Part 14 (2/2)

When it is decided to build a certain s.h.i.+p, the first thing to be done is to draw it on paper. The Admiralties of the world, and also the great s.h.i.+pbuilders, have each their own chief designer installed in a big, light, quiet office fitted with large strong, flat tables at which work a number of draughtsmen.

The naval authorities tell the ”chief” in general terms what they want the s.h.i.+p to be capable of, and he determines its size and form. Then the draughtsmen work out his ideas on paper, themselves deciding upon the minor details, until they have produced exact representations of the s.h.i.+p which is to be. Some draughtsmen deal with the actual hull of the s.h.i.+p, while others design the various fittings and minor details, all working, of course, under the constant supervision of the chief.

In this connection one may perhaps allude to a matter which the general public often seems to misunderstand--the work and functions of a draughtsman. I have heard people say of a boy that he is good at drawing so they think of making a draughtsman of him. Now the point is that the actual drawing is perhaps the least important part of a draughtsman's work. He has to know _what to draw_. He is given just a rough idea of something and from that he has to produce a perfect design, bearing in mind that the thing to be made must well fulfil its purpose, must be easy and cheap to construct, must be strong enough yet not too heavy, must be made of the most suitable material and so on. He has to possess a good deal of the knowledge of the skilled workman, he has to be something of a scientist and a good mathematician in addition to his ability to make neat and accurate drawings. So, you see, these men whose minds conceive the details of our great s.h.i.+ps are men of long training and experience, with far greater knowledge and skill than we sometimes give them credit for.

Anyway, there they stand, each at his own table, bending over his own drawing-board, each doing his own particular share towards producing the perfect s.h.i.+p.

But when all is said and done, there are limitations to the cleverness of the cleverest among us, so the next step, after the draughtsmen have done their best, is to test what they have done by experiment.

Years ago a certain Mr. William Froude interested himself in the question of the best shapes for s.h.i.+ps, and he found that by making an exact model of a s.h.i.+p and then drawing that model through water it was possible to foretell just how that s.h.i.+p would behave. He built himself a tank for the purpose of these experiments at Torquay, where he lived, and by its aid he added a very important chapter to the science of s.h.i.+pbuilding.

Nowadays the Admiralty have a large and well-fitted tank at Portsmouth, the United States Navy have one at Was.h.i.+ngton, private s.h.i.+pbuilders have the use of a national tank at Bushey, near London, while several of the large firms have tanks of their own.

The national tank at Bushey, by the way, was given to the nation by Mr.

Yarrow, a famous s.h.i.+pbuilder, in memory of Mr. Froude, it being called the ”William Froude Tank” in recognition of the great work done by him.

Now these tanks may be described as rather elongated swimming-baths.

Such a structure is generally a little narrower than the average bath, but it is longer and much deeper.

At one end there are miniature docks in which the models float when not in use, while at the other there is a sloping beach upon which the waves caused by the models expend their energy harmlessly.

Along each side there runs a rail upon which are supported the ends of a travelling bridge. Driven by electric motors, this bridge can run to and fro from end to end of the tank, and its purpose is to drag the models through the water.

Carried upon the bridge is a platform which bears a number of instruments, chief among which is a self-recording dynamometer.

Now a dynamometer is an instrument for measuring the force of a ”pull,”

and when we call it self-recording we mean that it automatically takes a record of a series of pulls or of a varying pull. In this case there projects below the bridge a lever, to the end of which the model under test is attached. As the bridge rushes along it pulls the model through the water by means of this lever, and the force which is expended in doing so is recorded in the form of a wavy line upon a sheet of ruled paper.

If the model slips through the water very easily there is little pull upon the lever and the line drawn by the pen of the instrument remains low down upon the chart. If, however, much power is needed and the pull is a strong one the pen moves and the line rises towards the top of the paper. Any change, whether increase or decrease, is thus shown by the rise or fall of the ink line.

One model can be thus tried at various speeds and its behaviour noted under different conditions. Other matters can be investigated too, such as whether or not the bow rises in the water or falls when the boat is in motion, also how much such rise or fall may amount to.

The suitability of a certain shape of vessel, moreover, can to a certain extent be seen by observing the commotion which it makes in the water.

Everyone has noticed the way in which a s.h.i.+p throws up a wave at its bows, and that bow-wave, as it is termed, represents so much energy being wasted. The power of the engines is absorbed to a certain extent in making that wave. It is impossible to make anything which when forced through the water will not make some wave, but certain forms cause less of it than others, and the designer of a s.h.i.+p seeks to find that form which will make the smallest bow-wave.

In like manner the eddies which a s.h.i.+p leaves in its wake are the result of wasted energy, and the s.h.i.+p must be so shaped that they too will be reduced to a minimum.

s.h.i.+pbuilders find that there are three things which r.e.t.a.r.d a s.h.i.+p's movement: skin friction, or friction between the water and the sides of the s.h.i.+p; wave making at the bow and eddy making at the stern. The first depends largely upon the smoothness of the s.h.i.+p's surface, the second and third depend upon its shape. If a model behaves badly in the tank the fault may be either too much wave making or too much eddy making, and which of these it is the dynamometer does not of course tell. In many cases the experienced eye of the tank officials furnishes the clue to the trouble, but in some cases a cinematograph is used to make a complete series of photographs of the model and the water around it as it rushes from end to end. These can then be studied in conjunction with the chart and the cause of the fault discovered.

The real aim, it is obvious, of all these tank experiments is to find out the lowest horse-power necessary to drive the s.h.i.+p, or the best form of s.h.i.+p to get the highest speed out of a given horse-power.

The cost of keeping up these large tanks and making the models and conducting the experiments is very great, for not only are the premises very large (I know one in which the water alone cost nearly a hundred pounds) but a highly skilled staff is necessary. The saving effected in the cost of s.h.i.+ps and the superior efficiency of the s.h.i.+ps makes it well worth while however.

There is still one other point about this matter which will possibly be puzzling the observant reader. What are the models made of and how are they made? They are made of paraffin wax, and a very important department of the experimental tank is that where the models are formed.

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