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Report of the Committee, consisting of James R. NAPIER, F.R.S., Sir
W. THOMSON, F.R.S., W. FROUDE, F.R.S., and OSBORNE REYNOLDS (Secretary), appointed to investigate the effect of Propellers on the Steering of Vessels.
[PLATE I.] The Committee commenced operations by printing the following Circular, and sending copies of it to the Admiralty and to those shipowners with whom the individual members of the Committee were personally acquainted, or those who in their opinion were likely to assist in the investigation :
“ THE BRITISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE.
“ Experiments on the Turning of Screw Steamers. « At the Meeting of the British Association in Bristol last year, a paper was read by Professor Osborne Reynolds, in which it was shown, from experiments upon models, that in a steamer when the screw is in motion, the direction in which the rudder tends to turn the ship depends on whether the screw is driving ahead or astern, and is independent of the actual motion of the ship through the water; for instance, if when a ship has headway on the screw is reversed, then the action of the rudder is the same in direction as that of a ship going astern; or if the ship have sternway on, and the screw be started to drive her ahead, then the rudder acts as if she were going ahead.
“ After the discussion of the paper, Mr. James R. Napier, Sir William Thomson, Mr. W. Froude, and Professor Reynolds were appointed a Committee to carry the investigation further, and particularly to ascertain if the same results would be obtained when the experiments were made with fullsized ships.
“In order to collect sufficient data to establish a general conclusion, the Committee are anxious to obtain the assistance of such shipowners and captains of ships as may be willing to aid them.
“ The Committee accordingly ask that certain trials and observations may be made, and the results, together with the name, size, tonnage, and condition of loading of the ship, as well as the depth of immersion of the screw, the date and name of the officer in charge, may be forwarded to Professor Reynolds, Owens College, Manchester, or to any of the Members of Committee.
“It is also particularly requested that the kind of screw and the number of blades may be stated, and whether the screw is right- or left-handed. By a right-handed screw is understood one in which the upper blades move from port to starboard when driving the ship ahead.
6. The following are the trials requested :
“ Trial I.—That when the ship is going full speed ahead, the screw should be suddenly reversed and the rudder put hard over, as if to turn the ship to starboard of her course, and careful notice taken as to the way in which the ship turns before all headway is lost.
“ Trial II.-The same repeated with the rudder set in the opposite direction.
“ Trial III.—That when the ship is going fast astern the screw should suddenly be started to drive her ahead, and the rudder put hard over to the same side as in Trial I.
“ Trial IV.--Trial III, repeated with the rudder in the opposite direction. o Trial V.-That the ship should be driven full speed ahead with the helm amidships, and notice taken as to the direction in which the ship turns under the action of the screw.
“ Trial VI.--That the ship should be driven full speed ahead, then the screw reversed, with the helm amidships, and notice taken in which direction the ship turns." "May 3, 1876."
After sending the Circular the Committee received a communication from the Secretary to the Admiralty, to the effect that the Admiralty had ordered the experiments to be made, and that the results should be forwarded.
As the result of their application to private owners, the Committee obtained the use of three vessels, upon which the following trials were made.
Experiments made with the · Valetta,' belonging to the Earl of Glasgow, Captain R. Hunter, on the 6th June, between Wemys Bay and the Cumbrae.
The Valetta' measures 80 tons, and was drawing during the trials 5' 6" forward and 6' 6" aft. Her screw, which is right-handed, is 5' 6" in diameter, and during the trials was immersed about 1'; it is 3-bladed, and has a pitch of 8' 6". When at full speed the · Valetta’ makes about 9} knots an hour.
During the trials the seconds were called out by Mr. James R. Napier. Mr. Bottomley, who was acting for Sir William Thomson, watched the angles through which the boat turned, by means of a dumb compass, while the signals for turning and stopping the vessel were given by Professor Reynolds.
The first trial was of the effect which the screw exerted to turn the ship with the helm amidships. When at full speed she turned to port at the rate of about 70 per minute, or, as it is usually expressed, she carried a port helm. However, as the speed of the engines was reduced the tendency to turn the ship to port was reduced, and when going very slow (about 5 miles an hour) the ship turned slightly in the opposite direction. When going fast the screw churned air into the water, but not when it was going slow.
The effect of the screw to turn the ship with the helm amidship, although appreciable, was not of sufficient magnitude to be taken into account in the results of the subsequent experiment. And as this effect was almost the same with the wind on either bow, it was evident that, although the wind was blowing with some little force, its effect to turn the vessel was also unimportant.
