BENEDEK , Z.

English

Periodica Polytechnica (Budapest University of Technology and Economics)

CONTRIBUTION TO THE RELATION BETWEEN THRUST DEDUCTION AND FRICTION By Z. BE='iEDEK Department of Hydraulic .\Iachines. Polyteclmical Lnin·rsity. Budapest (Reccind December 28. 1965) Presented by Prof. Dr. B. BALOGH Symhols 11 model s('ale A wetted "urface of ship (m") A" propeller disk area (m") A" CF frictional resistance coefficient k coefficient for determining: the nominal speed causing: the resistance k cF(Vx!1',Y L length (m) R resistance (kp) Rc Reynolds' number Rc RF component of the viscous resistance (kp) I thrust deduction coefficient T thrust (kp) ship speed or model speed (m s -1) FA propeller advance speed (m S-I) t'n nominal speed of the water flowing through the propeller disk area (m s -1) !'x nominal speed causing the viscous resistance (m s -1) )' kinematic viscosity (m" S-I) Q water demity (kp m -·1 s") 11 and b constants calculated for a given model For the usual design method of a ship propeller 'we must know the \"alue of the thrust deduction coefficient which gi\"es us information on the mutual influence of ship and propeller. The thrust deduction coefficient, as is known is T-R t=---T 'where T is the thrust of the propcller, R is the resistance of ship without any acting propeller, both at the same ship speed. The value of t is determinable only with the aid of model experiment, apart from simple approximati\"e relations used for its precalculation. ,Ve usually made two kinds of measurements with the model of the ship and of the propeller. The resistance of ship body 'without propeller is measured and the thrust of the self-propelled ship model, both at different ship speeds. 5 Periodica Polytechnica ),1. Xj'!.. 142 The thrust deduction fraction can be calculated with the aid of these measured yalues. However, in case we make our ship model according to different scales, we get different values of t for the same ship speed. E.g.: The Victory model family has becn investigated by the NSMB. This investigation gave the follow-ing yalues for 11 knots ship speed with the measuring of models with different scales (a) [1]. a 50 40 30 23 18 6smooth 6rollgh 0.209 0.175 0.183 0.210 0.214 0.30,1 0.212 [The largest model ·was a motorboat. Its surface was the same as the surface of the paraffin-wax model in one senes of mf'asures (smooth), and it ats ~------~~------~--------~------~ M ~ W ~ M~~ Fig. 1. Figures 1 and 2 are copies from [1] and [2] was the same as the usual ship surface in service condition (rough), in the other series of measurements. The other models (a = 18-50) were the usual paraffin models.] This means, that we ought to calculate with a scale effect when using the thrust deduction fraction determined by model experiment. Therefore, the thrust deduction fraction was investigated in connection with several quantities. In Fig. 1 values of t are plotted of the Victory model family as the func-tion of Reynolds' number [1]. In Fig. 2 the yalues of c~ T-R Q!2.v1.A are plotted against T CT = ----Qj2· v1. A RELATIO.Y BETWEKY THR[-ST DEDl-CTIO_Y A.YD FRICTIOII- 143 where Z:A is the intake velocity of the propeller, A is the wetted surface of the ship body, Q is the density of water [2]. Whereas the first plotting gives a very scattered set of values of t, in thE' second case it was possible to make two linear functions for the mentioned 1rf3. Cc I VICTDRY !I,C[ 10 15 20 I03C~ Fig. 2 8 103Cr VICTORY draught.A' 7 6 5 " -1 5 6 Fig. 3 coefficients by analysing the values determined for the same ship speed. One of these relations was made for the smooth models, and the other for the rough ship. In the second formula C F is the frictional resistance coefficient. The results of the Victory model family are also seen in Fig. 3. Thrust cOE'fficient T CT =----Q/2· z:2 A (where z: is the ship speed) is given as the function of resistance coefficient R CR = ----Q/2 v 2 A according to TELFER's method [3]. 