16 O. H. Oakley 
Escort Research Ship 
The escort research ship (see Fig. 1) is an actual design which was prepared to provide 
a research vehicle emphasizing quietness and minimum motion. Hydrodynamic quietness 
was sought by using a lightly loaded propeller and locating it deep and at the end of a 
nacelle well removed from the influence of the hull. Fig. 1 also shows the large nacelle or 
dome located at the bow to house sonar transducers and to provide damping and added mass 
which tend to produce favorable pitch characteristics. 
The aim of this unusual hull form was to avoid resonance in pitch by raising the natural 
pitch period. It was hoped by this to permit operation in the supercritical region with 
attendant low pitch response. This is illustrated in Fig. 16, which shows a single degree 
of freedom type of response for a damped system and indicates the supercritical range in 
which it is desired to operate. 
| RESONANCE — 
( AMPLITUDE ) 
f 
SUPERCRITICAL 
MOTION 
FORCING FREQUENCY 
NATURAL FREQUENCY 
Fig. 16. The supercritical region of motion, showing the region of 
operation under conditions of favorable pitch characteristics 
The longitudinal moment of inertia of the water plane was made small; this, combined 
with maximizing the longitudinal mass moment of inertia of the hull by moving heavy 
weights toward the ends and the added mass due to the water entrained by the nacelles, 
tended to produce a long pitching period. 
The early models tested of this concept were largely exploratory and investigated the 
locations of nacelles, etc. Model tests showed excellent pitch characteristics for the con- 
figuration then under study. The short-dashed line in Fig. 17 shows the very favorable 
behavior in pitch. This figure shows the pitch response in regular waves and is similar to 
the previous figure except that the forcing frequency or period of encounter is represented 
as a function of speed. In this instance the wave corresponds to the maximum energy wave 
in a Neuman spectrum for a state 5 sea. 
