CHAPTER VII 



EFFECT ON DELIVERED HORSEPOWER (DHP) OF 

 VARIATION IN LCB POSITION 



Propulsion experiments with the LCB series models were carried out in order to find 

 the effect of the changing hull form on propulsive efficiency and on the dhp required to drive 

 the equivalent ships. 



The models were fitted with a keel piece and streamlined balanced rudder, as shown 

 in Figure 36. The rudder had an area equal to 1.5 percent of the product ofi LBP and draft, 

 with a standard EPH strut section having a maximum thickness-to-chord ratio of 0.167, the 

 maximum occurring at a point 0.35 of the chord from the leading edge. 



A standard propeller diameter of 0.7 of the draft was chosen so that one propeller 

 could be used for each block coefficient series, since draft remained constant with changes 

 in LCB location. Five propellers were made, each having four blades and following the well- 

 known Troost B-type as used by the NSMB in its systematic propeller series experiments. ^^ 

 The rake was reduced from 15 to 6 degrees to suit the Series 60 aperture. The principal 

 particulars of the propellers are given below for a ship with LBP of 600 ft, and a typical 

 drawing is shown in Figure 37. 



PROPELLER PARTICULARS 



Model block coefficient 



0.60 



0.65 



0.70 



0.75 



0.80 



Propeller Number 



3378 



3380 



3376 



3379 



3377 



Diameter D, ft 



22.40 



23.20 



24.00 



24.89 



25.82 



Pitch P, ft over outer disk 



24.08 



25.52 



26.40 



25.51 



23.75 



P/D 



1.075 



1.100 



1.100 



1.025 



0.920 



Blade Area Ratio 



0.550 



0.525 



0.500 



0.475 



0.450 



Blade Thickness Fracticm 



0.045 



0.045 



0.045 



0.045 



0.645 



Rake, deg 



6 



6 



6 



6 



6 



Number of blades 



4 



4 



4 



4 



4 



The pitch ratios were chosen to give maximum efficiency for the given diameter and 

 power absorption on the parent models, and the revolutions per minute (rpm) were derived 

 from the experiment results. No bilge keels were fitted, and studs were used for turbulence 

 stimulation. The tests were run in the Continental method at a loading corresponding to the 

 ship self-propulsion point with a ship correlation allowance of +0.0004. The models were 

 fitted with standard TMB pendulum dynamometers, and measurements were made of speed, 

 thrust, torque, and revolutions per minute (rpm). The data were extrapolated to apply to a 

 ship with LBP of 600 ft by the method outlined in Reference 58. The propulsive efficiencies 



VII-1 