These preliminaries having been settled, the ship was driven full speed ahead, then the screw reversed as suddenly as possible, and immediately the engines began to turn astern the rudder was put hard over. At first on reversal the engines turned but slowly, and it was not until the boat had lost some of her way that they turned full speed astern.
Four observations were taken in this way with the helm to port, two with head to wind, and two before the wind; and similar observations were taken with the helm to starboard. All four observations with the helm to port gave nearly the same results, and so with the helm to starboard.
The mean results were as follows:
With the helm ported (which, had the engines been going ahead, would have brought the ship's head round to starboard at a rate of nearly 2o a second) the vessel at first, while the screw was turning but slowly, commenced turning to starboard, and had turned through 5° in 9 seconds; she then commenced turning to port; and in 16 seconds more, when she had nearly lost all way, she had returned 13° to port or about go to port of her original direction, i.e. in the opposite way to that in which she would have turned had the screw been kept on ahead.
With the helm to starboard, at the end of 10 seconds she had turned through 6° to port, and in 14 seconds more, when she had nearly lost way, she had come back 14° to starboard or go to starboard of her original direction ; that is, as before, in the opposite way to that in which she would have turned had the screw been kept on ahead.
With this ship, therefore, although the reversing of the screw did not at once reverse the action of the rudder, it greatly reduced its effect, and reversed it in time for the ship to have turned go out of her course before she had come to rest—that is, go out of the direction in which she headed on the reversal of her screw; and considering that, during the 25 seconds in which she was stopping, had her screw been kept on ahead she would have turned through some 50°, the effect of reversing the engines was to bring the ship some 58° out of the direction she might have occupied.
Experiments with the Hopper Barge, No. 12, belonging to the Clyde Navigation Trust, Captain J. Barrio, on June 7, off Kilcreggan, Rosneath.
These experiments were conducted in a similar manner to those on the · Valetta,' the same members of the Committee taking part in them.
The barge when loaded carries 400 tons of mud, is 140 feet long, was drawing during the first set of experiments 11' 6" aft and 9'6'' forward, and when light, during the second set, 8' 2" aft or 4 ft. forward. The top of the propeller is 8' 6" from the bottom of the keel. The screw, which is righthanded, has three blades, and is 8 feet in diameter and 16 feet pitch.
The first set of experiments were made with the barge head to windward, the wind being of much the same force as on the previous day. The mud was then discharged, and the barge put before the wind, and the experiments repeated.
When loaded and going to windward with the helm amidships, the barge sheered first to port and then to starboard. This was apparently owing to the screw churning the water intermittently ; when the wake was apparently clear the boat turned to starboard, and when the screw was churning air into the water she turned to port.
When the screw was reversed with full way on, and afterwards the helm put hard over either to port or starboard, the action of the rudder was always reversed, and was very decided. It required 1 minute for the screw to bring the boat to rest, and during that time she turned from 35° to 60°; moving slowly at first, and more rapidly as her speed diminished.
The reverse action of the rudder was therefore inuch more decided than in the case of the · Valetta,' which was accounted for by the fact that the screw was reversed to full speed at once, the engineer being an old locomotive engine-driver accustomed to reverse suddenly, besides which the boat being much heavier allowed more time for the operation.
When the boat was going full speed astern, the screw reversed to full speed ahead, the action of the rudder was the same in direction as if she had been going ahead, but it was very slow.
When the barge was steaming full speed ahead with the rudder hard over, she turned at the rate of 1° in 1 second. * With this vessel, therefore, the effect of reversing the screw was to cause her to turn through more than 30° from the direction in which she headed when the reverse action set in; and considering that in the same time she would have turned through 60° in the opposite direction had the engines been kept on ahead, the effect of reversing was to turn her through 90° from the position shç would have occupied had the engines kept on ahead.
Experiments with the Steam Yacht Columba,' belonging to His Grace the Duke of Argyll, June 29, in Gare Loch, the weather very fine, with little wind.
Tho draught of the vessel was 10 feet aft and 8' 2" forward. She was fitted with a Griffith's screw 7'1" in diameter and 12' pitch. The experiments were witnessed by Mr. James R. Napier and his son, Mr. Robert T. Napier. When the vessel was going full speed ahead (about 10 knots) the engines were reversed, and the helm immediately put to starboard ; the vessel turned to starboard until her forward way was lost, the time between the reversal of the engines and the stopping of the ship being about 1 minute.
When the vessel was going full speed ahead the helm was set to port, and shortly after the screw reversed. The vessel turned to starboard at first, and then to port until all way was lost. The turning to starboard at first was the natural result of the helm having been ported before the screw was reversed.
