1842.J 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



405 



NOTES ON STEAM NAVIGATION. 



Paddle floats are usually either of fir 3 in. thick or elm 2i in. thick. 

 Each float should consist of only one piece, either from bein'o- cut off a 

 plank as wide as the float, or from the pieces being bolted together 

 edgeways if consisting of more than one. These' bolts should go 

 right through the float edgewavs with a head on the one end and 

 clinched on a ring at the other. Tliere should be 3 in each float, and 

 in distributing them care should be taken that thev do not come in 

 the way of the bolts which attach the floats to the arms. 



Paddle bolls and plates constitute an item of considerable expendi- 

 ture, especially if the ship's carpenter is negligent in screwing up 

 the bolts firmly. New floats are particularly liable to become loose 

 from the compression of the timber, and the bolts should therefore be 

 tightened on every convenient opportunity. It is not uncommon to 

 nick the thread of each paddle bolt with a chisel to prevent the nut 

 from unscrewing, bnt this will seldom be necessary if the bolts be pro- 

 perly screwed up. When a float is broken, which wil! sometimes 

 occur in a heavy sea, some of the paddle bolls will generally be lost, 

 but none ought to be lost from anv other cause. The carpenter in 

 getting new bolts should always fetch back the old ones, if only to 

 show that there has been no considerable loss from negligence. On 

 one of the outward voyages of the British Queen she lost every float 

 on the weather wheel, and was placed in considerable peril in conse- 

 quence. This we are inclined to attribute to the circumstance of the 

 bolts not having been tightened after thev had been shifted. The 

 vessel on the occasion to whicli we refer had been in dock, and all the 

 floats had been removed to enable her to go in. When she came out 

 the floats were replaced, and we doubt not the bolts were screwed as 

 tight as possible. This was in London: the vessel then started for 

 Southampton, where the bolts should all have been tightened before 

 she put to sea ; but this we believe was not done, and the exposed 

 wheel lost all its floats in consequence. Some of the floats indeed 

 were probably broken by the violence of the waves, but the greater 

 number we suspect were lost from the slackness of the bolts. 



Against the rapid erosion of the paddle bolts by the sea water we 

 know of no defence except frequent painting. The wheels should be 

 well painted with red lead about four times a year. Red lead is, we 

 think, a better protection to the iron than coal tar, and is much more 

 manageable in its application. It will penetrate into all the joints 

 and minute interstices which coal tar will not do, and will also adhere 

 to the iron more firmly. Coal tar will sometimes peel off, especially 

 if the wheel has been at all wet at the time of its application ; and 

 the iron will not untrequently rust beneath the coal tar. 



As paddle bolts are an article of large consumption, it is of course 

 desirable to produce them with as little labour as possible. The 

 practice of the London engineers is to wind a bar of square iron round 

 an upright mandril fixed in the ground in the same manner as the 

 links of cables are made, and then to cut every round of the spiral in 

 two places ; so that every round of the spiral will make two bolt=. 

 The mandril must of course be of the figure formed bv placing two 

 bolts with the point of the one touching the hook of the other. The 

 nuts of the paddle bolts should not be hexagonal but square, and the 

 carpenter should be provided with two or three good stout spanners 

 that will ht on the nuts easily. These spanners should be single 

 ended ; the handles should be straight, and furnished with a hole at 

 the end, through which a rope yarn should be rove. Whenever the 

 spanner is used, the rope yarn should be tied tothe rim of the wheel 

 so as to prevent the spanner from being lost should it happen to slip 

 or fall from the man's hands into the water. Sufficient length of rone 

 yarn should of course be left to permit the spanner to be wrought 

 without impediment. The nuts of the paddle bolts may be quite well 

 stamped in a die ; a good stout punching press will answer very well 

 tor the compressing power. 



Horses pon-er.— There is a blessed uncertainty among engineers as 

 to what area of piston and length of stroke constitutes an. p ; in fact 

 every engineer appears to have a measure of his own, whereby 

 about as much confusion is introduced into engine making as would 

 arise among the haberdashers if every shopkeeper had a yard mea- 

 sure of greater or less length than his neighbour. A horse power is 

 as much a conventional unit as a pound avoirdupois, and we think it 

 a disgrace to the Institution of Civil Engineers that it has not accu- 

 rately defined what a horse power is. Mr. Watt, it is true, determined 

 a horse power to be 33,00olb. raised one foot high in a minute ; but, 

 however useful such a standard may be for the power exerted by an en- 

 gine with the power exerted by horses or any other species of prime 

 mover, it is wholly useless for commercial purposes. In short it may 

 inform you of the power actually exerted by any engine, but not of 

 ine number ot horses power which a given diameter of cylinder and 



length of stroke are equivalent to. If a man purchases an ent'iue h° 

 IS charged at the rate of so many pounds sterling per horse "power, 

 and how is he to ascertain whether he has that number of horses 

 power or not? He has no means of finding out, because it has not 

 been defined what a Hominat horse power is, and every en<rineer 

 makes It a different quantity. Thus the West India mail packels are 

 all said to be of the same power, but he will be very far out who shall 

 conclude from thence that all the cylinders are of the same diameter. 

