468 



NATURE 



\_Au^ust 29, 1878 



Report of the Covimittce consisting of yamcsR. A'apier, F.R.S., 

 Sir W. Thompson, F.R.S., W. Froude, F.R.S., J. T. Bottomley, 

 and Osborne Rtyftolds, F.R.S., Sec., appointed to investigate the effect 

 of Propellers on the Steering of Vessels, — The Committee are now 

 of opinion that the work for which they were originally brought 

 together has been fully accomplished. The importance of the 

 effect of the reversed screw on the action of the rudder has been 

 fully established, as well as the nature of its effect completely 

 ascertained. Also, for two years, the Committee have urged the 

 results of their work upon the attention of the Admiralty and 

 the various marine boards, and although they regret that, as yet, 

 they have failed to obtain that general recognition of the facts 

 brought to light which their vital importance demands, they 

 consider that this will surely follow, and that as a Committee 

 they can do no more than publish the reports of the trials and 

 the conclusions to which they have been led. 



The following is a summary of the conclusions which have 

 been established, and it is interesting to notice that the conclu- 

 sions drawn by Prof. Reynolds from experiments on models, 

 have been fully confirmed by the experiments on full-sized ships. 



Summary of the Results of the Trials of the Effect of the Reversed 

 Scniv on the Steering during the Time a Vessel is stopping Haself. 



It appears, both from the experiments made by the Committee 

 and from other evidence, that the distance required by a screw 

 steamer to bring herself to rest from full speed by the reversal 

 of her screw, is independent or nearly so of the power of the 

 engines, but depends on the size and build of the ship, and 

 generally lies between four and six times the ship's length. It is 

 to be borne in mind that it is to the behaviour of the ship during 

 this interval that the following remarks apply :— ■ - ■ 



The main point the Committee have had in view has been to 

 ascertain how far the reversing of the screw, in order to stop a 

 ship, did or did not interfere with the action of the rudder daring 

 the interval of stopping, and it is as regards this point that the 

 most important light has been thrown on the question of handling 

 ships. It is found an invariable rule that, during the interval in 

 which a ship is stopping herself by the reversal of her screw, the 

 rudder produces none of its usual effects to turn the ship, but 

 that, under these circumstances, the effect cf the rudder, such as 

 it is, is to turn the ship in the opposite direction from that in 

 which she would turn if the screw were going ahead. The 

 magnitude of this reverse effect of the rudder is always feeble, 

 and is different for different ships, and even for the same ship 

 under different conditions of loading. 



It also appears from the trials, that, owing to the feeble in- 

 fluence of the rudder over the ship during the interval in which 

 she is stopping, she is then at the mercy of any other influences 

 that may act upon her. Thus the wind, which always exerts an 

 influence to turn the stem (or forward end) of the ship into the 

 wind, but which influence is usually well under control of the 

 rudder, may, when the screw is reversed, become paramount, 

 and cause the ship to turn in a direction the very opposite of that 

 which is desired. Also, the reversed screw will exercise an in- 

 fluence, which increases as the ship's way is diminished, to turn 

 the ship to starboard or port, according as it is right or left- 

 handed ; this being jarticularly the case when the ships are in 

 light draught. 



These several influences, the reversed effect of the rudder, the 

 effect of the wind, and the action of the screw, will determine 

 the course the ship takes during the interval of stoppmg. They 

 may balance, in which case the ship will go straight on : or any 

 one of the three may predominate, and determine the course of 

 the ship. 



The utmost effect of these influences when they all act in con- 

 junction, as when the screw is right-handed, the helm star- 

 boarded, and the wind on the starboard side is small as 

 compared with the influence of the rudder as it acts when the 

 ship is steaming ahead. In no instance has a ship tried by the 

 Committee been able to turn with the screw reversed on a circle 

 of less than double the radius of that on which she would turn 

 when steaming ahead. So that even if those in charge could 

 govern the direction in which the ship will turn while stopping 

 she turns but slowly, whereas, in point of fact, those in charge 

 ■have little or no control over this direction, and imless they are 

 exceptionally well acquainted with their ship they will be unable 

 even to predict the direction. 