In the trials on this ship no measurements were made of the angles turned through. The direction of turning, however, was the same as before, the reversing of the screw at once reversing the effect of the rudder.
In all three of these vessels, therefore, the same effect on the steering was produced by the reversing of the screw when the vessel was at full speed.
The importance of this effect may perhaps be best seen from the diagrams (Plate I.), showing the various positions occupied by the · Valetta' and the barge compared with those they would have occupied had the screws not been reversed.
In these diagrams the directions of the vessels correspond with the actual measurements during the trials ; the positions and distances travelled being estimated from the known speed of the vessels. It had been the intention of the Committee to use one of Mr. Napier's pressure logs in order to ascertain exactly the positions of the vessels during the trial, but this intention was not carried out.
Diagram 1 shows the courses run by two ships after the reversing of the screw until they had lost all way compared with the courses they would have run had they continued under full steam, the helm being hard to port.
A glance at this diagram is sufficient to show what a fatal mistake it must be when a collision is imminent to reverse the screw, and then use the rudder as if the ship would answer to it in the usual manner.
But perhaps, as regards collisions, the most important result is that shown in diagram 2--namely, the positions of the ships when they have not lost more than half their way, and when, as regards the distance run, the cffect of reversing the screw is but small.
As is shown in this diagram, it appears that whether the reversing of the screw reverse the action of the rudder or not, the rudder is nearly powerless to turn the ship, and that she will turn not only more rapidly, but in less room when going full speed ahead.
Before closing their Report, the Committee desire to express their thanks to the Earl of Glasgow, the Clyde Navigation Trust, and His Grace the Duke of Argyll, for the use of their ressels, and to the officers and crews who assisted in making the arrangements and conducting the experiments.
On the Investigation of the Steering Qualities of Ships.
By Prof. OSBORNE REYNOLDS. : [A communication ordered by the General Committee to be printed in extenso.] The primary object of using steam power in ships is to enable them to pass quickly over long distances. Under normal circumstances rapidity and certainty in mancuvring are matters of secondary importance; but circumstances do arise under which these powers are of vital importance. Experience has taught those who go down to the sea in steam-ships that their greatest danger is that of collision ; and fogs are feared much more than storms. That there must always be danger when long ships are driven at full speed through crowded seas in a denso fog cannot be doubted; but this danger is obviously increased manyfold when those in command of the ships are under the impression that a certain motion of the helm will turn the ship in the opposite direction to that in which it does turn.
The uncertainty which at present exists in the manœuvring of large ships is amply proved by the numerous collisions which have occurred between the ships of our own navy while endeavouring to execute ordinary movements under the most favourable circumstances, and with no enemy before them. These accidents may be, and have been, looked upon as indicating imperfections in the ships or the manner in which they were handled; but it must be admitted that the ships are the best and best found in the world, and that they are commanded by the most skilful and highly trained seamen alive. And if peaceable ships fail in their mancuvres when simply trying not to hurt each other, what will be the case of fighting ships when trying to do all thoy can to destroy each other? If the general impression as to the important part which the ram is to play in the naval combats of the future is over realized, then certainty in manæuvring must not only be of very great importance (this it has always beon in sea fights), but it must occupy the very first place in the fighting qualities of the ship.
Now the results of the investigation of the effect of reversing the propellers on the action of the rudder appear to show that, however capricious the behaviour of ships has hitherto seemed, it is in reality subject to laws; and that by a series of careful trials the commander of a ship may inform himself how his ship will behave under all circumstances.
The experiments of the Committee on large ships have completely established the fact to which it was my principal object last year to direct attention, namely, that the reversing of the screw of a vessel with full way on very much diminishes her steering-power, and reverses what little it leaves; so that where a collision is imminent, to reverse the screw and use the rudder as if the ship would answer to it in the usual manner is a certain way of bringing about the collision. And to judge from the accounts of collisions, this is precisely what is done in nine cases out of ten. In the paper of to-day I find the following (August 22, 1876):
“ The Fatal Collision off Ailsa Craig.-The Board of Trade inquiry into the collision between thë steamer • Owl' and the schooner-yacht. Madcap' was continued at Liverpool yesterday. Two passengers by the Owl' were recalled, and spoke to some of the facts of the collision. The night was not misty, though some rain had fallen. They saw the green light of the yacht shining brightly after the collision, William Maher, third officer of the • Owl,' said it was the chief officer's watch at the time of the collision, There were five able seamen in the watch. Witness and the chief officer