| ITT. | H (shearing force) 
HM approximate line r 
| H I of shear ! 
| 1 | 1 I 
| | 
= approximate shear 
! area A 
(a) shear stress = + (b) 
Figure 39. Graphical analogy between laboratory soil shear box (a) and a 
track on undisturbed soil, (b). 
It is extremely difficult to predict the traction that will be available 
in a Saturated submerged soil. Although the shear strength of an undisturbed 
soil sample may be determined, the effect of agitation caused by a vehicle 
moving through the soil—water interface is to mix the soil and water. In the 
case of soft clays whose particles are very small and light, the whole mass of 
disturbed soil can be mixed with the water and become liquid. In this state, 
the cohesion is destroyed and no traction will be available. Hydromechanical 
methods may then be the only means of locomotion under these circumstances 
(Bekker, 1956, p. 142). 
Propellers produce thrust, or tractive effort, by changing the 
momentum of the fluid in which they are submerged. A vehicle working 
on the ocean bottom using propellers for propulsion would not have to 
depend on the highly unpredictable bottom soil for movement. Another 
advantage of a submerged propeller is that it can be made to produce a 
dependable amount of thrust in any direction, including the vertical. 
Another hydrochemical device for producing thrust is the water 
jet. This is essentially a pump that creates a water jet and by so doing has 
a thrust exerted upon it which is the propelling force. In fact, a propeller 
does the same thing and thus Is one form of water jet propulsion (Streeter, 
1948, p. 114). 
The propelling force, F, of a water jet unit is 
F = pQAV (15) 
in which p is the fluid density, O is the quantity of fluid moved in cubic feet 
per second, and AV is the absolute velocity of the fluid. 
49 
