most coals.
574. Q.--What is the cause of this peculiarity?
A.--The cause is, that when the screw is so proportioned in its length as to be most suitable for propelling vessels in calms, it is too short to be suitable for propelling vessels which encounter a very heavy resistance. It follows, therefore, that if it is prevented from pursuing its spiral course in the water, it will displace the water to a certain extent laterally, in the manner it does if the engine be set on when the vessel is at anchor; and a part of the engine power is thus wasted in producing a useless disturbance of the water, which in paddle vessels is not expended at all.
575. Q.--If a screw and paddle vessel of the same mould and power be tied stern to stern, will not the screw vessel preponderate and tow the paddle vessel astern against the whole force of her engines?
A.--Yes, that will be so.