54 
PACIFIC SCIENCE, Vol. XII, January, 1958 
inch, of which the mouth intercepts 9 in. The 
remaining 9 in. is divided between the chan- 
nels lateral to the model. Each of these nar- 
rows from 4 Vi in. to 2Vi in. 
Figure 8 is a composite drawing and shows 
the flow lines at an estimated 4 kt. (left half) 
and 1 to 2 kt. (right half). A positive pressure 
gradient builds up lateral to the model, as a 
result of the restriction to flow due to the 
narrowing of the channels, and is probably 
responsible for the inwardly sloping flow lines 
at, and preceding, the leading edge of the 
mouth. There is a similar, but stronger, inflow 
towards the hole in the front of the boss. A 
positive pressure gradient again would seem 
to be the cause, but arising in the nature of 
the flow around the boss, and along the 
channels of mouth and valve chambers. The 
effects of the gradients become more evident 
at the faster rate of flow. It is possible that 
the representation of the flow lines preceding 
the leading edges of the mouth of the model 
are not what would obtain in open water with 
the catcher. However, flow lines into the hole 
in the boss are distorted by pressure develop- 
ing as a result of the design of the mouth and 
valve chambers, and in this case the effect 
would probably also occur with the catcher. 
Should either gradient persist during towing, 
a zone of higher pressure would precede the 
mouth. This feature is undesirable, although 
the end result may well be to reduce the actual 
mouth area to an effective one more nearly 
equivalent to the area of the orifice controlling 
the flow into the catcher, namely the valve 
(see later) . 
Flow through the mouth and valve cham- 
bers (Fig. 8) follows the channels "lateral” to 
the boss in clean and definite lines. Com- 
pression of the lines towards the outer surface 
of the channel, obvious at the faster rate, 
suggests that more water flows along this 
margin. At the higher rate, the lines leave the 
margin at a position preceding the valve and 
tend to straighten. The faring at the after end 
of the boss deflects the flow towards the valve 
where this occurs. Inside the boss, the main 
Fig. 8. Composite drawing showing flow lines for 
estimated speeds of 4 kt. (left half) and 1-2 kt. (right 
half). 
flow forms a distinct pattern near the spindle, 
but strong turbulence is evident beyond this. 
The main flow assists effectively in forcing 
the water from mouth and valve chambers 
into the valve opening. Even so, and largely 
because there is a sharp change of direction 
in flow from valve chamber into the valve, 
cavitation and turbulence are induced on the 
inner face of the valve. Therefore the valve 
may not be accepting water to its full capacity. 
These tests demonstrate that the flow pat- 
terns in mouth and valve chambers are satis- 
factory, evidence which is of value in assessing 
whether this design is functioning adequately. 
It is regretted that higher rates of flow were 
not available for further tests. 
Efficiency of the Catcher 
Relative efficiencies between the catcher 