 Messrs. Ma.idsiay & Co.'s 400 h.p. is about 72 in. diameter of cy- 

 linuer. 



Messrs. Miller & Co.'s 400 h.p. is about 74 in., which is just about as 

 reasonable as that one man's foot should be twelve inches and another 

 only ten. Again a given sized cylinder, if devoted to the propulsion 

 of a vessel through the water, is rated difterentiv from what it would 

 be If used for any purpose on land. And in Messrs. :\rauds|av's 

 marine engines, while a cylinder of 47.i in. diameter and 5 ft. stroke is 

 rated at bU h.p., if on the side lever plan, a 4« inch cylinder and oft. 

 bin. stroke is only rated at 75 H.p., if on their double cylinder con- 

 struction, buch capricious variations appear to us extremely re- 

 prehensible, and are calculated, we think, to give a false impres- 

 sion of the force of a new kind of engine or defraud purchasers of 

 a part of their measure in the old kind. The power of an engine 

 ought manifestly to be some function of the cubical capacity of the 

 cylinder, and that function should not be determined bv abstract or 

 scientific views, but by the intent to equalize the cost of production 

 for the same power whatever be the stroke and diameter. In other 

 words au engine of the same power should cost the ^ame money 

 whether its stroke be longer or shorter than that of other engines 

 any increment in the stroke being so compensated for by a decrement 

 in the diameter, so as to keep the expense of production unaltered. 

 This standard, it is true, would no longer be a measure of the power 

 exerted, but of the expense incurred, and therefore "horses power" 

 might be an inappropriate name for it. But the term horses 

 power IS not even now expressive of the real power an engine exerts 

 at least in common language. The actual power is sometimes de- 

 termined for scientific purposes by means of the indicator, but it is by 

 the nominal power that engines are bought and sold and always 

 spoken of, unless when the contrary is expressly stated. The term 

 horse power is therefore as applicable to the proposed new measure 

 as to the existing one, yet we think a better term might be invented. 



The following is Boulton and Watt's rule for determining the no- 

 minal horses power. 



Let D = the diameter of the cylinder in inches. 



V = half the velocity of the piston in feet per minute. 

 (D=— 4DJV , ^ 



' 2650 ~ number of nominal horses power. 



But how is V to be determined before perhaps the encrine has 



been made ? Boulton and Watt fixed upon an empirical velocity for 



each ditferent length of stroke. The several velocities are as follow. 



stroke. velocity. 



ft. in. ft, • 



•2 IGO 



2 ^5 170 



^0 ISO 



^ ^ lilO 



4 200 



j ^ 210 



5 . . . . . . , , _ _ 220 



And so on with 10 feet of additional velocity for every G inches of 

 additional stroke. Yet this rule does not give results a'nswerable to 

 Uie dimensions observed by Boulton and Watt in their marine engines. 

 Ihus the original engines of tlie Thames and Shannon constructed by 

 Boulton and Watt, were rated at SO h.p., the cylinders beino- 47^ in. 

 in diameter, and the length of the stroke 4ft. (Sin." (47-.".)= — 4 r47-5) = 

 20G(j-25 X 105 = 2171)30 -f- 2(i50 = S3 h.p. nearly, instead of 80. 

 Land engines of 43-4 in. diameter of cylinder and S ft. stroke, makincr 

 lb double strokes in a minute, were rated by Boulton and Watt at 

 SO H.p.^ The average eftective pressure on the piston is rateil at 

 ,1,^ '' „''^''*1"'"''' '"'^''' ""f" the power may be thus computed, 

 r432j- X -7854 = 148(0-2 X by 7 and 2G6, and H- by 33,UiiU = about 

 80 HP. In marine engines a greater area of piston is allowed to re- 

 present a horse power than in land engines, because the motion of the 

 piston is supposed to be slower, but the eflective force is calculated a 

 little higher, or at 7-3 per square inch. 



To heighten all this perplexity, M. Pambour proclaims to all- 

 nations that the boiler is the true measure of an engine's power, and 

 with the forwardness proper to a little knouUdg.-, maim lins this to 

 be a discovery of his own. All engineers and most amateurs have 

 long known that the power produced by au engine is proportional 

 simply to the steam expended, provided there be no accidental cir- 



3 1. 