5* 144 Z. BE'\"EDEK We can dra-w t·wo straight lines in this case. One of them contains the measured values of the two motorboats (a 6) and the other goes through the points of the paraffin models and the rough motorboat. The motorboat was investigated in open water while the other models were investigated in a model tank. Consequently with the wall effect we can account for the increase of the difference between the two lines in the direction of the largpr model. The model family of the Victory ship was investigated ,\·ith the aim to give some method for the extrapolation of the measured data of an individual shipmodel to the ship. Thus, it is necessary to continue this work. id id Id ~ .. ~~~ T Self-propelled condition Overload condition (river lug) Fig. ·1 The real thrust of a ship's propeller, acting on a ship, must be equal to the resultant force of all other active forces. Giying the results of resistances of the self-propelled ship hody, it seems according to our experiments, that the thrust is not equal to the ship-resistance which we can measure in the towed condition of the ship, ,,-ithout active propeller. Thus, we must assume that the resistances of the ships are different in the self-propelled and in the towed conditions. This difference is the result of the fact that the streamlines are different in the neighbourhood of the ship at the two mentioned conditions. Let us assume that there is an ideal two-dimensional stream around the ship's hull in the horizontal plane of the propeller axis. We can draw the streamlines in both the to'wed and the self-propelled conditions (Fig. 4). Behind the ship we can find higher velocity in the jet of the propeller, and consequently lower velocities out of it. Because of the action of the propeller, there are higher velocities immediately near the surface of the stern than in tllf~ aboye-mentioned towing condition. If the propeller load is not yery high as in self-propelled conditions, the growth of velocities along the stern has greater importance than the decrease of velocities farther from the ship body. RELATIO.Y BETlT'EES THR[;ST DED[;CTIOli ASD FRICTIO,Y 145 But, the latter is also very important if the propeller load is yery high and if the thrust is a multiple of the own resistance of the ship body. (In the case of a river tug.) The difference of resistance has the same three components as the ship resistance in to"wed condition (R): frictional, pressure and wave making com-ponents. The frictional and the pressure components are dependent on the Reynolds' number, the roughness of surface and the form of the ship body. Thus both can be investigated in the same way. Together "we call them "viscous" resistance components. According to earlier research work, we have arrived at the conclusion, in the case of self-propelled ships at lower speed that the viscous component is the most important part of the above-mentioned resistance difference [2]. This is also shown by the picture of stream lines. The differences of the relative velocities are higher immediately hy the ship hnll and farther the picture of stream lines is the same in the to"wed and self-propelled conditions. Thus, approximately the same waves are made hy the ship in the two cases, but there is a difference in the viscous resistance. It is disputable whether the thrust measured on the propeller shaft is equal to the resistance of the self-propelled ship hody. The screw, mainly when it is behind the stern in the case of a single-screwship, does not work