One other result of the experiments was to make patent what was not very generally known, that the reversal of the engines of a screw steamer has but inconsiderable effect on stopping her way ; and that the distance required to bring a screw steamer to rest mainly depends upon her size, weight, form, and speed. The distance may roughly be put down as five or six times her own length.
These peculiarities should be well known and be well considered by all in charge of screw steamers, as it is quite plain that "full speed astern " may often bring about a collision, whereas steaming ahead and using the rudder might avoid it. This may be especially the case if a steamer is approaching another, or approaching a shore, in an oblique direction. Say that the ship or shore bears on her port bow six or seven lengths off, and by continuing she would strike the ship, or in the case of the shore go aground; if she ported her helm and reversed, she might be carried stem on to the very object she hoped to avoid ; whereas by porting her helm and steaming ahead she might have cleared it, or by only stopping the engines, and not reversing them, she might have gone clear.
A screw steamer will generally turn in a circle whose radius is about four times her own length, and stop in a distance equal to about six times her own length ; thus far, if the object to be avoided were six lengths off, it could be cleared by turning under full speed. This, however, supposes that the steamer can be given her full rudder power (about thirty-five degrees) at once; whereas the fact is that it takCs a very considerable time to get a rudder hard over when a steamer is at full speed, and very seldom is there power enough at the helm—perhaps only one man—at the moment to do it. Of course, by stopping the engines the power gained by the
helmsman over the rudder would be increased ; and generally it would appear to be the wisest plan, when a screw ship gets into such a position that it is almost certain she will strike some object ahead, for her to stop her engines and put the helm over just as if the engines were still turning ahead. Of course, the engines could be reversed, and the helm put the other way, to assist the effect of the backward motion of the screw on the turning ; but, unless the person in charge is thoroughly acquainted with the behaviour of the ship under all conditions, simply stopping the engines and making as much use of the rudder as possible would appear to be the wisest course, as, if the screw is not revolving, the vessel will, of course, steer like an ordinary sailing ship.
As before said, very few steam yachts behave exactly the same under the influence of the rudder during stern way, or under the influence of the screw alone with the rudder amidships ; and it will be found incumbent in most cases to make an experiment with every steamer in order to discover her peculiarities. In the first place, it should be ascertained how far she will go before losing way when the engines are stopped from full speed ahead, and from half speed ahead ; also the distance she will traverse before losing way whilst assisted by the engines being reversed. Next it should be ascertained how many compass points the vessel will turn before losing way; and how many points she will turn and in what direction, before losing way, whilst the engines are being reversed. Also, the steering should be thoroughly tested whilst the steamer has stern way on.