Steam boats have a very considerable rolling mo tion. owing to the small proportion their breadth bears to their length, and to the height of the common cen tre of gravity of the principal weights. This motion arises from a deficiency in stability, and it would be advantageous therefore to adopt that form for the body most conducive to that very desirable quality. It is also of importance to have the greatest displacement with the least direct resistance, that is, with the least area of the midship section. Supposing the area of the midship section and the breadth to he given, the condition here alluded to, is in favour of a form, full near the load water line, and lean below. In such a body also, the centre of gravity of the displacement is high, which is favourable to the stability. It more over enables the body forward and aft to be made finer than could be the case with a flat-floored mid ship section. The rising of the floor must, however, be limited by the consideration, that if the engines and other material weights are raised by it, the advan tages might be counterbalanced by the effect this would have in raising the centre of' gravity of the ves sel. There is one great advantage in the extra draught of water, resulting from the rising floor, viz. that the keel, which, by its direct opposition to the water must tend very much to diminish the rolling motion, is at a greater distance from the axis of rotation, and consequently has a proportionally greater effect. The rising floor is now generally adopted in the English steam boats.
We have already remarked in a former part of this article, that the form of the sides between wind and water has a very material effect on the rolling of the vessel, and the observation equally applies to steam boats. For this purpose, the moment of stability should increase rapidly but uniformly, and as the vessel performs its alternate oscillations, the centre of gravity of the displacement should remain in the same transverse section. The form of the body also above and below the plane of flotation, should so ac cord with the position of the centre of gravity, as to cause the different oscillations of the vessel to be per formed with the axis of rotation in the same constant plane. The elevation of the chimney, moreover, should be diminished as much as other circumstan ces will allow, in order that its weight, by raising the centre of gravity of the vessel, does not diminish in too great a degree the stability. The momentum also that the chimney acquires by its almost incessant vi brations, not only increases the rolling of the vessel, but creates also the chance of its being carried away, if the stability be not very well graduated. Not only indeed for the comfort of the passengers, and the per fect case and security of the engines, but also for the general advantage of the vessel, ought the motions and strains of a steam boat to he rendered as mode rate and uniform as possible.
In the English steam boats, the engines are so adapted as to have the axis of the paddle wheel gene rally below the surface of the deck. In the American
steamers on the contrary, it is as generally above, and even some of their boats which are destined for mer chandise have, according to M. Marestier, their en gines on the deck. The sides of those vessels being, however, in general nearly vertical for some distance both above and below the water section, it would be advantageous with regard to easiness of motion, to endeavour to adjust the different weights so that the centre of gravity of the boat should be as nearly as possible in the plane of the deck.
In the earlier steam boats it was usual to give great comparative length, in imitation it is said of the rela tive proportion of row gallies. Thus in the following table, it will be remarked, that the length of the Cler mont is to its breadth as 9.3 to 1; whereas the Con necticut, which had precisely the same length, had its breadth so increased as to present the relation of 4..2 to 1. The Clermont was constructed in 1807, and the Connecticut at a much later period. But the En terprize presents an in this particular of a still more striking kind, her length being 24.38 me tres and her breadth 8.84, the two elements present ing the ratio of 2.8 to I. The objects and destinations of these boats are without doubt very different; but it will be apparent, that in a mechanical structure like a steam boat, wherein the weights are so very une qually distributed, the length ought not to exceed the breadth in any thing like the ratio first mentioned. In steam boats intended for river navigation, the length may without much impropriety be increased, because the strains are much less considerable than in the open sea. In the construction of steamers for rivers, some attention should be paid to length on account of the space necessary for turning them—a circumstance which may sometimes be productive of inconvenience..
The report of M. Marestier is replete with numer ous and important tables, one of which we introduce, for the purpose of illustrating the relative dimensions of the length and breadth. The vessels are arrang ed according to the numerical relations of these di mensions, and not as At Marestier has given them, according to the places at which they were built.
breadth, however, being i.5.3 metre less than the same dimension of the Robert Fulton, but her length twu thirds of a metre more.
With respect to the draught of steam vessels, there is, however, no necessity for its being so considera ble as in sailing vessels, because their great length and straight of breadth in the event of their using sails, supply the place of depth, any useless de gree of which serves only to increase the resistance; neither can there be any advantage in a difference of draught of water forward and aft in boats construct ed with a rising floor; but, probably with flat floors, it may be requisite to assist the action of the water on the rudder.