in a homo-geneous velocity distrihution. Thus, the thrust is varying in time, and the thrust measured may be different from the real average thrust. Yet the dif-ference is negligible as compared to the error of the other measured data in the inyestigation of ship models. The difference of the measured thrust and ship resistance (T - R) is always proportional to the difference of the resistance in self-propelled and towed conditions. The difference of resistance is approximately equal to the difference of their viscous components. Thus, let us assume that T R where Tp is the yiscous resistance in the self-propelled condition and Rp the viscous resistance in the towed condition. In the towed condition the yiscous resistance is where Q is the density of water, 1; the ship speed, A the wetted surface of ship, Cp the viscous resistance coefficient. In this formula v is not the real yelocity in the neighbourhood of the ship hull. The real velocities are very different along the ship's length. We can say that v is only a nominal velocity from the viewpoint of yiscous resistance. But it is true that the real velocities are in proportion to the ship's yelocity. 146 Z. BE.YEDEK Let us write the viscous re"istance of the self-propelled "hip in the same way 'where Vx will be the nominal velocity. with which we can obtain the effective viscous resistance of the self-propelled ship. This Vx nominal velocity and 'with vx, the real local velocities in the neighbourhood of the "hip depend on the velocity of thc ship (1') and the velocity of the water flo'wing through the pro-peller (1'0)' It is also important to know on what size of the ship surface the local velocities are changed in consequence of the action of the propeller. Thus ho'w many parts of surface of the ship have other local velocities in the self-propelled and in the to\\-ed conditions. It depends on the ship's form (afterbody), the posi-tion of the propeller relative to the ship, the proportion of the \I'etted surface and the mass of 'water flowed through the propeller (Ao . 1"0) mainly depends on the local frictional coefficient between the water and surface. When geometrically similar ships are investigated (in the case of a model family), the effect of ship-form and position of propeller can be omitted. We can observe the other componcnts in this case. Consequently, in the case of the investigation of a model familv we can write: The velocity of the water flowing through the propeller (1"0) is proportional to the velocity in the jet behind an ideal propeller having the same thrust. Accordingly 'Wl' can use this velocity for our investigation. The thrust of an ideal propellel' is: T + '1~ 2 (l'U - r) From thi" equation " T ", t" - ---. -,- I" 0- I') 4. Q/ -..;' 0 If we get the thrust coefficient T eT = -----Q/2 v2 A we may write This formula contains the ship speed, the wetted surface and the pro-peller disk area. Our function for determining Vx then becomes simpler RELATIO_'Y BETTFEES THRL'ST DEDL'CTIO.Y ~L'YD FRICTIO!Y 147 Last, we can carry Vo to the other side. So we can write: v For the determination of this function, the values of ~ from the measured Vo results of the Victory model family were calculated. These could be determincd from the measurcments of the ship speed, thrust coefficients (CT) and resistance coefficients (CR)' We can then calculate thc viscous resistance coefficient CF' TIH' value of CF is calculated here -with TELFER'S formula Cl' = 1· 2 - 3.65 100 The values of constants were determined by the analysis of the Victory model family, thcreforc it gives exactly the viscous part of the model resistance. CF is also calculated with the ITTC 1957 0.07.5 CF = --------(lg Rc - 2)2 but the result~ of the following calculations are the same with both methods. In the case of tilt' "rough motorboat". instead of the C F (for paraffin-wax models) was obtained, eR rough -- eR ~moolh where eR rough and eR sl1100th arc the total resistance coefficients for the two different models. As assumed formerly, the total difference of the thrust and the resistance IS equal to the viscous section of the resistance difference: T - R = T F - RF = 9/2 CJ.