154 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[Mat, 



being secured to bits in the centre of tlie raft. When the sea strikes 

 the front of the inclined plane, the framework will yield, and the 

 hinges will prevent that sudden check which the cable tiglitening 

 would give to the work. Were they not there, the frame would 

 almost instantly resume its place ; the buoyancy of the shutters takes 

 all weight from the hinges. The principal use of the hinges to the 

 shutters is for the more easy recovery of the moorings. A small chain 

 attached to the shutters will enable them to be weighed at pleasure ; 

 when at the surface they are easily hung to lighters. The iron 

 plates on the shutters under the moorings are to prevent the wood 

 being chafed away by the friction. Each shutter is proposed to be 

 30 ft. long; the raft can be increased to any height, though GO ft. 

 broad would be suBicient for the heaviest sea that ever ran. Unlike 

 a beach, tliere would be no back run of water; and a raft of such 

 breadth would have the effect, from its own hold in the water, of ren- 

 dering the whole front stationary, at least this, I imagine, would be so. 

 I should think 20 ft. perpendicular depth would also be deep enough 

 for any weather. One of moderate dimensions, tried in a moderate 

 sea, would prove its efficacy. It is an idea formed after considerable 

 reflection, upon seeing plans of others, and hearing objections urged. 

 If it should prove successful, 'twill be a double pleasure ; and the 

 lives it may be the means of saving will not be the least portion of 

 such gratification. 

 March, 1842. 



ON THE ECONOMY OF FUEL IN MARINE ENGINES, AS 



PRODUCED BY EXPANSION. 



By J. G. Laurie. 



In prosecuting the investigation of expansion, I now come to 

 consider it with reference to economy of fuel, as applied in marine 

 engines. In former numbers of this Journal I have investigated its 

 effect in locomotives; but important as economy of fuel is in them, 

 it is of vastly greater importance in marine engines, the result in the 

 one case being only a diminished expenditure, and in the other the 

 accomplishment of passages otherwise quite unattainable, [n sea- 

 going steam vessels, subjected to varying weather and tempestuous 

 voyages, the application of expansion is as beautiful in theory as 

 beneficial in practice. By its means the power of the engine both 

 statical and dynamical is most beautifully adapted to the necessities 

 of the vessel, for by extending or contracting the degree of expansion 

 in favourable or adverse weather, the mean statical force of the 

 engine and the resistance of the vessel are, within extended limits, 

 exactly proportional ; and in each case the dynamical power is greatly 

 increased over what it otherwise could be, and that too in the way (in 

 the one case being by increased statical force, and in the other by 

 speed,) in which it is most available. 



In marine engines there are two means in use for effecting expan- 

 sion; the one being by the D valve, and the other by a separate or 

 expansion valve, which in some engines are used singly, and in others 

 jointly. 



The slide valve, as wrought by an eccentric, and constructed with 

 only sufficient cover to keep it tight, seems admirably fitted to the 

 purpose for which it was introduced, but does not seem so capable of 

 carrying out the principle of expansion, for, in the first place, it does 

 not afford the means of varying the extent of expansion, and, in the 

 next place, when expansion is to be carried to any considerable extent, 

 it opens and shuts the ports in a manner that has the effect of virtually 

 diminishing the size of the engine. In proof of 



this, by referring to my communication in No. •* P 



47 of this Journal, it will be found proved that, 

 if A B C D be a steam cylinder, A D the stroke 

 of the piston^2rf; the radius of the eccentric 

 =a ; the cover of the valve on the steam side 

 =c; distance from commencement of stroke to 

 commencement of expansion^rra' ; distance from 

 commencement of stroke to termination of eX' 

 panaion=i ; 