It is easy to see, therefore, that if on approaching danger the 

 screw be reversed, all idea of turning the ship out of the way of 

 the danger must be abandoned. She may turn a little, and those 



in charge may know in ^^hich direction she will turn, or may 

 even, by using the rudder in an inverse manner, be able to in- 

 fluence this dii-ection, but the amount of turning must be small, 

 and the direction very uncertain. The question, therefore, as to 

 the advisability of reversing the screw is simply a question as to 

 whether the danger may be better avoided by stopping or by 

 turning. A ship cannot do both with any certainty. 



Which of these two courses it is better to follow must depend 

 on the particular circumstances of each particular case, but the 

 following considerations would appear to show that when the 

 helm is under sufficient command there can seldom be any doubt. 



A screw steamship when at full speed requires five lengths, 

 more or less, in which to stop herself; whereas, by using her 

 rudder and steaming on at full speed ahead, she should be able 

 to turn herself through a quadrant without having advanced five 

 lengths in her original direction. That is to say, a ship can 

 turn a circle of not greater radius than four lengths, more or 

 less (see Hankow, Valetta, Barge),^ so that, even if running at 

 full speed directly on to a straight coast, she should be able to 

 save herself by steaming on ahead and using her rudder after 

 she is too near to save herself by stopping ; and any obliquity in 

 the direction of approach or any limit to the breadth of the 

 object ahead is all to the advantage of turning, but not at all to 

 the advantage of stopping. 



There is one consideration, however, with regard to the ques- 

 tion of stopping or turning which must, according to the present 

 custom, often have weight, although there can be but one opinion 

 as to the viciousness of this custom. This consideration is the 

 utter inability of the officers in charge to make any rapid use of 

 their nidder so long as their engines are kept on ahead. It is 

 no uncommon thing for the largest ships to be steered by as few 

 as two men ; and the mere fact of the wheel being go arranged 

 that two men have command of the rudder renders so many 

 turns of the wheel necessary to bring the i-udder over that, even 

 where ready help is at hand, it takes a long time to turn the 

 wheel round and round so as to put a large angle on the rudder. 



The result is, that it is often one or two minutes after the 

 order is heard before there is any lai-ge angle on the rudder, and 

 of course, under these circumstances, it is absurd to talk of 

 making use of the turning qualities of a ship in case of emer- 

 gency. The power available to turn the rudder should be pro- 

 portional to the tonnage of the vessel, and there is no mechanical 

 reason why the rudder of the largest vessel should not be 

 brought hard over in less than fifteen seconds from the time 

 the order is given. Had those in charge of steamships efficient 

 Control over their rudders, it is probable that much less would be 

 heai'd of the reversing of the engines in cases of imminent danger. 



Report of the Committee on the best Means of Drjclcfing 

 Light from Coal Gas. Read by Mr. T. Will?.— This first part of 

 the report was by Dr. Wallace, and referred to cannel gas, the 

 standard quality of which was assumed to be twenty-six candles. 

 That was the quality delivered by most of the Scottish gas com- 

 panies, while in London the illuminating power was sixteen 

 candles ; in Liverpool, Manchester, and Carlisle, twenty candles ; 

 and in most of the other cities in England and Ireland about 

 fourteen candles. The report was largely occupied by a tabu- 

 lated series of results obtained with different kinds of burners, 

 such as the rat-tail, onion or fish-tail, batwing, and argand, and 

 also the influence of globes of different sizes, shapes, and 

 materials. The report was strongly in favour of cannel rather 

 than common gas, on account of its comparatively small in^ 

 fluence on the atmosphere of apartments and the smaller pro- 

 portion of sulphur it contained. The report also advocated the 

 burning of gas at a comparatively low pressure, and the use of 

 district governors to equalise the pressure in different levels of 

 towns, and of regulators in houses and street lamps, to give the 

 exact pressure calculated to give the best photometric results. 



Fou7-teenth Report of the Committee for Exploring Kent's 

 Carer n, Devonshit-e. (Abstract.) 



The Tortuous Gallery.— \\htn their thirteenth report was 

 drawn, at the end of July, 1 877,- the Committee had completed 

 the exploration of the " Tortuous Gallery," with the exception 

 of the portions of it termed the "Terminal Chamber" and the 

 " Recess." On entering the Chamber its floor -was found to be 

 a complete pavement of blocks of limestone, the removal Of 

 which disclosed an almost horizontal bed of the typical breccia 

 —the most ancient deposit yet found in the cavern._ It was ex- 

 cavated to the customary depth of four feet, but without reach- 

 . ' B.A. Reports, 1876-77. 