- A(v~ -- v~) or divided with Q/2 r2A But (1 + ACT.) . A,) 148 Z. BESEDEK and so Calcnlated the valnes of and plotted against C F a linear function was obtained (Figs 5 and 6) 3 2 2 VICTORY draughl.A,---,,-~-F·--"------3 5 Fig. 5 3.5~------------~------~-~ ,oJk J 2,5 2 VICTORY draught"B' +a=23 i o a=18 +o=6smoolh J Fig. 6 5 5 :o!e, RELATIOS BETWEES THR[;ST DEDUCTIOS ASD FRICTIO_Y 149 Table I Londcd condition Light ('ondition -"------------"~-------Scale l' knot 10' (er-CR) Difference 10' (Cr-CR) Difference mea$tttl..'d calculated per cent measured calculated I per cent 10 1.30 1.13 -13.1 1.30 1.20 7.7 11 1.23 1.12 9. 1.22 1.19 2.5 12 1.17 1.12 4.3 1.21 1.20 0.8 6 13 1.15 1.15 0 1.01 1.18 -16.8 ~mooth 14 1.17 1.17 0 1.22 1.27 -:- 4.1 15 1.20 1.21 -- 0.8 1.29 1.29 I 0 16 1.22 1.25 --- 2.5 1.35 1.32 2.2 17 1.19 1.35 -11.7 1.38 1.39 0.7 9 1.61 1.53 5 10 1.61 1.57 2.S 1.75 1.70 2.8 11 1.62 1.62 0 1.71 1.68 1.5 6 12 1.68 1.65 1.8 1.69 1.70 0.5 rough 13 1.75 1.73 1.1 1.70 1.73 1.8 14- 1.87 1.84 1.6 1.7.3 1.76 1.7 15 1.78 1.8l 1.7 16 1.97 1.96 0.5 10 1.03 0.96 6.8 0.9+ LOS -11.7 11 1.03 0.97 5.8 1.27 1.14 -10.2 12 0.99 0.99 0 1.00 1.07 7 18 13 1.06 1.0+ 1.9 0.78 14 1.1-1 1.08 5.3 1.04 1.14 9.6 15 1.12 1.11 1.3 1.03 1.15 +11.6 16 1.07 1.13 5.6 1.01 Ll8 ---16.8 17 1.09 -12 1.14 1.26 --10.5 10 1.09 -11 1.09 1.09 0 11 1.12 -11.6 1.06 1.05 12 1.08 5.6 1.07 1.07 0 23 13 1.08 1.05 2.8 1.06 1.10 3.8 14 1.15 1.08 6.1 1.13 1.14· 0.9 15 1.15 1.12 2.6 1.19 1.17 1.7 16 1.20 1.19 0.8 1.19 1.19 0 17 1.29 1.30 0.8 1.25 1.27 1.6 30 10 0.96 -;-14.3 1.07 1.04 2.8 11 1.00 6.+ 1.02 1.03 1 150 Z. BE.YEDEK (Table I cont.) Loaded condition Light condition Sl'uJe ,. 10" (er-eR) knot 10" DiffereIlt'c (er-CR) Difference m.ea;;ured (~aI('lilated per cent nH~a:::nrcd caieulatf'd per cent 12 1.07 1.0.J, 2.B 1.06 1.06 0 13 1.05 1.01 3.8 1.06 1.11 1.7 30 14 1.08 1.10 1.9 1.12 1.14 1.8 15 Ll6 1.16 0 1.20 1.18 1.7 16 1.20 1.20 0 1.21 1.21 I) 17 1.32 1.34 1.5 1.25 1.25 11 10 0.96 0.99 .. 3.1 1.25 Ll6 7.2 11 1.01 1.02 ... 1 1 ~"7 1.H -10.2 12 1.02 1.05 2.9 1.31 Ll8 9.9 .j.O 13 1.04 1.09 -1.8 1.26 1.20 ·LR 1·1 1.08 1.12 .. 3.7 1.22 1.19 2.5 15 LlO Ll4 .. 3.6 1.25 l.23 1.6 16 1.H Ll8 3.5 1.27 1.25 1.6 17 1.28 1.32 3.1 1.H l.3.J, 7 ~ ~--~----10 1.23 1.07 -13 1.21 1.22 --. 0.8 11 1.26 Ll2 -11.1 1.23 Ll.; - 6.5 12 1.29 LlB 3.5 1.2-1 Ll8 ·k8 50 13 1.22 l.20 1.6 1.24 1.26 1.6 l-t 1.16 Ll8 - . 1.7 1.33 1.30 2.3 15 1.H Ll9 ·1·..1 1.39 1.36 2.2 16 Ll6 1.26 --. 3.6 1.36 1.34 1.:; 17 1.:32 1.42 7.6 1.36 lA2 1..1 Thus, nominal yelocity 1\. for the calculation of T F is the following: 1'61' a + bJ' er: A ') (a 'I ..L eT' -- +- b An ' ,er: ! With this, the difference and the thrust deduction fraction RELATIOS BETTFEES THRUST DEDUCTIOS ASD FRICTIOS 151 Using the measured results of CT and CR from the model experiments of tht' Victory model family [1], [2], the following constants were obtained: in loaded condition A a = 1.205 b = 0.347 in light condition B a = 1.295 b = 0.380 By comparing, the coefficient of the resistance-difference-mcans was calculated from the following equation: The percentages of errors of (CT CR) in the table and k in the diagrams did not show any regular differences in the cases of the investigated field of speeds. The errors in the table [the errors of the (CT - CR)] are also equal to the error::: of the