Part 60 (1/2)
Dee 1366 Firefly 1364 Firebrand, as altered 1295 Phito 1215 Monarch 1323 Magnet 1310 Meteor 1490 Carron 1287 Average 1381
Great Western 127 Archimedes 129
This list shows that the result in the 'Great Western,' with which s.h.i.+p I have made the comparison, is in itself a favourable one, and that compared with many others the 'Archimedes' would stand much better.
This apparent superiority of the screw over the paddle as regards the resistance offered to it by the water may at first appear startling, but there is a great mistake committed in a.s.suming that the action of the screw is a very oblique action, tending rather to drive the water laterally with a rotatory motion than to push it steadily backwards.
Having witnessed and carefully observed the degree and the nature of the disturbance in the water caused by the screw, and comparing this with the violent displacement of the water by the action of paddle-boards, even under the most favourable circ.u.mstances, I no longer feel surprised.
The ma.s.s of water pushed backwards by the action of the screw appears to be very large, spreading from the screw probably in the form of an inverted cone, but there is little or no appearance of any rotatory motion, and the surface of the water is not put into rapid motion as in the case of the paddlewheel, which may be observed to impart a considerable velocity to the water, probably for a small depth only, but over a very large s.p.a.ce.
As regards the oblique action also, a great mistake appears to have been generally made, and very naturally made, by most persons when first considering the working of the screw. It is generally a.s.sumed that the inclined plane formed by the thread of the screw strikes the particles of water at that angle and with the velocity of the revolution of the screw, but it is forgotten that the screw is moving forward with the s.h.i.+p, and therefore that the angle at which the water is struck by the plane is diminished by all that much that the s.h.i.+p with the screw advances--indeed, it is evident that if the s.h.i.+p advanced the whole amount of the pitch of the screw, the screw, oblique as it appears, and rapidly as it revolves, would not strike the water at all, but simply glide through.
The angle at which any given part of the screw does in fact strike the water is only equal to the difference between the angle to which that part of the screw is formed and the angle or direction in which it moves by the compound motion of the revolution of the screw and of the forward motion of the s.h.i.+p and screw; and, contrary to one's hastily imbibed notions of the action of the screw, this angle at which the plane of the screw is driven against the particles of water, is in such a screw as that of the 'Archimedes' very nearly equal over the greater portion of the surface, diminis.h.i.+ng to nothing at the centre; and the motion imparted to the water, although perpendicular to the plane of the screw in point of direction, is small in extent or velocity, being also nearly the same over the whole surface of the screw, except close to the centre, where it is infinitely small.
In the 'Archimedes' screw, which appears to the eye so oblique, and the centre part of which would appear to act flat against the water, only causing it to revolve, the outer circ.u.mference being 18 feet and the slip 1 foot 8 inches, the angle at which this outer edge acts upon the water is only one in 11.
The total amount of motion imparted to the water at right angles to the plane of the screw by one entire revolution even at the outer edge is not quite equal to the slip, being only 167 foot. The rotatory motion is still less, the total distance to which any particle of water is displaced laterally, or at right angles to the axis of the s.h.i.+p, by one entire revolution of the screw being at the outer edge only 069 foot, and the maximum distance being in any screw only equal to half the slip, and occurring at that part of the screw where the circ.u.mference is equal to the advance of the s.h.i.+p due to one revolution. This maximum of lateral motion is 09 foot, and takes place at 099 foot, or about 1 foot from the centre. In this mode of considering the direction at which the particles of water are acted upon by the plate of the screw I have taken no notice of the effect of the friction upon the surface of the screw, which, causing to be carried with it a film of water, will modify more or less according to the degree of smoothness of the surface the effect of the screw upon the water; and towards the centre this friction, however smooth the surface may be made, will gradually become equal to, and at last greater than, the propelling effect of that part of the screw; but this defect applies only to a very small portion of the whole area of the screw, and the absence of any very violent impulse to the water in a direction approaching to a right angle with the axis of the vessel, and which has always been a.s.sumed as an unavoidable evil in the screw, will account for the absence I have observed upon of any apparent rotatory motion.
I would not pretend, however, to advance these circ.u.mstances which I have observed, or these reasonings, as arguments whereon to found an opinion of the action of the screw, the facts as proved by the experiments are what I rely upon; but it is satisfactory to be able to account for the results by circ.u.mstances actually observed, and the reasons which suggest themselves.
The effect of a propelling surface in the form of a screw, and moving at a certain velocity, as compared with an equal surface moving at the same velocity but applied in the shape of paddle-boards, having been ascertained, it remains to determine the comparative power required to give motion to that surface.
The difficulty of determining this with any degree of accuracy from any experiments which we could make on board the 'Archimedes' was very great, but considering such results as I could obtain in conjunction with experiments which I have since made in our own works, and with the results upon steamboats recorded by others, and of those of experiments made by Colonel Beaufoy on the resistance of bodies in water, I think we may arrive at approximate conclusions sufficiently accurate for our purpose, and which may safely be relied upon.
In the case of the 'Archimedes' the engines were certainly not effective well-working engines, the proportions of the gearing or wheel-work between the engine and the screw was bad--such that the engine could not attain its proper speed--the friction of the gearing (which, whether it be a source of resistance necessarily attending the use of the screw or not, I shall consider afterwards) was very great, and the surface of the screw itself, which I had an opportunity of examining out of water, was so rough as necessarily to create very much more friction than would be caused by a tolerably smooth metallic surface. With all these sources of resistance, and under these unfavourable circ.u.mstances, the power calculated for the effective pressure on the piston and without deduction for friction or other causes, which, for the sake of distinction hereafter, I shall call the gross power, was about 145 horses, the speed of the vessel being about 8? knots per hour, as actually measured by the land, and full 9 knots as measured with great care by heaving the common log, the mids.h.i.+p section being, as before stated, 122 feet, and the lines of the vessel not so good as those of fast boats; comparing this with the gross power of the 'Great Western'
engines when propelling that vessel at the same velocity, with the advantage of better lines and the other advantages arising from greater dimensions, there does not appear any such discrepancy as to indicate any loss of power by the use of the screw in the 'Archimedes'; on the contrary, the power expended in the 'Great Western' is actually as great as that in the 'Archimedes,' as compared with their relative mids.h.i.+p sections--and if any great allowance is to be made for the circ.u.mstances which I have referred to of larger dimensions and better lines, there would appear to be actually less power expended in proportion to the dimensions and form of the 'Archimedes' than in the 'Great Western.'
The results obtained with the 'Great Western,' which as regards speed are similar to those of the 'Archimedes,' are necessarily taken from experiments made when she was rather deep, and the speed thereby reduced to 79 knots; but I have compared these with results reduced by calculations from experiments at higher speeds, and I find them agree satisfactorily--indeed, at the draft and consequent immersion of paddles when in this state, I consider the 'Great Western' as very nearly at her best as regards economy of power and effect produced. I should observe that the particular experiments from which the following calculations are deduced were made with the 'Great Western' in smooth water in the Severn. I have added also some calculations deduced from data given by Tredgold as to the performance of the 'Ruby,' a good boat with immense surface of paddle-board.
The comparison stands thus:
+--------------------------------------+---------+-----------+-----+
GREAT
WESTERN
ARCHIMEDES
RUBY
+--------------------------------------+---------+-----------+-----+
ACTUAL DIMENSIONS:
Mids.h.i.+p section
520
122
63
Area of board immersed
230
---
64
Area of a disc of diameter of screw
---
26
--
RELATIVE DIMENSIONS AND POWER:
Area of propelling surface, mids.h.i.+p
section being = 1
0442
0213