Sec. 78.5 



MODEL-TESTING PROGRAM FOR A SHIP 



871 



While tests with a stock propeller i'uniisli cmly 

 an approximation to the shaft power and rate 

 of rotation for a range of ship speeds, they serve 

 to confine the range of the probable wake and 

 thrust-deduction fractions within rather narrow 

 limits. With average values inside these Umits it 

 is possible to recalculate the shaft power and 

 rate of rotation for a propeller other than the 

 stock model by the short method described in 

 Sees. 70.21 through 70.38. The propeller designer 

 is able to undertake a final design with far more 

 assurance than if he were forced to approximate 

 the wake and thrust-deduction fractions and other 

 design factors by less precise methods. 



Since there are two propeller diameters repre- 

 sented in the alternative ABC designs, 20 and 

 24 ft, it is necessary to determine the availability 

 of two stock wheels which have the same scale 

 ratio as the two models. 



Assuming that the ship models are to have an 

 LwL of 20 ft, ample for the testing of a prehminary 

 design if the propellers are sufficiently large, the 

 scale ratio X (lambda) is 510/20 = 25.5. This gives 

 values of X"' of 5.0498, X' of 650.25, and X' of 

 16,581.4. Dividing the propeller diameters by 

 25.5 gives 0.7843 ft, or 9.412 in, for the 20-ft 

 wheel and 0.9412 ft, or 11.294 in., for the 24-ft 

 wheel. The first preliminary propeller design by 

 the chart method, similar to that described in Sec. 

 70.6, indicates optimum P/D ratios of 0.975 and 

 1.045, respectively. (A later calculation, quoted in 

 full in Sec. 70.6, gave an optimum P/D ratio 

 for the transom-stern wheel of 1.02). With 

 4-bladed propellers the blade width need not 

 be large nor the thickness great. Reasonably 

 modern blade sections, of airfoil type near 

 the hub and ogival type near the tips, are available 

 in stock, as are propellers with small or zero rake. 



An examination of the TMB stock hst reveals 

 two right-hand 4-bladed propellers having the 

 following characteristics: 



TMB TMB 



Item 229/t 1986 



Diameter of model propeller, in 9 . 652 1 1 . 40 



Full-scale diameter for X of 25.5, ft 20 . 5 24 . 22 



Pitch-diameter ratio . 98 1 . 05 



Mean-widtii ratio 0.238 0.213 



Blade-thickness fraction 0.038 0.047 



The corresponding full-scale diameters are shghtly 

 larger than contemplated in the hull design. 

 However, as it appears that the 20-ft wheel for 

 the transom-stern hull is on the small side, and as 

 further study of the tunnel stern indicates that 



the tip clearanc(! may to a<lvaiitag(; !«! loss than 

 the 1.0 ft originally planned, these two propellers 

 are considered suitable for the preliminary self- 

 propulsion tests. The larger wheel has a rake of 

 approximately 6 deg aft, but some brief sketching 

 indicates that this is small enough not to interfere 

 with its performance ahead of the four strut arms. 

 It appears, therefore, that a standard waterlinc 

 model length of 20 ft can be used. The scale ratio 

 X is then fixed at 25.5. 



In view of the relatively large wake velocities 

 to be expected around the inside of the tunnel in 

 the arch-stern design, a reduced pitch for the 

 outer blade sections of the propeller is definitely 

 indicated. This prevents overloading the tip 

 regions and the formation of strong tip vortexes. 

 Since TMB model propeller 1986 was designed 

 for a single-screw vessel with normal stern and 

 has constant pitch for 0.5 to 1.0 R, it will be rather 

 heavily loaded in that region when run under the 

 arch-stern model. 



78.5 Displacement and Draft Conditions. The 

 ABC models are to be run at a relatively advanced 

 stage of the prehminary design, when it is known 

 what appendages are to be carried and their sizes 

 and shapes are rather well determined. Their 

 total volume is readily calculated, as is the addi- 

 tional volume below the 26-ft DWL due to the 

 shell plating. It is decided therefore to run all tests 

 in the designed-load condition at a model weight 

 correspondmg to the total finished ship weight, 

 ■with plating and all appendages, when floating 

 at the designed waterUne in salt water having a 

 specific volume of 34.977 ft^ per long ton of 

 2,240 lb, at 59 deg F, 15 deg C. 



Since each model is to be built to the molded 

 lines of the respective hull design, is to have no 

 representation of plating, and is to carry no 

 appendages in the bare-hull condition, it may 

 be expected to float shghtly deeper than the 

 designed draft when ballasted to the total weight 

 prescribed. There are several reasons for this 

 apparently illogical procedure: 



(1) Tolerances and unavoidable errors in fairing 

 the preliminary fines, in making the early volume 

 calculations, and in shaping the model render it 

 almost a coincidence when the model floats at 

 exactly the correct draft 



(2) The change in draft due to the added volume 

 of the shell plating and normal appendages is 

 usually insignificant. For the two ABC hulls, the 

 change is less than 2 inches on the ship. 