thrust deduction fraction: below 3 per cent in 57 per cent of the cases between 3 -- 7 per cent In 22 per cent of the cases between 7--,12 per cent in 18 per cent of the cases above 12 per cent In 3 per cent of the cases Thf' m.ean valnes of the errors In load condition 4.56 per cent In light condition 3.92 per cent. These are not yery high, compared with the error of the measurement. In Fig. 7 the measured points are given [1] and the curve [2] of the thrust coefficient (CT), plotted against the ship speed (v m ). in the case of the model a = 18 in loaded condition. We can say, that our assumption is practically true in the case of the Victory ship 1', = 10 ~~I7 1,/. As the errors are not higher than the errors of the measurcment, and there is no effect of Froude's number in the inyestigated field of :;:hip speed, Fr=-.-I, r VgL 0.14 -- 0.:24, where l' m/s, Lm\ I the difference of the thrust and resistance (T - R) is equal to the difference of the viscous resistance in self-propelled and towed conditions. 152 5,5 Z. BKYEDEK VICTORY draughl.A" a =18 model 16 Fig. 2.0 Vm m/s The "Victory ship was inyestigated In two conditions. The principal dimensions of the ship are as follows: Load condition Light condition Lp III 133.045 133.045 L III 135.562 133.177 E m 18.898 18.898 T (mean yalue) III 8.687 6.809 .J In:~ 15019 11370.3 A. In::! 3687 3164 D (dialll. of screw) 111 5.3 5.3 .J cB = LET 0.6876 0.6575 A. cA L(E -i- 2T)- 0.750 0.731 D T 0.610 0.669 Though it is premature to make a determination of the effect of the ship-form and the relative position of propeller from these two investigated model families, it is interesting to note that the constants a and b in the for-mula of k may he determined in the following way: 0.623 a= b = 0.568~ T 0.623 = 1.208 "'''' 1.205 0.6876·0.750 __ 0_._62_3 ___ = 1.296 ?d 1.295 0.6565·0.731 bload = 0.568·0.610 = 0.347 blight = 0.568·0.669 0.380 RELATIOS BE1'ff"EEX THRCST DEDCCTfOX .·LYD FRICTIOX 153 Conclusion It may be seen from the investigation of the measured results of the Victory model family that thc caiculated yalues of k from the model ex-periment of a ship without high propeller load, plotted against C F, gives a linear extrapolator. If ,"ve repeat the model experiment with the same model having different roughness, ,"ve can get the extrapolator more exactly. 'Vith this extrapolator we can obtain the value of k for the ship and determine the eT thrust coefficient with the aid of CR resistance coefficient eT = or the resistance coefficient 'with a good approximation. Summary The thrust deductiun of a model family of a singlt' screw ship is a linear function of the viscous resistance roefficicllt: This linear function gives us the possibility to determine the real value of the thrust deduction of a ship, from the results of measured model data without any scale effect. References 1. L.nn1EREl'i, \V. P. A.- YAl'i }lA:\"E:\", J. D.-LAP, A. J. W.: Scale effect experiments of Victory ship and models (part I). Transactions of the Royal Inst. of ::\aval Arch. 1955. 2. LAP, A. J. W.-VA:\" ~lA:\"E:\". J. D.: Scale effect experiments of Victory ship and models (part II). Transactions of the Royal Inst. of :Naval Arch. 1962. 3. TELFER, E. V.: The reconciliation of model data. measured mile results and service perfor-mance of ships. ::\orth East Coast Inst. of Ellgrs. and Shipbuilders 1960. Zolt£m BE:-\EDEK, Budapest, XI., Sztoczek u. 2-4·. Hungary 

CONTRIBUTION TO THE RELATION BETWEEN THRUST DEDUCTION AND FRICTION

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CONTRIBUTION TO THE RELATION BETWEEN THRUST DEDUCTION AND FRICTION

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Periodica Polytechnica (Budapest University of Technology and Economics)