**Case 3**:

step 1. Surge (Propeller I and II work together, propeller III powered off);

step 2. Brake (Propeller I and II powered off, Propeller III works to produce brake force).

In case 1, timing of step 1 is about 10*s*, and step 2 takes about 12*s*. And timing of case 2 is relatively the same with case 1, because of the same hydrodynamics characteristics of turning right and left. In case 3, it takes 15*s* reaching a stable speed, and the brake effect happens in about 3*s* which is effective for low speed underwater vehicles. From Fig.20, we can see,

Fig. 20. Experimental Results of Horizontal Motion

the experimental results fit well with simulation results in surge stage, but when the vehicle rotating, errors become large. The reason of this is because the simulation experiment only considered linear damping force and quadratic damping force, but in reality, there are other hydrodynamic forces act on the vehicle.
