Design DSGN 313 - ROVs - Some Initial Thoughts and Hints

ROVs tend to come in 2 types:

1. Tend to have thrusters for each required direction, maximum speed tends to be slow and directional control is poor.
The flow of water to the propellers is usually limited leading to poor performance.
A vertical thruster can overcome slight positive or negative buoyancy.
Should be able to manoeuvre in restricted spaces.
Skids make settling on a flat bottom easy.

2. Easier to get a streamlined shape, good flow over propellers and consequently better performance, but if it is a single cylinder then contra rotating propellers are needed to prevent the device rotating. Some designs have two cylinders (separated by some structure) each of which has its own propeller. There is no need for contra rotating propellers or rudders as dirctional control is by adjusting propeller speed and reversing one propeller.
If propellers are only fitted at the stern, then adjustable hydrovanes are needed to control depth. Unless buoyancy is adjustable, hovering is likely to be difficult and if a current is present will be impossible unless facing it (or stern to if propellers are reversible).
Single cylinder devices have limited ability to manoeuvre in restricted spaces.
Packaging the internal components may be more difficult in a cylindrical container, but it can easily be made to withstand a much greater pressure than a rectangular box.

General Points
Check stresses for both yielding AND buckling.
The theoretical equations for buckling assume perfect geometry, etc. Even quite minor variations in wall thickness can seriously reduce the sustainable external pressure capacity.
Solid Works COSMOS Works can be used to carry out a buckling analysis. The buckling factor computed is the multiplication factor that needs to be applied to the load you have specified to produce buckling.

Voltage loss down a long length of wire can be considerable.

Designing to resist unidirectional pressure is not difficult, however beware if you have a situation where the pressure can (at different times!) act in both directions - this is more tricky.

Over designing a gland joint to give you a big margin on water tightness is likely to result in too much friction, or even being unable to turn the shaft with the motor. This must be considered when designing seals and specifying motors.

Dr David J Grieve, 14th September 2006 (previous: 30th September 2002).