172 



KNOWLEDGE. 



May. 1911 



total resistance of the model has been found by 

 towing experiments the \\a\e- and eddy-making 

 resistance, -which is not mathematicalh' ascertainable, 

 can be found bv subtracting the frictioiial resist- 

 ance from the* total resistance. 



Newton's law of mechanical similarities, first 

 applied b\- Froude in the calculation of the resist- 

 ance of ships, and therefore called " Froude's Law," 

 enunciates the theorem that if two geometricalh- 

 similar ship bodies are mosing at corresponding 

 speeds through the water, the wave- and eddy- 

 making resistances of these bodies are proportional 

 to the third powers of their linear measurements 

 or disjilacements. 



.Vs soon as the wave- and eddy-making resistance 

 of the model has been found by experiments it 

 need only be multiplied by a'^ — the third power of 

 the proportional displacement of ship and model — 

 in order to obtain the corresponding resistance of 

 the ship. If the frictional resistance of the ship is 

 added to this resistance the total resistance of the 

 the ship can be found hv a simple mathematical 

 formula. 



It will bo readilv understood that in experiments 

 of the nature indicated the most important instru- 

 ment (if precision is the towing carriage, the 

 mechanism which registers the resistance to towing 

 set up by the wax models. The carriage at 

 Teddington spans the \\ hole w idth of the tank and is 

 borne on four wheels. thirt\-six inches in diameter, 

 which travel on rails built on the sides of the tank. 

 It is made entirely of steel and the illustration 

 (see Figure 1) gives a better idea of it than an\- 

 description. Its dimensions are thirty-two feet six 

 inches by thirt\-four feet six inches and its weight 

 including all motors and electrical equipment, 

 but excluding the model dynamometer and other 

 apparatus, amounts to fourteen-and-a-half tons. The 

 carriage is driven hv four motors and can be run at 

 more than four hundred registered speeds, towing 

 the models at rates varying from one foot to fifteen 

 feet a second. The dynamometer and towing 

 apparatus are made to follow the methods evolved 

 by Froude, at Haslar. and allow tlie model some 

 small amount of movement while being towed. It 

 is not possible in the space at our disposal to 

 describe the electrical measuring apparatus. It must 

 suffice to sa\' that this apparatus, (so sensiti\'e that 

 if the length or breadth of the wax model were 

 altered bv a thousandth part, the resultant alteration 

 of its resistance would be registered by the dials on 

 the towing carriage) is fitted with several drums. The 

 records taken on the main drum consist of time, 

 distance and resistance. The first of these is given 

 by an electric clock, which makes and breaks 

 contact ever\- half second. The distance towed 

 is measured by a trigger, fitted to the fore girder, 

 striking against points twenty feet apart, fixed to 

 the rail sleepers at the side of the tank. This trigger 

 is caused to swing and so complete an electro- 

 magnetic circuit. Records of the trim of the model 

 can be taken, when required, on separate drums. 



There are two tanks at Teddington. The Great 

 Tank (see Figure 4) is of dimensions as follows : — 



Length 549-ft. over all. 



Breadth 30-t't. on water line. 



Depth of water.... IJ-ft. i-in. alont; the middle line. 



In the tank walls an oljservation window has been 

 fitted on either side, about mid-length of the tank. 

 An arc lamp is to be fitted on one side of the tank 

 to illuminate the water through the window, so as to 

 enable a passing model to be obser\-ed under water, 

 and any phenomena to be photographi^'d. 



A smaller tank (see Figure 2). sixt\--four-and-a-half 

 feet in length, and of five feet width, has been made 

 for the purpose of in\'estigations with running water. 

 Experiments can be made in this tank imder two sets 

 of conditions — either in the still water and the model 

 being towed : or with the model held on a dvna- 

 mometer arm and the water made to flow past at 

 an}- \elocity. For the second of these purposes a 

 rotary pump has been fitted at one end of the tank, 

 and provision has been made for reducing eddies. 

 This pump will give a velocity of about three 

 iniles an hour to the water, and the ^•elocit\• can be 

 regulated as desired. Special apparatus has been 



designed for castnij. 



the wax models — in lengths 



varying up to twenty-five feet — and for making 

 them exactly to scale. The initial programme of 

 experiments is as follows : — 



A standard model will be made to the same lines 

 as that used at the Haslar Experiment Works, and 

 the results of the experiments with this model will 

 be compared with those at Haslar. 



Further experiments will be made with (sav three) 

 models of widely different t\'pes alread\- tried at 

 Haslar, to ensure the general accurac\- of the 

 a[i()aratus and methods adopted. 



On the satisfactory com[)letion of this work, and 

 with a view to a series of experiments on mercantile 

 forms, a preliminary set of experiments will be made 

 on some typical forms of passenger-cargo and cargo 

 N'essels having as w ide a variation of block coefficient 

 and form as possible. 



In conjunction with the foregoing, some experi- 

 ments will be made to test the effect produced on the 

 diverging system of waves by varying the bow form. 

 These experiments will start on the lines suggested 

 by Lord Raxleigh {Phil, .l/.r.t/., September, 1909). 



.\s further work to be undertaken at an earh- date, 

 the Committee have appro\-ed the suggestion to 

 explore thoroughly the wake in the afterpart of 

 several typical models. Experiments would be 

 made to gi\-e the direction of flow, the pressure, and 

 the velocity head in the streams. 



It has also been agreed that it is desirable to 

 repeat, and, if possible, extend, Mr. W. Froude's 

 experiments on friction : it is hoped to make 

 observations on planks up to at least one hundred 

 feet in length. 