, 2i(a-c«) --y., „, , ,, a'd+d\/ai-a^c- 

 0= — ^-5 -^=2aX(l— c^) when a=l h— ■ : 



o- ■ a 



d-\-d^/ i^f.2 or for a particular and perhaps an extreme case, take 



3 7 3'3 



c=- and then o'=2iXT7; and !»^2dx-r which proves that when 

 4a 16 4 



there is cover on the steam side only of the valve, and when that cover 



is sufficient to expand the steam from 1 to 1"9, the whole available 



volume of the cylinder for receiving the steam from the boiler, and 



3'3 



for expanding that steam, does not exceed — of the cylinder, nor does 



it come to be quite so much as that, for at the same moment that ex» 

 pansion terminates on the one side of the piston, compression of the 

 enclosed vapour commences on the other, which can be viewed either 

 in the light of detracting from the power already developed by the 

 piston, or of diminishing (which it does in reality) the available part 

 (a') for receiving fresh steam from the boiler. If less cover be given 

 to the valve, the evil here pointed out is also less ; and when the cover 

 is small, it seems to be quite overbalanced by a property which shall 

 be hereafter explained. But if, on the other hand, greater cover be 

 given them, it is more than proportionally increased, and comes to be 

 of serious amount; to avoid which the valve is frequently constructed 

 so that it shall have cover on the eduction side as well as on the steam 

 side. If these covers be equal, then at the same time that the steam 

 is cut off from the side of the piston, the eduction is cut off from the 

 other, in consequence of which, although the available part (6) is 

 increased by giving cover to both sides of the valve, the available 

 part (a') is decreased, and, in addition, the period of time for which 

 the eductingside of the piston is in communication with the condenser 

 is greatly shortened ; if, however, the cover on the eduction side be 

 less than the cover on the steam side, this effect of compression of the 

 enclosed vapour will be diminished, and the period of time for con- 

 densation lengthened, but so will also the available part (5) of the 

 cylinder be diminished. Hence betwixt the advantage of increasing 

 the part (6) of the cylinder, and the disadvantage of diminishing (o') 

 and shortening the period of time for condensation, there is some 

 extent of cover of the eduction side of the valve, which shall in the 

 greatest possible degree secure the one and avoid the other. The 

 exact magnitude of this might (but with great difliculty, from the 

 nature of the analysis, which is lying before me,) be ascertained ana- 

 lytically ; but though it were ascertained with all conceivable precision, 

 still the plan of expanding by the slide valve has the effect of dimin- 

 ishing the available size of the cylinder, and, therefore, to employ 

 the steam produced by a given boiler, that steam being expanded to 

 a given extent, will require a cylinder, and, consequently, an engine, 

 larger wlien that engine expands by the D valve than when it expands 

 by a valve which does not diminish the available volume of the cylin- 

 der, which, even though there were no other reason, would, in engines 

 of considerable magnitude, in which alone expansion is much used 

 and is most serviceable, prevent the adoption of expanding by the 

 slide valve, and give a decided preference to a separate one being 

 employed for the purpose. 



To expand by a separate or expansive valve there have been almost 

 numberless schemes, the principal of which are throttle, slide, and 

 conical valves, of various constructions and variously situated, each of 

 which has in its turn had some claim to superiority over every other. 

 The great aim of the various plans seems to have been to cut off the 

 steam close to the cylinder ports, for the attainment of which, in many 

 cases, means are resorted to that are both difficult of execution and 

 troublesome to keep in order. The evil produced by placing the ex- 

 pansion valve at some distance from the cylinder port is two-fold. In 

 the first place, such an arrangement fixes a limit to the extent of 

 expansion depending on the ratio of the volume of the cylinder to 

 the volume of the steam betwixt the cylinder port and the expansion 

 valve ; and the next place, the effective power of the steam through- 

 out the stroke (or in other words, the area of the indicator diagram,) 

 is diminished, the amount of which diminution may be thus estimated. 

 Let volume of steam cylinder;=o; volume of steam betwixt cylinder 

 port and expanBion_^valve:=:6_; pressure _of steam at commencement 



